Chapter VI: Bacteria in Milk, Milk Products, and Other Foods (1)

Injurious micro-organisms in foods are, fortunately for the consumers, usually killed by cooking. Vast numbers are, as far as we know, of no harm whatever. Alarming reports of the large numbers of bacteria which are contained in this or that food are generally as irrelevant as they are incorrect. Bacteria, as we have seen, are ubiquitous. In food we have abundance of the chief thing necessary to their life and multiplication--favourable nutriment. Hence we should expect to find in uncooked or stale food an ample supply of saprophytic bacteria. There was much wholesome truth in the assertions made some two years ago by the late Professor Kanthack, to the effect that good food as well as bad frequently contained large numbers of bacteria, and often of the same species. It is well that we should become familiarised with this idea, for its accuracy cannot be doubted, and its usefulness at the present time may not be without its beneficial effect.

Nevertheless, it is well we should know the bacterial flora of good and bad foods for at least two reasons. First, there is no doubt whatever that a considerable number of cases of poisoning can be traced every year to food containing harmful bacteria or their products. To several of the more notorious cases we shall have occasion to refer in passing. Secondly, we may approach the study of the bacteriology of foods with some hope that therein light will be found upon some important habits and effects of microbes. There can be little doubt that food-bacteria afford an example of association and antagonism of organisms to which reference has already been made. Any information that can be gleaned to illumine these abstruse questions would be very welcome at the present time. But there is a still further, and possibly an equally important, point to bear in mind, namely, the economic value of microbes in food. In a short account like the present it will be impossible to enter into hypotheses of pathology, but we shall at least be able to consider some of these interesting experiments which have been conducted in the sphere of beneficial bacteria.

The injurious effects of organisms contained in foods has been elucidated by the excellent work of the late Dr. Ballard. From the careful study of a number of epidemics due to food poisoning, this patient observer was able, without the aid of modern bacteriology, to arrive at a simple principle which must not be forgotten. Food poisoning is due either to bacteria themselves or to their products, which are contained in the substance of the food. In cases of the first kind, bacteria gaining entrance to the human alimentary canal, set up their specific changes and produce their toxins, and by so doing in course of time bring about a diseased condition, with its consequent symptoms. On the other hand, if the products, sometimes called _ptomaines_, are ingested as such, the symptoms set up by their action in the body tissues appear earlier. From these facts Dr. Ballard deduced the simple principle that if there is no incubation period or, at all events, a comparatively short space of time between eating the poisoned food and the advent of disease, the agents of the disease are products of bacteria. If, on the other hand, there is an incubation period, the agents are probably bacteria.

It is necessary to mention two other facts. Dr. Cautley[53] has recently been engaged in isolating from poisoned foods the different species of bacteria present. It would appear that these are limited, as a rule, to two or three kinds. As regards disease, the organisms of suppuration are the most common. Liquefying or fermentative bacteria are frequently present, the _Proteus_ family being well represented. In addition there are, according to circumstances, a number of common saprophytes. Now, as we have pointed out, these organisms may act injuriously by some kind of cooperation, or they may by themselves be harmless, and pathological conditions be due to the occasional introduction of pathogenic species.

The other fact, requiring recognition from anyone who proposes to study the bacteriology of foods, is that a certain appreciable amount of the responsibility for food poisoning rests with the tissues of the individual ingesting the food. There is ample evidence in support of the fact that not all the persons partaking of infected food suffer equally, and occasionally some escape altogether. We know little or nothing of the causes of such modification in the effect produced. It may be due to other organisms, or chemical substances already in the alimentary canal of the individual, or it may be due to some insusceptibility or resistance of the tissues. Be that as it may, it is a matter which must not be neglected in estimating the effects of food contaminated with bacteria or their products.

_Milk._ There are few liquids in general use which contain such enormous numbers of germs as milk. To begin with, milk is in every physical way admirably adapted to be a favourable medium for bacteria. It is constituted of all the chief elements of the food upon which bacteria live. It is frequently at a temperature favourable to their growth. It is _par excellence_ an absorptive fluid. A dish of ordinary water and a dish of newly drawn milk laid side by side, and under similar conditions of temperature, will rapidly demonstrate the difference in degree of absorptivity between the two fluids. Yet, whilst this general fact is true, we must emphasise at the outset the possibility and practicability of securing absolutely pure sterile milk. Recently some milking was carried out under strict antiseptic precautions, with the above sterile result. The udder was thoroughly cleansed, the hands of the milker washed with corrosive sublimate and then pure water, the vessels which were to receive the milk had been carefully sterilised, and the whole process was carried out in strict cleanliness. The result was that the sample of milk remained sweet and good and contained no germs. It should be stated that the first flow of milk, washing out the milk-ducts of the udder, was rejected. This fact of the sterility of cleanly drawn milk is not a new one, and has been established by many bacteriologists. Milk, then, is normally a sterile secretion. How does it gain its enormously rich flora of bacteria?

_Sources of Pollution of Milk._ These are various, and depend upon many minor circumstances and conditions. For all practical purposes there are three chief opportunities between the cow and the consumer when milk may become contaminated with bacteria:

1. At the time of milking.

2. During transit to the town, or dairy, or consumer.

3. After its arrival.

_Pollution at the Time of Milking_ arises from the animal, the milker, or unclean methods of milking. It is now well known that in tuberculosis of the cow affecting the udder the milk itself shows the presence of the bacillus of tubercle. In a precisely similar manner all bacterial diseases of the cow which affect the milk-secreting apparatus must inevitably add their quota of bacteria to the milk. To this matter we shall have occasion to refer again. There is a further contamination from the animal when it is kept unclean, for it happens that the unclean coat of a cow will more materially influence the number of micro-organisms in the milk than the popularly supposed fermenting food which the animal may eat. It is from this external source rather than from the diet that organisms occur in the milk. The hairy coat offers many facilities for harbouring dust and dirt. The mud and filth of every kind that may be habitually seen on the hinder quarters of cattle all contribute largely to polluted milk. Nor is this surprising. Such filth at or near the temperature of the blood is an almost perfect environment for many of the putrefactive bacteria.

