Chapter I: Structure, Growth and Distribution

=Relation of bacteriology to dairying.= The arts which have been developed by mankind have been the outgrowth of experience. Man first learned by doing, _how_ to perform these various activities, and a scientific knowledge of the underlying principles which govern these processes was later developed.

The art of dairying has been practiced from time immemorial, but a correct understanding of the fundamental principles on which the practice of dairying rests is of recent origin. In working out these principles, chemistry has been of great service, but in later years, bacteriology has also been most successfully applied to the problems of modern dairying. Indeed, it may be said that the science of dairying, as related to the problems of dairy manufacture is, in large degree, dependent upon an understanding of bacteriological principles. It is therefore essential that the student of dairying, even though he is concerned in large measure with the practical aspects of the subject, should acquire as complete an understanding of these principles as possible.

While bacteriology is concerned primarily with the activities of those microscopic forms of plant life known as the bacteria, yet the general principles governing the life of this particular class of organisms are sufficiently similar to those governing the molds and other types of microscopic life that affect milk and its products to make it possible to include all of these types in a general consideration of the subject.

=Nature of bacteria.= The vegetable kingdom to which the bacteria belong consists of plants of the most varying size and nature. Those of most common acquaintance are the green plants varying in size from those not visible to the naked eye to the largest trees. Another class of plants known as fungi or fungous plants do not contain chlorophyll, the green coloring matter, but are usually colorless and, as a rule, of small size; among them are included such forms as the mushrooms, smuts, rusts and mildews, as well as the molds and yeasts. The bacteria are closely allied to this latter class. When first discovered they were thought to be animals because of the ability of some forms to move about in liquids.

The bacteria, like other kinds of living organisms, possess a definite form and shape. They are the simplest in structure of all the plants, the individual organism consisting of a single cell. The larger and more highly organized forms of life are made up of many microscopic cells, and the life of the individual consists of the work of all the cells. The bacteria are very comparable to the single cells of the higher plants and animals, but in the case of the bacteria the single cell is able to exist apart from all other cells and to carry out all of its life processes including reproduction.

=Forms of bacteria.= With the multicellular organisms much variation in form is possible, but with these single-celled organisms the possible variation in form is greatly limited. Three well marked types occur among the bacteria: the round or coccus form (plural cocci); the rod-shaped or bacillus (plural bacilli); and the twisted or spirillum type (plural spirilla). Most organisms of special significance in dairying belong to the coccus or bacillus group.

=Size of bacteria.= The bacteria, as a class, are among the smallest of living objects. None of them are individually visible to the naked eye, and they can be so seen only when clumps or masses are formed in the process of growth.

While there is considerable relative variation in size, yet in actual dimensions, this difference is so small as to make careful microscopic determinations necessary. An average diameter may be taken as about one thirty-thousandth of an inch, while the length varies naturally several fold, depending upon whether the type under observation is a coccus or a bacillus.

It is very difficult to conceive of the minuteness of the bacteria; the following may give some idea of their size. In a drop of cream ready for churning may be found as many as 10,000,000 and in a piece of fresh cheese as large as a cherry there may be as many living bacteria as there are people on our earth. While the bacteria are very minute, the effect which they exert in milk and other dairy products is great on account of their enormous numbers.

=Manner of growth.= The cells of which all plants and animals consist increase in numbers by the division of each cell into two cells through the formation of a division wall across the cell. The new cells divide and the plant or animal continues to grow. The same cell division occurs in the bacteria but since the bacteria are single celled, division of the cells means an increase in numbers rather than growth as in the higher forms of life.

The bacteria increase in numbers by the division of each cell into two cells. (After Novy.)]

In the case of those bacteria that have a greater length than diameter, the new wall is formed at right angles to the long axis of the cell. As soon as the division is complete each cell is a complete individual, capable of carrying on all of its life processes. The cells may, however, cohere and thus form distinctive groupings that may serve to identify certain types. Some of the cocci form long chains and the term _streptococcus_ is applied to such. Other groupings may be similar to a bale of twine or they may be massed in clusters with no regularity distinguishable.

