Читать книгу The Fundamentals of Bacteriology - Charles Bradfield Morrey - Страница 13

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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.

Fig. 41.—Streptospirillum grouping.

Fig. 42.—Diplobacillus grouping.

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.

Fig. 43.—Streptobacillus grouping.

Fig. 44.—Typical diplococcus grouping. Note that the individual cocci are flattened on the apposing sides.

Fig. 45.—Long streptococcus grouping.

Fig. 46.—Short streptococcus grouping.

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.

Fig. 47.—Staphylococcus grouping. The large flat masses are staphylococcus grouping. Diplococcus grouping, tetrads and short streptococci are also evident.

Fig. 48.—Sarcina grouping.

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—round or spherical.	diplococcus—in 2’s.streptoccus—in chains.tetracoccus, tetrads—in 4’s.staphylococcus—irregular flat masses.sarcina—regular, solid packets, multiples of 8.
bacillus—rod-shaped or cylindrical.	diplobacillus—in 2’s.streptobacillus—in chains.
spirillum—spiral-shaped.	diplospirillum—in 2’s, little used.streptospirillum—in chains, little used.