The milker is also a source of risk. His hands, as well as the clothes he is wearing, can and do readily convey both innocent and pathogenic germs to the milk. Clothed in dust-laden garments, and frequently characterised by dirty hands, the milker may easily act as an excellent purveyor of germs. Not a few cases are also on record where it appears that milkers have conveyed germs of disease from some case of infectious disease, such as scarlet fever, in their homes. But under the more efficient registration of such disease which has recently characterised many dairy companies, the danger of infection from this source has been reduced to a minimum. The habit of moistening the hands with a few drops of milk previous to milking is one to be strongly deprecated.

Professor Russell recounts a simple experiment which clearly demonstrates these simple but effective sources of pollution:

"A cow that had been pastured in a meadow was taken for the
experiment, and the milking done out of doors, to eliminate
as much as possible the influence of germs in the barn air.
Without any special precaution being taken the cow was
partially milked, and during the operation a covered glass
dish, containing a thin layer of sterile gelatine, was exposed
for sixty seconds underneath the belly of the cow in close
proximity to the milk-pail. The udder, flank, and legs of
the cow were then thoroughly cleaned with water, and all of
the precautions referred to before were carried out, and the
milking then resumed. A second plate was then exposed in the
same place for an equal length of time, a control also being
exposed at the same time at a distance of ten feet from the
animal and six feet from the ground to ascertain the germ
contents of the surrounding air. From this experiment the
following instructive data were gathered. Where the animal was
milked without any special precautions being taken there were
3250 bacterial germs _per minute_ deposited on an area equal
to the exposed top of a ten-inch milk-pail. Where the cow
received the precautionary treatment as suggested above, there
were only 115 germs per minute deposited on the same area.
In the plate that was exposed to the surrounding air at some
distance from the cow there were 65 bacteria. This indicates
that a large number of organisms from the dry coat of the
animal can be kept out of milk if such simple precautions as
these are carried out."[54]

The influence of the barn air, and the cleanliness or otherwise of the barn, is obviously great in this matter. As we have seen, moist surfaces retain any bacteria lodged upon them; but in a dry barn, where molecular disturbance is the rule rather than the exception, it is not surprising that the air is heavily laden with microbic life. Here again many improvements have been made by sanitary cleanliness in various well-known dairies. Still there is much more to be done in this direction to ensure that the drawn milk is not polluted by a microbe-impregnated atmosphere.

The risks in _transit_ differ according to many circumstances. Probably the commonest source of contamination is in the use of unclean utensils and milk-cans. Any unnecessary delay in transit affords increased opportunity for multiplication; particularly is this the case in the summer months, for at such times all the conditions are favourable to an enormous increase of any extraneous germs which may have gained admittance at the time of milking. Thus we have (1) the milk itself affording an excellent medium and supplying ideal pabulum for bacteria, (2) a more or less lengthened railway journey or period of transit giving ample time for multiplication, (3) the favourable temperature of summer heat. We shall refer again to the rate of multiplication of germs in milk.

Lastly, many are the advantages given to bacteria when milk has reached its commercial destination. In milk-shops and in the home there are not a few risks to be added on to the already imposing category. Water is occasionally, if not frequently, added to milk to increase its volume. Such water of itself will make its own contribution to the flora of the milk, unless indeed, which is unlikely, the water has been recently and thoroughly boiled before addition to the milk. Again, it is impossible to suppose that in small homes--perhaps of only one room--where the milk stands for several hours, pollution is avoidable. From a hundred different sources such milk runs the risk of being polluted.

Before proceeding, a word must be said respecting the first milk which flows from the udder in the process of milking, and which is known as the _fore-milk_. This portion of the milk is always rich in bacterial life on account of the fact that it has remained in the milk-ducts since the last milking. However thorough the manipulation, there will always be a residue remaining in the ducts, which will, and does, afford a suitable nidus and incubator for organisms. The latter obtain their entrance through the imperfectly closed teat of the udder, and pass readily into the milk-duct, sometimes even reaching the udder itself and setting up inflammation (_mastitis_). Professor Russell states that he has found 2800 germs in the fore-milk in a sample of which the average was only 330 per cc. Schultz found 83,000 micro-organisms per cc. in the fore-milk, and only 9000 in the mid-milk. As a matter of fact, most of this large number belong to the _lactic-acid fermentation_ group, and the fore-milk rarely contains more than two or three species, and still more rarely any disease-producing bacteria. Still, they occur in such enormous numbers that their addition to the ordinary milk very materially alters its quality. Bolley and Hall, of North Dakota, report sixteen species of bacteria in the fore-milk, twelve of which produced an acid reaction. Dr. Veranus Moore, of the United States Department of Agriculture,[55] concludes from a large mass of data that freshly drawn fore-milk contains a variable but generally enormous number of bacteria, but only several species, the last milk containing, as compared with the fore-milk, very few micro-organisms. The bacteria which become localised in the milk-ducts, and which are necessarily carried into the milk, are for the greater part rapidly acid-producing organisms, _i. e._, they ferment milk-sugar, forming acids. They do not produce gas. Still their presence renders it necessary to "pasteurise" as soon as possible. Dr. Moore holds that much of the intestinal trouble occurring in infants fed with ordinarily "pasteurised" milk arises from acids produced by these bacteria between the drawing of the milk and the pasteurisation.