=Spores.= Just as ordinary plants form resistant structures, known as seeds, capable of retaining vitality under conditions unfavorable for growth thereby perpetuating the species, so with certain of the bacteria, definite structures, known as _spores_, that are analogous in some respects to the seeds of the higher plants, are produced within the mother cell. The spores are exceedingly resistant to the influence of an unfavorable environment, such as heat, cold, drying, and even chemical agents. It is this property of the spores which makes it so difficult to destroy the bacterial life in the process of sterilizing milk. The property of spore-formation is fortunately confined to a comparatively small number of different species of bacilli.

=Movement.= Many of the bacteria are provided with vibratory organs of locomotion, known as _cilia_ (singular cilium) which are variously distributed on the surface of the cell. By the movement of these relatively long, thread-like appendages the individual cell is able to move in liquids. It must be remembered, when these moving cells are observed under the microscope, that their apparent rate of movement is magnified relatively as much as their size.

=Conditions for growth.= All kinds of living things need certain conditions for growth such as food, moisture, air and a favorable temperature. The bacteria prefer as food such organic matter as milk, meat, and vegetable infusions. Those living on dead organic matter are known as _saprophytes_, while those which are capable of thriving in the tissues of the living plant or animal are known as _parasites_. Certain of the parasitic forms are capable of causing disease in plants and animals. In the first group are embraced most of the bacteria that are able to develop in milk or its products, such as those forms concerned in the spoiling of milk or its fermentation. It is true that milk may contain disease-producing bacteria coming either from a diseased animal or from a diseased human being. It is also true that some of such harmful forms are able to grow in milk, such as the organisms causing typhoid fever and diphtheria.

=Food.= The bacteria like all other plants must have their food in solution. Where they apparently live on solids, such as meats, fruits, etc., they dissolve the food substances before utilizing the same. If the solutions are highly concentrated, as in the case of syrups, preserves and condensed milk, the bacteria cannot readily grow, although all of the necessary food ingredients are present. When such concentrated solutions are diluted, bacterial growth will take place and the solutions will spoil.

Each cell is an individual organism, magnified 1250 diameters.]

Generally speaking the bacteria grow best in a neutral or slightly alkaline solution rather than in acid liquids.

=Temperature.= One of the most important conditions influencing the rate of growth of bacteria is the temperature. Each form has a _minimum_ temperature below which growth can not take place; also a _maximum_ above which growth is again impossible. For the majority of species the minimum temperature ranges from 40 to 45 deg. F. the maximum from 105 to 110 deg. F. Growth takes place most rapidly at the optimum temperature, which, for each species, lies close to the maximum temperature at which growth can occur. Most of the bacteria of importance in the dairy grow well at from 70 to 100 deg. F.

There are forms that can grow below the freezing point of water when they are in solutions that do not freeze at this temperature. There are still other bacteria that can grow at 140 deg. F. a temperature that is quickly fatal to most forms. These are of importance in the dairy since they limit the temperatures at which milk can be stored for long periods of time.

=Air supply.= Living organisms, both plant and animal, require air or oxygen for the combustion of their food and for the production of energy. Most bacteria use, as do the green plants and animals, the free oxygen of the air for their respiration. Such organisms are called _aerobic_ or air-living. A much smaller group possess the power of taking oxygen from organic compounds such as sugar and the like and therefore are able to live under conditions where air is excluded. These are called _anaerobic_ bacteria. A large number of bacteria are able to live either in the presence or in the absence of free oxygen. Most of the bacteria of importance in the dairy are of this nature.

