not end to end, but side by side in groups. Cocci thus collected are designated staphylococci (from araqui, a grape), because viewed microscopically the cocci resemble bunches of grapes. (Fig. 7, k.) When the bacteria appear joined in pairs, they are spoken of as diplococci (Fig. 7, b) or diplobacilli. A number of bacteria divide in two or three successively vertical direc

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tions. This mode of division has been Fig. 6.-Streptococci. observed only among cocci, and there thus

result plate-cocci (tetragenus, tetrad arrangement) and the ball-of-twine-like packets known as sarcina. (Fig. 7, i.)

In the process of cell-division an essential developmental difference is observed between bacilli and cocci. The bacilli become double their ordinary size before they are ready for multiplication; while the cocci break up, without preliminary increase in size, into two hemispheres (or in the case of tetrad arrangement into four quadrants, and in the case of sarcina-formation into eight octants), and spheric cocci then form from the products of division.

Fig. 7.-Diagram illustrating the morphology of cocci: a, Coccus or micrococcus ; diplococcus; c, d, streptococci; e, f, tetragenococci or merismopedia; g, h, modes of division of cocci; i, sarcina; j, coccus with flagella; k, staphylo cocci (McFarland).

Vibrios also form pseudofilaments consisting of numerous spiral cells, and, besides, though seldom, of long spirals consisting each of a single cell. Division occurs among these in much the same manner as among bacilli.

For every bacterium there is a temperature at which it thrives best (temperature-optimum), as well as a temperaturelimit above, and beyond which it can not survive (temperature-maximum), and one below, at which it will just live (temperature-minimum). Among pathogenic bacteria the

temperature-optimum is mostly about 37° C. (98.6° F.). There are microorganisms, however, that live at o° C. (32° F.), and exhibit all of their vital manifestations, such as color-formation, light-development, peptonization of gelatin, etc. Forster was the first to obtain such bacteria in pure culture from street-dirt, garden-earth, sea-water, and from the surface of luminous sea-fish. On the other hand, there are found, widely distributed in the earth, the air, the water, the contents of the intestine, etc., bacteria that under aerobic conditions require for their growth temperatures of from 50° C. (122° F.) to 75° C. (167° F.)-socalled thermophilic bacteria; under anaerobic conditions, however, these also thrive only between 34° C. (93.2° F.) and 44° C. (111.2° F.).

The thermal death-point for individual mature bacteria at the upper limit, on short exposure, ranges for the different varieties between 55° C. (131° F.), 60° C. (140° F.), and 80° C. (176° F.). The death-point at the lower limit

a

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d

f

Fig. 8.-Diagram illustrating sporulation: a, Bacillus inclosing a small, oval spore; b, drumstick-bacillus, with terminal spore; c, clostridium, with central spore; d, free spores; e and ƒ, bacilli escaping froni spores (McFarland).

has been determined only for the smallest number of growths; many bacteria withstand a temperature of o° C. (32° F.)—that is, freezing—without injury. These statements are naturally not applicable to thermophilic and glacial microorganisms.

In addition to dichotomous division some bacteria, especially the bacilli, and only exceptionally spirilla, and perhaps sarcinæ, proliferate through the formation of special permanent bodies or spores. (Fig. 8.) In the interior of the cell there forms an ovoid, intensely bright, and refractive body, the spore, which is enveloped in a firm capsule, the spore-membrane. One cell generally contains only one spore, which sometimes is situated in the middle, sometimes at an extremity. In the latter event, the rod appears swollen and club-shaped at the corresponding extremity, and it is designated a drumstick-bacterium. (Fig. 8, b.) If the swelling of the bacillus takes place at the middle in consequence of

sporulation, a spindle-shaped figure results, which is known as a clostridium.

At a later stage the body of the bacillus disintegrates, and the spore is set free. If this finds its way into a suitable nutritive medium of dead or living matter, it germinates, becomes a rod, the rod multiplies by dichotomy, and the whole process again terminates with the formation of new spores. The manner and the method in which the spore becomes transformed into a rod vary among different bacteria. Full details, in so far as they are of importance for clinical bacteriology, will be discussed in the special section of this work. In general the process of development is such that the spore first takes up water, then swells, next doubles in size, and loses its marked refractive property. The spore-membrane ruptures in the center or at one pole; the young bacillus slips through the rent, retaining for some time the capsule of the permanent body.

Spore-formation and spore-germination take place only within certain temperature-limits, which differ for each individual bacterium. In the mode of sporulation just described the spore develops from the interior of the protoplasmic ground-substance of the rod, and this form of fructification is therefore designated endogenous spore-formation, in contradistinction from arthrogenous spore-formation, where individual segments of the cell-chain assume sporequalities without passing through any intermediate stage. In external appearance these particular cells may differ in no respect from other bacteria of the same chain. Sometimes they become somewhat larger and brighter and more highly refractive and surrounded by a firmer capsule.