_The Number of Bacteria in Milk._ From all that has been said respecting the sources of pollution and the favourable nidus which milk affords for bacteria, it is not surprising that a very large number of germs are almost always present in milk. The quantitative estimation of milk appears more alarming than the qualitative. It is true some diseases are conveyed by bacteria in milk, but on the whole most of the species are non-pathogenic. Nor need the numbers, though serious, too greatly alarm us, for, as we shall see at a later stage, disease is a complicated condition, and due to other agencies and conditions than merely the bacteria, which may be the _vera causa_. In addition to the fact that the high numbers have but a limited significance, we must also remember that there is no uniformity whatever in these numbers. The conditions which chiefly control them are (1) temperature, (2) time.

_The Influence of Temperature._ We have already noticed, when considering the general conditions affecting bacteria, how potent an agent in their growth is the surrounding temperature. Generally speaking, temperature at or about blood-heat favours bacterial growth. Freudenreich has drawn up the following table which graphically sets forth the effect of temperature upon bacteria in milk:

3 hours. 6 hours. 9 hours. 24 hours.

59° F. 1 + 2.5 5 163
77° F. 2 18.5 107 62,100
95° F. 4 1,290 3,800 5,370

This instructive table claims some observations. It will be noticed that at 59° F. there is very little multiplication. That may be accepted as a rule. At 77° F. the multiplication, though not particularly rapid at the outset, results finally, at the end of the twenty-four hours, in the maximum quantity. These were probably common species of saprophytic bacteria, which increase readily at a comparatively low temperature. During the subsequent hours, after the twenty-four, we should expect a decline rather than an increase in 62,000, owing to the keen competition consequent upon the limitation of the pabulum. From a consideration of these figures we conclude that a warm temperature, somewhat below blood-heat, is most favourable to multiplication of bacteria in milk; that the common saprophytic organisms multiply the most rapidly; that, in the course of time, competition kills off a large number.

Let us take another example, from Professor Conn:

77° F. 95° F.
2 hours after milking (liquefied the
plate of gelatine) 1,275,000
6 " " " 14,620,000 45,900,000
9 " " " 36,550,000 57,800,000
24 " " " 13,702,000,000 13,812,500,000

[Bacteria per cub. inch.]

These almost incredibly large figures illustrate much the same points, particularly the rapid multiplication at blood-heat, and the later rise at 77° C.

_The Influence of Time_ is not less marked than that of temperature, as the following table will show:

Milk drawn at 59° C. = 153,000 m.o. per cub. in.
After 1 hour = 616,000 " " " "
" 2 hours = 539,000 " " " "
" 4 " = 680,000 " " " "
" 7 " = 1,020,000 " " " "
" 9 " = 2,040,000 " " " "
" 24 " = 85,000,000 " " " "

(Conn.)

Freudenreich gives another example, as follows:

Milk drawn at 15.5° C. = 27,000 m.o. per cc.
After 4 hours = 34,000 " " "
" 9 " = 100,000 " " "
" 24 " = 4,000,000 " " "

Concerning these figures little comment is necessary. But here again, we may remember that this rapid multiplication continues only up to a certain point, after which competition brings about a marked reduction.

The effect of temperature and time has been illustrated by Dr. Buchanan Young's recent researches, laid before the Royal Society of Edinburgh. He estimated that in the Edinburgh milk supply three hours after milking there were 24,000 micro-organisms per cc. _in winter_; 44,000 _in spring_; 173,000 _in late summer and autumn_. Again, he found that five hours after milking there were 41,000 micro-organisms per cc. in country milk, and more than 350,000 micro-organisms per cc. in town milk. Many London milks would exceed 500,000 per cc.[56]

There is no standard or uniformity in the numerical estimation of bacteria in milk. A host of observers have recorded widely varying returns due to the widely varying circumstances under which the milk has been collected, removed, stored, and examined. Nor is it possible to establish any standard which may be accepted as a _normal_ or healthy number of bacteria, as is done in water examination. Bitter has suggested 50,000 micro-organisms per cc. as a maximum limit for milk intended for human consumption.

But owing to differences of nomenclature and classification, in addition to differences in mode of examination at present existing in various countries, it is impossible to state even approximately how many bacteria and how many species of bacteria have been isolated from milk. Until some common international standard is established mathematical computations are practically worthless. They are needlessly alarming and sensational. And it should be remembered that great reliance cannot be placed upon these numerical estimations. They vary from day to day, and even hour to hour. Furthermore, vast numbers of bacteria are economic in the best sense of the term, and the bacteria of milk are chiefly those of a fermentative kind, and not disease-producers.

_Kinds of Bacteria in Milk._ It is clear from the foregoing that the only valuable estimation of bacteria in milk is a qualitative one. The kinds commonly found may be classified thus:

1. Non-pathogenic; fermenting and various unclassified micro-organisms.

2. Pathogenic; tuberculosis, typhoid, cholera, scarlet fever, diphtheria, and suppurative diseases have all been spread by the agency of milk.