=Rate of growth.= When there is an abundant supply of food and when the temperature conditions are favorable, the bacteria increase in numbers with astounding rapidity. It has been determined by actual experiment that the process of cell division under favorable conditions takes place in a few moments. Barber has shown that one of the forms of bacteria constantly found in milk will divide in 17 minutes at 98 deg. F. and that a single organism kept at this temperature for ten hours would increase to 1,240,000,000. If the temperature is reduced to 50 deg. F., the time required for division is increased to several hours. The explanation for the rapid spoiling of milk that is not well cooled is thus apparent. The initial rapid rate of increase cannot be maintained for any length of time as the conditions become more and more unfavorable as growth continues, due to the accumulation of the by-products of the cell activity. Thus, the growth of acid-forming organisms in milk becomes checked by the formation of acid from the fermentation of the sugar.

=Detrimental effect of external conditions.= Environmental conditions of a detrimental character are constantly at work tending to repress the activity of bacteria or to destroy them. These act more readily on the vegetating cells than on the more resistant spores. It is of the utmost importance that those engaged in dairy work be familiar with these antagonistic forces since it is constantly necessary to repress or to kill outright the bacteria in milk and other dairy products. In many lines of dairy work it is likewise important to be familiar with the conditions favorable for bacterial growth.

=Effect of cold.= While it is true that chilling largely prevents fermentative action, and actual freezing stops all growth processes, still it does not follow that exposure to low temperatures will effectually destroy the vitality of bacteria, even in the growing condition. Numerous non-spore-bearing species remain alive in ice for a prolonged period, and experiments with liquid air show that even a temperature of-310 deg. F. maintained for hours does not kill all exposed cells.

=Effect of heat.= High temperatures, on the other hand, will destroy any form of life, whether in the vegetative or latent spore stage. The temperature at which the vitality of the cell is lost is known as the _thermal death point_. This limit is dependent not only upon the nature of the organism, but upon the time of exposure and the condition in which the heat is applied. In a moist atmosphere, the penetrating power of heat is great, consequently cell death occurs at a lower temperature than in a dry atmosphere. An increase in time of exposure lowers the temperature point at which death occurs.

For growing organisms, the thermal death point of most species ranges from 130 deg. to 140 deg. F. for ten minutes. When spores are present, resistance is greatly increased, some forms being able to withstand steam at 212 deg. F. from one to three hours. In the sterilization of milk, it is often necessary to heat for several hours, where a single exposure is made, to destroy the resistant spores, that seem to be more abundant under summer than winter conditions. Steam under pressure is a much more effective agent, as the temperature is thus raised considerably beyond 212 deg. F. An exposure of twenty minutes, at a temperature of 230 deg. to 240 deg. F. will kill all spores. Where heat is used in a dry state, it is much less effective, a baking temperature of 260 deg. to 300 deg. F. for an hour being necessary to kill spores. This condition is of the utmost importance in the destruction of bacteria in the dairy and creamery.

=Effect of drying.= The spore-bearing bacteria withstand effects of desiccation without serious injury, and many of the non-spore-producing types retain their vitality for some months. The bacteria found in the air are practically all derived from the soil, and exist in the air in a dried condition, in which they are able to remain alive for considerable periods of time. In a dried condition, active cell growth is not possible, but when other conditions, such as moisture and food supply are present, resumption of growth quickly begins. This property is also of importance in the dairy as in the preparation of dry starters for creameries and cheese factories.

=Effect of light.= Bright sunlight exerts a markedly injurious effect on bacterial life, both in a spore and in a growing condition. Where the direct sunlight strikes, more or less complete disinfection results in the course of a few hours, the effect being produced by the chemical or violet rays, and not by the heat or red rays of the spectrum. This action, however, does not penetrate opaque objects, and is therefore confined to the surface. In diffused light, the effect is much lessened, although it is exerted to some extent. Sunlight exerts a beneficial effect on the general health and well-being of animal life, and is a matter of importance to be taken into consideration in the erection of buildings for animals as well as for people.