Attempts have been made, upon the basis of these variations in fructification, to divide bacteria into two great classes (1) those with endogenous spore-formation (endosporous bacteria) and (2) those with arthrogenous sporeformation (arthrosporous bacteria). Among the latter class the cocci principally are grouped. However desirable it might be to classify the bacteria according to an intrinsic principle, it must be emphasized that the classification mentioned appears at least premature, inasmuch as our knowledge concerning the arthrogenous spores is as yet but slight and uncertain.

The spores represent permanent forms of the bacteria. Owing to the concentrated nature of their plasma, they are

extremely resistant; much more so than the mature bacteria themselves. They withstand for years drying and all atmospheric influences. Nature possesses but one agency capable of rendering them harmless—namely, the direct action of the rays of the sun, insolation. In order to destroy the spores of anthrax-bacilli, which are not even among the most resistant, exposure to dry heat of 140° C. (284° F.) for three hours, or to steam of 100° C. (212° F.) for a few minutes, is necessary. Far greater resistance is displayed by the spores of Flügge's peptonizing milk-bacteria, as well as by some members of the group of haybacilli and potato-bacilli, which may survive exposure to live steam for five, six, and even for sixteen hours.

The question as to the existence of pleomorphic bacteria of varied form has not yet been decided with certainty. Pleomorphism has been conceded by some authorities to the proteus obtained by Hauser from decomposing fluids. It is, however, quite probable that the spheric bodies in the cultures of the proteus result in consequence of the retarded growth in the presence of progressive division. If the actinomyces are included among bacteria, there is no alternative but to admit the occurrence of pleomorphism. The actinomyces-fungus, however, is no longer grouped by most authorities among bacilli. Kruse, in Flügge's "Textbook," makes a special group of streptothricea, of which the actinomyces may be considered the most important representative. These streptothriceæ stand midway between the filamentous fungi and the true bacteria, and they are characterized by the formation of filaments and especially by the ramification resulting in consequence of their germination. In old cultures the filaments break up into bodies resembling bacilli, spirilla, and cocci, and there is thus yielded an appearance of pleomorphism. A further peculiarity of the streptothriceæ is the formation of airhyphæ that give rise to germ-cells (spores) by segmentation. These are probably not analogous to bacterial spores, as they are destroyed by exposure for five minutes to a temperature of 75° C. (167° F.). Nevertheless, they withstand higher degrees of temperature than the filaments, which undergo destruction at 60° C. (140° F.). As the result of personal investigation, we are of the opinion that the actinomyces-group represents a subdivision of the filamentous fungi.

Of late, attention has been directed to ramification, clubformation, and granular disintegration occurring among tubercle-bacilli and diphtheria-bacilli, and strongly suggestive of streptothriceæ. Upon the basis of such observations a number of authorities have concluded that the exciting agents of diphtheria and of tuberculosis, together with the whole actinomyces-group, belong to the hyphomyceta. Such classification must, however, be considered as premature; although the presumption is justified that the tuberclebacillus and the diphtheria-bacillus stand in kindred relationship to the ray-fungus and the group of organisms represented by it.

Involution-changes must not be confounded with variability in form of growth. The former take place when the nutritive medium is exhausted and the bacteria begin to die; the cells then swell, become thick and plump, show deficiencies, and undergo disintegration and other changes of allied nature. Whether all of the resulting peculiar forms are to be attributed to degenerative changes is a question that must, at least, be left open for the present. It is quite possible that the very large, so-called giant-cells result rather in consequence of especially favorable vital conditions. Other involution-manifestations appear to be due to special functions of the bacteria, such as their fermentative activity. In this manner are perhaps to be explained the involutionforms of the acetic bacterium, and especially of numerous microbes that are cultivated in media containing grape-sugar.

Bacteria are ubiquitous: they are found everywhere; only the internal organs of the human and the animal body not in communication with the atmosphere are free from them. Some of the pathogenic germs confine their activities to certain living organisms, so that their area of distribution is, in consequence, a restricted one.

For their sustenance the bacteria require preformed organic carbon-compounds, as most of them, by reason of their deficiency in chlorophyl, are capable of utilizing the carbon dioxid of the atmosphere. They require also nitrogen-compounds, which they can obtain from organic substances, as well as from inorganic nitrates and ammoniacompounds. It need scarcely be added that water is necessary for the development of the bacteria. The nutritive material for the bacteria must be feebly alkaline or neutral, as in general they do not develop so well upon acid media.