1. _The Fermentation Bacteria_

At the most we can make a merely provisional classification of these processes. Many of them are intimately related. Of others, again, our knowledge is at present very limited. It may be advisable, before proceeding, to consider shortly what are the constituents of milk upon which living ferments of various kinds exert their action. A tabulation of the chief constituents would be as follows:

{ (1) Water 87.5 per cent.
Ordinary { (2) Milk-sugar 4.9 " "
fresh milk = { (3) Fat 3.6 " "
100 per cent. { (4) Proteids (casein, etc.) 3.3 " "
{ (5) Mineral matter 0.7 " "
-----
100.0

Another mode of expressing average milk constitution would be thus:

Fat 4.1 per cent.
Solids not fat 8.8 " "
----
12.9 " "

It is probably too obvious to need remark that milks vary in standard, but the above figures may be taken as authentic averages.

_Milk-sugar, or Lactose_ (C_{12}H_{24}O_{12}). This is an important and constant constituent of milk. It forms the chief substance in solution in whey or serum. Milk-sugar approximates to dextrose in its action on polarised light. By boiling with sulphuric acid it is converted into dextrose and galactose.

_Fat_ occurs in milk as suspended globules, and by churning may be made into butter.

_The Proteids_ include casein, albumen, lactoprotein, and a small quantity of globulin. These are the nitrogenous bodies.

_Mineral Matter._ The ash of milk, obtained by careful ignition of the solids, contains calcium, magnesium, potassium, sodium, phosphoric acid, sulphuric acid, chlorine, and iron, phosphoric acid and lime being present in the largest amounts.

(1) _Lactic Acid Fermentation._ If milk is left undisturbed, it is well known that eventually it becomes sour. The casein is coagulated, and falls to the bottom of the vessel; the whey or serum rises to the top. In fact, a coagulation analogous to the clotting of blood has taken place. In addition to this, the whole has acquired an acid taste. Now, this double change is not due to any one of the constituents we have named above. It is, in short, a fermentation set up by a living ferment introduced from without. The constituent most affected by the ferment is the milk-sugar, which is broken down into lactic acid, carbonic acid gas, and other products.

For many years it has been known that sour milk contained bacteria. Pasteur first described the _Bacillus acidi lactici_, which Lister isolated and obtained in pure culture. Hueppe contributed still further to what was known of this bacillus, and pointed out that there were a large number of varieties, rather than one species, to be included under the term _B. acidi lactici_. We have already seen that these bacilli do not as a rule liquefy gelatine, form spores, are non-motile, and are easily killed by heat.

When a certain quantity of lactic acid has been formed the fermentation ceases. It will recommence if the liquid be neutralised with carbonate of lime, or pepsine added. Since Pasteur's discovery of a causal bacillus for this fermentation, other investigators have added a number of bacteria to the lactic acid family. Some of these in pure culture have been used in dairy industry to add to the butter a pure sour taste, a more or less aromatic odour, and a higher degree of preservation.

(2) _Butyric Acid Fermentation._ This form of fermentation is also one which we have previously considered.

Both in lactic and butyric fermentation we must recognise that in the decomposition of milk-sugar there are almost always a number of minor products occurring. Some of the chief of these are gases. Hydrogen, carbonic acid, nitrogen, and methane occur, and cause a characteristic effect which is frequently deleterious to the flavour of the milk and its products. Most of the gas-producing ferments are members of the lactic acid group, and are sometimes classified in a group by themselves. In cheese-making the gases create the pin-holes and air-spaces occasionally seen.

(3) _Curdling Fermentations without Acid Production._ Of these there are several, caused by different bacteria. What happens is that the milk coagulates, as we have described, but no acid is produced, the whey being sweet to the taste rather than otherwise. Digestion of casein may or may not take place.

We must now mention several fermentations about which little is known. They are designated by terms denoting the outward condition of the milk, without giving any information respecting the real physiological alteration which has occurred.

(4) _Bitter Fermentation._ Some bitter conditions of milk are due to irregularity of diet in the cow. Similar changes occur in conjunction with some of the acid fermentations. Weigmann and Conn have, however, shown that there is a specific bitterness in milk due to bacteria which appear to produce no other change. Hueppe suggests that it may be due in part to a proteid decomposition resulting in bitter peptones. There seems to be some evidence for supposing that the bitter bacteria produce very resistant spores, which enable them to withstand treatment under which the lactic acid succumbs.

(5) _Slimy Fermentation._ This graphic but inelegant word is used to denote an increased viscosity in milk, and its tendency when being poured to become ropy and fall in strings. Such a condition deprives the milk of its use in the making of certain cheeses, whilst in other cases it favours the process. In Holland, for example, in the manufacture of Edam cheese, this "slimy" fermentation is desired. _Tættemœlk_, a popular beverage in Norway, is made from milk that has been infected with the leaves of the common butter wort, _Pinguicula vulgaris_, from which Weigmann separated a bacillus possessing the power of setting up slimy fermentation. There are, perhaps, as many as a dozen species of bacteria which have in a greater or less degree the power of setting up this kind of fermentation. In 1882 Schmidt isolated the _Micrococcus viscosus_, which occurs in chains and rosaries, affecting the milk-sugar. It grows at blood-heat, and is not easily destroyed by cold. Its effect on various sugars is the same. The _M. Freudenreichii_, the specific micro-organism of "ropiness" in milk, is a large, non-motile, liquefying coccus, which can produce its result in milk within five hours. On account of its resistance to drying, it is difficult to eradicate when once it makes its appearance in a dairy. The organism used in making Edam cheese is the _Streptococcus Hollandicus_, and in hot milk it can produce ropiness in one day. A number of bacilli have been detected by several observers and classified as slime fermentation bacteria. The _Bacillus lactis pituitosi_, a slightly curved, non-liquefying rod, which is said to produce a characteristic odour, in addition to causing ropiness, brings about some acidity. _B. lactis viscosus_ is slow in starting its fermentation, but maintains its action for as long as a month. Many of the above organisms, with others, produce "slimy" fermentation in alcoholic beverages as well as in milk.