=Effect of chemicals.= A great many chemical substances exert a more or less powerful toxic action on various kinds of life. Many of these are of great service in destroying bacteria or holding them in check. Those that are toxic and result in the death of the cell are known as _disinfectants_; those that merely inhibit, or retard growth are known as _antiseptics_. All disinfectants must of necessity be antiseptic in their action, but not all antiseptics are disinfectants, even when used in large amounts. Disinfectants have no place in dairy work, except to destroy disease-producing bacteria, or to preserve milk for analytical purposes. The so-called chemical preservatives used to "keep" milk depend for their effect on the inhibition of bacterial growth. In this country, most states prohibit the use of these substances in milk. Their only function in the dairy should be to check fermentative and putrefactive processes outside of milk and so keep the air free from taints.

=Products of growth.= All bacteria, as a result of their growth in food substances, form more or less characteristic compounds that are known as _by-products_. The changes brought about are those of decomposition and are collectively known as _fermentations_; they are characterized by the production of a large amount of by-products as the result of the development of a relatively small amount of cell life. The souring of milk, the rotting of eggs, the spoiling of meats, the making of vinegar from cider are examples of fermentations caused by different bacteria.

If the substances decomposed contain but little sugar, as do animal tissues, the conditions are favorable for the growth of the putrefactive bacteria, and foul-smelling gases are formed. When sugars are present, as in milk, the environmental conditions are most favorable for the acid-forming bacteria that do not as a rule produce offensive odors.

Many of the bacteria form substances known as enzymes which are able to produce certain decomposition changes in the absence of the living cells, and it is by virtue of these enzymes that the organisms are able to break down such enormous quantities of organic matter. Most of these enzymes react toward heat, cold, and chemical poisons in a manner quite similar to the living cells. In one respect, they are readily differentiated, and that is, that practically all of them are capable of producing their characteristic chemical transformations under conditions where the activity of the cell is wholly suspended as in a saturated ether or chloroform atmosphere. The production of enzymes is not confined to bacteria, but they are found throughout the animal and plant world, especially in those processes that are concerned in digestion. Rennet, used in cheese making, is an example of an animal enzyme.

=Distribution of bacteria.= As bacteria possess greater powers of resistance than almost any other form of life, they are found very widely distributed over the surface of the earth. In soil they are abundant, because of the fact that all of the conditions necessary for growth are here best satisfied. They are, however, distributed with reference to the layers of the soil; the soil proper, i.e., that turned over by the plow, is extremely rich in them on account of the abundance of organic matter. But at the depth of a few feet they decrease rapidly in numbers, and in the deeper layers, from six to ten feet, or more, they are normally not present, because of the lack of proper food supply and oxygen. The fertility of the soil is closely associated with their presence.

The bacteria are found in the air because of their development in the soil below. They are unable to grow even in a moist atmosphere, but are so readily dislodged by wind currents from the soil that over land areas the lower strata of the air always contain them. They are more numerous in summer than in winter; city air contains larger numbers than country air. Wherever dried fecal matter is present, as in barns, the air contains many forms.

Water generally contains enough organic matter in solution, so that certain types of bacterial life find favorable growth conditions. Water in contact with the soil surface takes up many impurities, and is of necessity rich in bacteria. As the rain water percolates into the soil, it loses its germ content, so that the normal ground water, like the deeper soil layers, contains practically no bacterial life. Springs, therefore, are relatively deficient in germ life, except as they become contaminated with soil organisms, as the water issues from the ground. Wells vary in their germ content, depending upon manner of construction, ease of contamination at surface, etc. Wells are too frequently insufficiently protected from surface leachings, and consequently may contain all kinds of organisms found in the surface soil. Typhoid fever is very frequently disseminated in this way, as is cholera and a number of animal maladies.

While the inner tissues of healthy animals are free from bacteria, the natural passages, as the respiratory and digestive tracts, being in more direct contact with the exterior, become readily infected. This is particularly true with reference to the intestinal tract, and in the undigested residue of the food, bacterial activity is at a maximum. The result is that fecal matter of all kinds contains enormous numbers of organisms so that the pollution of any food medium, such as milk, with such material is sure to introduce elements that seriously affect its quality.