Chapter IV: Cell Groupings

It has been stated that bacteria reproduce by transverse division, that is, division across the long axis. Following repeated divisions the new cells may or may not remain attached. In the latter case the bacteria occur as separate isolated individuals. In the former, arrangements characteristic of the particular organism almost invariably result. These arrangements are best described as _cell groupings_ or _growth forms_.

In the case of spiral forms it is obvious that there is only one possible grouping, that is, in chains of two or more individuals adherent end to end. A chain of two spirilla might be called a _diplospirillum_ (διπλός = double); of three or more, a _streptospirillum_ (στρεπτός = necklace, chain) (Fig. 41). These terms are rarely used, since spirilla do not ordinarily remain attached. Likewise the bacillus can grow only in chains of two or more, and the terms _diplobacillus_ (Fig. 42), bacilli in groups of two, and _streptobacillus_ (Fig. 43), bacilli in chains are frequently used. Still the terms _thread_, _filament_, or _chain_ are more common for _streptobacillus_.

Since the coccus is spherical, _transverse_ division may occur in any direction, though in three planes only at right angles to each other. Division might occur in _one plane only_ as in spirilla and bacilli, or in _two planes only_ or in _all three planes_. As a matter of fact these three methods of division are found among the cocci, but only one method for each particular kind of coccus. As a result there may be a variety of cell groupings among the cocci. When division occurs in one plane only, the possible groupings are the same as among the spirilla or bacilli. The cocci may occur in groups of two--_diplococcus_ grouping (Fig. 44), or in chains--_streptococcus_ grouping (Figs. 45 and 46). When the grouping is in _diplococci_, the individual cocci most commonly appear as hemispheres with the plane surfaces apposed (Fig. 44). Sometimes they appear as spheres and occasionally are even somewhat elongated. The individuals in a streptococcus grouping are most commonly elongated, either in the same direction as the length of the chain, or at right angles to it. The latter appearance is probably due to failure to enlarge completely after division. Streptococci frequently appear as chains of diplococci, that is, the pair resulting from the division of a single coccus remain a little closer to each other than to neighboring cells, as a close inspection of Fig. 45 will show.

If division occurs in _two planes only_, there may result the above groupings and several others in addition. The four cocci which result from a single division may remain together, giving the _tetracoccus_ or _tetrad_ grouping. Very rarely all the cocci divide evenly and the result is a regular _rectangular flat mass_ of cells, the total number of which is a multiple of four. The term merismopedia (from a genus of algæ which grows the same way) is applied to such a grouping. If the cells within a group after a few divisions do not reproduce so rapidly (lack of food), as usually happens, the number of cells becomes uneven or at least not necessarily a multiple of four and the resultant _flat mass_ has an _irregular_, _uneven outline_. This grouping is termed _staphylococcus_ (σταφυλος = a bunch of grapes) (Fig. 47). It is the most common grouping among the cocci.

When division occurs in all three planes, there is in addition to all the groupings possible to one- and two-plane division a third grouping in which the cells are in _solid packets_, _multiples of eight_. The name _sarcina_ is applied to this growth form (Fig. 48). The individual cells in a sarcina packet never show the typical coccus form so long as they remain together, but are always flattened on two or more sides.

The above descriptions indicate how the method of division may be determined. If in examining a preparation the _sarcina_ grouping appears, that shows _three-plane division_. If there are no sarcina, but _tetrads_ or _staphylococci_ (rarely merismopedia), then the division is in _two planes_. If none of the foregoing is observed but only _diplo-_ or _streptococci_, these indicate _one-plane division_ only. Cocci show their _characteristic_ groupings only when grown in a liquid medium, and such should always be used before deciding on the plane of division.

As the above description shows, these terms which are properly adjectives describing the cell grouping, are quite generally used as nouns. Thus the terms a diplococcus, a tetrad, a streptococcus, etc., are common, meaning a bacterium of the cell form and cell grouping indicated.

CELL FORM. CELL GROUPING.

coccus-- {diplococcus--in 2’s.
round or spherical. {streptoccus--in chains.
{tetracoccus, tetrads--in 4’s.
{staphylococcus--irregular flat masses.
{sarcina--regular, solid packets, multiples of 8.

bacillus-- {diplobacillus--in 2’s.
rod-shaped {streptobacillus--in chains.
or cylindrical.

spirillum-- {diplospirillum--in 2’s, little used.
spiral-shaped. {streptospirillum--in chains, little used.