(6) _Soapy Milk._ This is still another form of fermentation, the etiology of which has been elucidated by Weigmann. The _Bacillus saponacei_ imparts to milk a peculiar soapy flavour. It was detected in the straw of the bedding and hay of the fodder, and from such sources may infect the milk. There is little or no coagulation.

(7) _Chromogenic Changes._ We have already remarked that colour is the natural and apparently only product of many of the innocent bacteria. They put out their strength, so to speak, in the production of bright colours. The chief colours produced by germs in milk are as follows:

_Red Milk._ _Bacillus prodigiosus_, in the presence of oxygen, causes a redness, particularly on the surface of milk. It was the work of this bacillus that caused "the bleeding host," which was one of the superstitions of the Middle Ages. _B. lactis erythrogenes_ produces a red colour only in the dark, and in milk that is not strongly acid in reaction. When grown in the light this organism produces a yellow colour. There is a red sarcina (_Sarcina rosea_) which also has the faculty of producing red pigment. One of the yeasts is another example.

It must not be forgotten that redness in milk may actually be due to the presence of blood from the udder of the cow. In such a case the blood and milk will be inextricably mixed together, and not in patches or a pellicle.

_Blue Milk_ is due to the growth of _Bacillus cyanogenus_. This is an actively motile rod, the presence of which does not materially affect the milk, but causes the milk products to be of poor quality.

_Yellow Milk._ _Bacillus synxanthus_ is held responsible for curdling the milk, and then at a later stage, in redissolving the curd, produces a yellow pigment.

_Violet and Green Pigments_ in milk are also the work of various bacteria.

2. _Various Unclassified Bacteria_

In milk this is a comparatively small group, for it happens that those bacteria in milk which cannot be classified as fermentative or pathogenic are few. The almost ubiquitous _Bacillus coli communis_ occurs here as elsewhere, and might be grouped with the gaseous fermentative organisms on account of its extraordinary power of producing gas and breaking up the medium (whether agar or cheese) in which it is growing. What its exact rôle is in milk it would be difficult to say. It may act, as it frequently does elsewhere, by _association_ in various fermentations. Some authorities hold that its presence in excessive numbers may cause epidemic diarrhœa in infants.

Several years ago a commission was appointed by the _British Medical Journal_ to inquire into the quality of the milk sold in some of the poorer districts of London. Every sample was found to contain _Bacillus coli_, and it was declared that this particular microbe constituted 90 per cent. of all the organisms found in the milk.[57] We record this statement, but accept it with some misgiving. The diagnosis of _B. coli_ four or five years ago was not such a strict matter as to-day. Still, undoubtedly, this particular organism is not uncommonly found in milk, and its source is unclean dairying. In the same investigation _Proteus vulgaris_, _B. fluorescens_, and many liquefying bacteria were frequently found. Their presence in milk means contamination with putrefying matter, surface water, or a foul atmosphere.

A number of water bacteria find their way into milk in the practice of adulteration, and foul byres afford ample opportunity for aërial pollution.

Another unclassified group occasionally present in milk is represented by moulds, particularly _Oidium lactis_, the mould which causes a white fur, possessing a sour odour. It is allied to the _Mycoderma albicans_ (_O. albicans_), which also occurs in milk, and causes the whitish-grey patches on the mucous membrane of the mouths of infants (_thrush_). These and many more are occasionally present in milk.

3. _The Disease-Producing Power of Milk_

The general use of milk as an article of diet, especially by the younger and least resistant portion of mankind, very much increases the importance of the question as to how far it acts as a vehicle of disease. Recently considerable attention has been drawn to the matter, though it is now a number of years since milk was proved to be a channel for the conveyance of infectious diseases. During the last twenty years particular and conclusive evidence has been deduced to show that milch cows may themselves afford a large measure of infection. The recent extensive work in tuberculosis by the Royal Commission has done much to obtain new light on the conveyance of that disease by milk and meat. The enormous strides in the knowledge of diphtheria and other germ diseases have also placed us in a better position respecting their conveyance by milk. Generally speaking, for reasons already given, milk affords an ideal medium for bacteria, and its adaptibility therefore for conveying disease is undoubted. We may now suitably turn to speak shortly of the outstanding facts of the chief diseases carried by milk.

_Tuberculosis._ It is well known that this disease is not a rare one amongst cattle. The problem of infective milk is, however, simplified at the outset by recognising the now well-established fact that the milk of tuberculous cows is only certainly able to produce tuberculosis in the consumers when the _tuberculous disease affects the udder_. This is not necessarily a condition of advanced tuberculosis. The udder may become affected at a comparatively early stage. But to make the milk infective the udder must be tubercular, and milk from such an udder possesses a most extraordinary degree of virulence. When the udder itself is thus the seat of disease, not only the derived milk, but the skimmed milk, butter-milk, and even butter, all contain tuberculous material actively injurious if consumed. Furthermore, tubercular disease of the udder spreads in extent and degree with extreme rapidity. From these facts it will be obvious that it is of first-rate importance to be able to diagnose udder disease. This is not always possible in the early stage. The signs upon which most reliance may be placed are the enlargement of the lymph-glands lying above the posterior region of the udder; the serous, yellowish milk which later on discharges small coagula; the partial or total lack of milk from one quarter of the udder (following upon excessive secretion); the hard, diffuse nodular swelling and induration of a part or the whole wall of the udder; and the detection in the milk of tubercle bacilli. The whole organ may increase in weight as well as size, and on _post-mortem_ examination show an increase of connective tissue, a number of large nodules of tubercle, and a scattering of small granular bodies, known as "miliary" tubercles. _Tuberculin_ may be used as an additional test. The udder is affected in about two per cent. of tuberculous cows.

There are a variety of causes in addition to the _vera causa_, the presence of the bacillus of tubercle, which make the disease common amongst cattle. Constitution, temperament, age, work, food, and prolonged lactation are the individual features which act as predisposing conditions; they may act by favouring the propagation of the bacillus or by weakening the resistance of the tissues. To this category must further be added conditions of environment. Bad stabling, dark, ill-ventilated stalls, high temperature, prolonged and close contact with other cows, all tend in the same direction.

Though there can be no doubt as to the virulence of tuberculous milk, it may be remembered with satisfaction that only about two per cent. of tuberculous cows have unmistakably tubercular milk. Even of this tubercular milk, unless it is very rich in bacilli and is ingested in large quantities, the risks are practically small or even absent. Practically the danger from drinking raw milk exists only for persons who use it as their sole or principal food, that is to say, young children and certain invalids. With adults in normal health the danger is greatly minimised, as the healthy digestive tract is relatively insusceptible. Moreover, dairy milk is almost invariably _mixed_ milk; that is to say, if there is a tubercular cow in a herd yielding tubercle bacilli in her milk, the addition of the milk of the rest of the herd so effectually _dilutes_ the whole as to render it almost innocuous.

But if for practical purposes we look upon all milk derived from tubercular udders as highly infective, we may adopt a comparatively simple and efficient remedy. To avoid all danger it is sufficient to bring the milk to a boil for a few minutes before it is consumed; in fact, the temperature of 85° C. (160° F.) prolonged for five minutes kills all bacilli. The common idea that boiled milk is indigestible, and that the boiling causes it to lose much of its nutritive value, is largely groundless.

Milk may become tubercular through the carelessness or dirty habits of the milker. Such a common practice as moistening the hands with saliva previously to milking may, in cases of tubercular milkers, effectually contaminate the milk. Again, it may become polluted by dried tubercular excreta getting into it. Such conveyances must be of rare occurrence, yet their possibility should not be forgotten.

An infant suckled by a tuberculous mother would run similarly serious risks of becoming infected with the disease.

In Liverpool, Dr. E. W. Hope, the Medical Officer of Health, has organised an admirable system of examination by skilled bacteriologists to find to what degree the Liverpool milk supply is contaminated with tubercle. The final result of this pioneer work, which ought really to be undertaken by every great corporation responsible to the citizens for a pure water and pure milk supply, is to the effect that in Liverpool 5.2 per cent. of the samples of milk taken from the city shippons contains tubercle bacilli. As regards the milk sent in from the country, the return is that 13.4 per cent. is contaminated with the bacillus of tubercle.

-------------------------------+--------------------------------+--------
TOWN SHIPPONS. | COUNTRY SHIPPONS. | TOTAL.
-------+-----------+-----------+--------+-----------+-----------+--------
Total. | Infected. | Per cent. | Total. | Infected. | Per cent. |
228 | 12 | 5.2 | 67 | 9 | 13.4 | 295
-------+-----------+-----------+--------+-----------+-----------+--------

Such results are very significant, and indicate the importance of all large corporations obtaining the service of systematic and periodic bacteriological examination of the milk supply. Nor are the results surprising, for when we remember the habits of the tubercle bacillus we cannot conceive a more favourable nurture ground than the typical byre. "Nothing worse than the insanitary conditions of the life of the average dairy cow," says Sir George Brown, late of the Board of Agriculture, "can be imagined." It will be obvious that the above facts make it incumbent upon responsible authorities to see that not a stone is left unturned to enforce cleanliness in all dairy work, isolation of diseased cows, and strict treatment of all infected milk.

_Typhoid Fever._ Jaccoud in France and Hart in England have shown that enteric fever (typhoid) is not infrequently spread by milk. An epidemic affecting 386 persons in Stamford, Conn., U.S.A., was traced to milk, 97 per cent. of the cases coming from one single milk supply. Dr. McNail recently recorded an outbreak of twenty-two cases of enteric, due to a polluted milk supply.

Within the last twelve months much attention has been drawn to a milk source of typhoid infection by the epidemic of typhoid at Bristol. Dr. D. S. Davies has pointed out that a brook received the sewage of thirty-seven houses, the overflow of a cesspool serving twenty-two more, the washings from fields over which the drainage of several others was distributed, and the direct sewage from at least one other, and then flowed directly through a certain farm. The water of this stream supplied the farm pump, and the water itself, it is scarcely necessary to add, was highly charged with putrescent organic matter and micro-organisms. This water was used for washing the milk-cans from this particular farm, otherwise the dairy arrangements were efficient. Part of the milk was distributed to fifty-seven houses in Clifton; in forty-one of them cases of typhoid occurred. Another part of the milk was sold over the counter; twenty households so obtaining it were attacked with typhoid fever, and a number of further infections and complications arose. This evidence would appear to support the fact that milk may act in the same way, though not in such a high degree, as water in the conveyance of typhoid fever.

It may be pointed out that specific typhoid is not a disease of animals; consequently no danger need be apprehended from milk if it is properly cared for _after_ it comes from the cow. Typhoid milk is almost invariably due to the addition of typhoid-infected water, either by way of adulteration or in the process of washing out the milk-cans. Cases have, however, been recorded in which there has been direct transmission to the milk from a person convalescing from the disease, and also indirect transmission by a milker serving also in the capacity of nurse to a patient in his own family.

Though the typhoid bacillus appears not to have the power of multiplying in milk, it has the faculty of existing and thriving in milk, even when it has curdled or soured, for a considerable time, and may thus infect milk products like butter and cheese. But infection by milk products may be eliminated as of too rare occurrence to deserve attention. The bacillus does not coagulate the milk like its ally the _Bacillus coli communis_, which is a much more frequent and less injurious inhabitant of milk.

_Cholera._ The cholera bacillus, as we have already pointed out, is unable to live in an acid medium. Hence its life in milk is a limited one, and generally depends on some alkaline change in the milk. Heim found that cholera bacilli would live in raw milk from one to four days, depending upon the temperature. D. D. Cunningham, from the results of a large number of investigations in India, concludes that the rapidly developing acid fermentations normally or usually setting in, connected with the rapid multiplication of other common bacteria and moulds, tend to arrest the multiplication of cholera bacilli, and eventually to destroy their vitality. Boiling milk appears, on the contrary, to increase the suitability of milk as a nidus for cholera bacilli, partly by its germicidal effect upon the acid-producing microbes, and partly because it removes from the milk the enormous numbers of common bacteria, which in raw milk cause such keen competition that the cholera bacillus finds existence impossible.

Professor W. J. Simpson, lately the Medical Officer of Health for Calcutta, has placed on record an interesting series of cholera cases on board the _Ardenclutha_, in the port of Calcutta, which arose from drinking milk which had been polluted with one quarter of its volume of cholera-infected water. This water came from a tank into which some cholera dejecta had passed. Of the ten men who drank the milk four died, five were severely ill, and one, who drank but very little of the milk, was only slightly ill. There was no illness whatever amongst those who did not drink the milk.

_Diphtheria._ Recent observations on the infectivity of diphtheria in milk by Schottelius have established the fact that milk is a good medium for the bacillus of diphtheria, but that it rarely acts as a vehicle for transmitting the disease. Klein has emphasised the possibility of this means of infection. In the first place, it is obvious that the milk may become infected from a human source--from pollution with diphtheritic discharges or dried "fomites." Secondly, from a variety of different quarters evidence has been forthcoming to throw some suspicion upon the cow itself as the agent. Klein states that "a new eruptive disease on the teats and udder of the cow," consisting of papules, vesicles, and induration, may be set up by the subcutaneous inoculation of a pure culture of the _Bacillus diphtheriæ_. In these eruptions a bacillus similar to the _B. diphtheriæ_ was demonstrated. On _a priori_ grounds this evidence substantiates a belief that diphtheria, in some form or other, may be a disease of cows. Other observers have not been able to confirm these observations, and the whole matter of cow diphtheria must remain for the present _sub judice_.

As long ago as 1879 W. H. Power traced an epidemic of diphtheria in North London to the milk supply. In 1887 the same authority studied another outbreak, and other observers have produced further evidence in favour of the conveyance of this disease by milk. Air infection of milk by the _Bacillus diphtheriæ_ probably occurs only very rarely, on account of the fact that the organism is readily killed by desiccation, and yet such is necessary before it can be airborne. The most frequent mode of infection of milk with this disease is from the throats, hands, bodies, or clothing of dairy workers suffering from a mild or acute form of the disease.

The specific and proved cases in which milk has acted as the vehicle of diphtheria are, it is true, comparatively few. Yet, nevertheless, the possibility of milk infection in this disease is not one which we can afford to neglect.

_Scarlet Fever._ Here again the evidence is not complete, chiefly owing to the fact that no specific organism of scarlet fever has yet been discovered. Many cases have, however, illustrated the undeniable conveyance of the disease by milk. Even before 1881 a number of milk epidemics of scarlet fever had been traced out. In 1882 these were further added to by Mr. W. H. Power's report concerning a series of cases in Central London. That report was remarkable for the introduction of a new feature, viz., the evidence produced in favour of the infection of milk from some disease of the cow. The Medical Department of the Local Government Board from that time took up a position of suspended judgment concerning the belief hitherto credited that milk could only be infected by _human_ scarlet fever. In 1886 there was a remarkable epidemic in Marylebone, and the theory was suggested by Dr. Klein and Mr. Power that the cow from which the milk was derived suffered from scarlet fever.

Into the extensive controversy which raged round "the Hendon disease," as it was called, affecting the cows supplying the Marylebone milk, we cannot here enter. It will be sufficient to say that a long discussion took place as to whether or not this Hendon disease was or was not scarlet fever. The difficulty of course largely arose from the fact before mentioned that we do not at present know the specific micro-organism of scarlet fever. The Agricultural Department supported the view of Professor Crookshank that the cow disease at Hendon was _cow-pox_, and Professor Axe further pointed out that there was evidence of the Hendon milk having been contaminated with human scarlet fever. Whichever conclusion was adopted, all were agreed upon one point, viz., that the disease had been conveyed from Hendon to persons in Marylebone by means of the milk.

Mr. Ernest Hart in 1897 published a very large number of records of scarlatinal milk infection from all parts of the country, and though the cause of the disease is obscure, there is now no doubt that it may be and is conveyed by means of milk.

_Other Diseases Conveyed by Milk._ In addition to the above, there are other diseases spread by means of polluted milk. From time to time exceptional cases have occurred in which a disease like anthrax has been spread by this means. But it is not to such rare cases that we refer. There are two very common diseases in which milk has been proved to play a not inconsiderable part, viz., _thrush_ and _diarrhœa_.

The mould which gives rise to the curd-like patches in the throats of children, and which is known as _Oidium albicans_, frequently occurs in milk. Soft white specks are seen on the tongue and mucous membrane of the cheeks and lips, looking not unlike particles of milk curd. If a scraping be placed upon a glass slide with a drop of glycerine and examined by means of the microscope, the spores and mycelial threads of this mould will be seen. The spores are oval, and possess a definite capsule. The threads are branched and jointed at somewhat long intervals. Milk affords an excellent medium for the growth of this parasite. Thus undoubtedly we must hold milk partly responsible for spreading this complaint. _Penicillium_, _Aspergillus_, and _Mucor_ are also frequent moulds in milk.

Professor MacFadyen[58] has given a full account of the ways in which milk becomes pathogenic, and his views have received further support from Professor Sheridan Delépine, who has examined more than one hundred samples of milk from Liverpool and Manchester. The result of this investigation has been that milk must be held to be one of the most potent causes of the summer diarrhœa of children. Indeed, a bacillus has been isolated identical with one which was apparently the cause of this complaint, which carries off such a large number of infants every summer. It resembles closely the _Bacillus coli communis_, which is an almost constant inhabitant of the alimentary canal, and is held by many bacteriologists to play, especially in conjunction with yeasts and other saprophytic organisms, an active rôle in the intestine of man.

In a recent official report[59] Dr. Hope, of Liverpool, states that "the method of feeding plays a most important part in the causation of diarrhœa; when artificial feeding becomes necessary, the most scrupulous attention should be paid to feeding-bottles." Careless feeding, in conjunction with a warm, dry summer, invariably results in a high death-rate from this cause. These two causes interact upon each other. A warm temperature is a favourable temperature for the growth of the poisonous micro-organism; a dry season affords ample opportunity for its conveyance through the air. Unclean feeding-bottles are obviously an admirable nidus for these injurious bacteria, for in such a resting-place the three main conditions necessary for bacterial life are well fulfilled, viz., heat, moisture, and pabulum. The heat is supplied by the warm temperature, the moisture and food by the dregs of milk left in the bottle; and the dry air assists in transit.

Before passing on to other matters, reference must be made to poisonous products other than bacteria which occur in milk and set up ill-health. Vaughan, of Michigan, pointed out at the London Congress of Hygiene in 1891 that he had separated a poisonous alkaloid, which he called _tyrotoxicon_. This, as its name denotes, was a toxic or poisonous substance, probably produced by some form of microbe. It may be taken as a type of the organic chemical substances frequently occurring in milk.

METHODS OF PRESERVING MILK

From the somewhat extensive category of diseases which may be milk-borne, it will be suitable now to speak of some of the means at our disposal for obtaining and preserving good, pure milk.

We considered at the commencement of this chapter the most frequent channels of contamination. If these be avoided or prevented, and if the milk be derived from cows in good health and well kept, the risk of infection is reduced to a minimum. But we have seen that much, if not most, of the pollution of milk arises after the milking process and during transit and storage preparatory to use. Bacteria are so ubiquitous that to prevent the entrance of any at all is almost beyond hope. Can anything be done to prevent their multiplication or to kill them in the milk? Fortunately the answer is in the affirmative.

There are two means at hand to secure these results. First, we may add to the milk various chemical or physical preservatives. Borax or boric acid, formaldehyde, salicylic acid, and other chemical bodies are used for this purpose. The commonest of these is that named first. The Food and Drugs Act (Section VI., 1875) permits the addition of an ingredient not injurious to health if the same is required for protection or preparation of the article in question. It is, however, a difficult matter to determine what amount of boric acid is injurious to health, for this differs widely in different persons. It has been laid down by one authority that even so small an amount as one-tenth per cent. might have inconvenient results, owing to its cumulative effect. _Formaldehyde_ is without doubt an excellent antiseptic, and the more its efficacy becomes known so much the more probably will it be used. The _salicylates_, which are mild antiseptics, have long been used as preservatives. These substances, then, can be added to milk in quantities not recognisable to the taste (salicylic acid about .75 grain, and boracic acid .4 grain, to the litre of milk). They will materially increase the time that milk will remain sweet, they will prevent a number of micro-organisms living in the milk, and will inhibit multiplication of others.[60] Secondly, it is possible very perceptibly to remove the infectivity of milk by filtration and temperature variations.

_Filtration_ has been practised for some time by the Copenhagen Dairy Company and by Bolle, of Berlin. The filters used consist of large cylindrical vessels divided by horizontal perforated diaphragms into five superposed compartments, of which the middle three are filled with fine sand of three sizes. At the bottom is the coarsest sand, and at the top the finest. The milk enters the lowest compartment by a pipe under gravitation pressure, and is forced upwards, and finally is run off into an iced cooler, and from that into the distribution cans. By this means the number of bacteria is reduced to one-third. The difficulty of drying and sterilising enough sand to admit a large turnover of milk is a serious one. This, in conjunction with the belief that filtration removes some of the essential nutritive elements of milk, has caused the process to be but little adopted. Dr. Seibert states that if milk be filtered through half an inch of compressed absorbent cotton, seven-eighths of the contained bacteria will be removed, and a second filtration will further reduce the number to one-twentieth. One quart of milk may thus be filtered in fifteen minutes.