Volume 3, Part 1, Slice 2 Part 2 (1/2)

[Sidenote: Spores.]

A very characteristic method of reproduction is that of spore-formation, and these minute reproductive bodies, which represent a resting stage of the organism, are now known in many forms. Formerly two kinds of spores were described, _arthrospores_ and _endospores_. An arthrospore, however, is not a true spore but merely an ordinary vegetative cell which separates and pa.s.ses into a condition of rest, and such may occur in forms which form endospores, _e.g._ _B. subtilis_, as well as in species not known to form endospores. The true spore or endospore begins with the appearance of a minute granule in the protoplasm of a vegetative cell; this granule enlarges and in a few hours has taken to itself all the protoplasm, secreted a thin but very resistive envelope, and is a ripe ovoid spore, smaller than the mother-cell and lying loosely in it (cf. figs. 6, 9, 10, and 11). In the case of the simplest and most minute Schizomycetes [v.03 p.0162] (_Micrococcus_, &c.) no definite spores have been discovered; any one of the vegetative micrococci may commence a new series of cell by growth and division. We may call these forms ”asporous,” at any rate provisionally.

[Ill.u.s.tration: FIG. 9.

A. _Bacillus anthracis._ (After de Bary) Two of the long filaments (B, fig.

10) in which spores are being developed. The specimen was cultivated in broth, and spores are drawn a little too small--they should be of the same diameter transversely as the segments.

B. _Bacillus subtilis._ (After de Bary.) 1, fragments of filaments with ripe spores; 2-5, successive stages in the germination of the spores, the remains of the spore attached to the germinal rodlets.]

[Ill.u.s.tration: FIG. 10.--_Bacillus subtilis_. (After Strasburger.) A.

Zoogloea pellicle. B. Motile rodlets. C. Development of spores.]

The spore may be formed in short or long segments, the cell-wall of which may undergo change of form to accommodate itself to the contents. As a rule only one spore is formed in a cell, and the process usually takes place in a bacillar segment. In some cases the spore-forming protoplasm gives a blue reaction with iodine solutions. The spores may be developed in cells which are actively swarming, the movements not being interfered with by the process (fig. 4, D). The so-called ”Kopfchenbacterien” of older writers are simply bacterioid segments with a spore at one end, the mother cell-wall having adapted itself to the outline of the spore (fig. 4, F). The ripe spores of Schizomycetes are spherical, ovoid or long-ovoid in shape and extremely minute (_e.g._ those of _Bacillus subtilis_ measure 0.0012 mm.

long by 0.0006 mm. broad according to Zopf), highly refractive and colourless (or very dark, probably owing to the high index of refraction and minute size). The membrane may be relatively thick, and even exhibit sh.e.l.ls or strata.

The germination of the spores has now been observed in several forms with care. The spores are capable of germination at once, or they may be kept for months and even years, and are very resistant against desiccation, heat and cold, &c. In a suitable medium and at a proper temperature the germination is completed in a few hours. The spore swells and elongates and the contents grow forth to a cell like that which produced it, in some cases clearly breaking through the membrane, the remains of which may be seen attached to the young germinal rodlet (figs. 5, 9 and 11); in other cases the surrounding membrane of the spore swells and dissolves. The germinal cell then grows forth into the forms typical for the particular Schizomycete concerned.

The conditions for spore-formation differ. Anaerobic species usually require little oxygen, but aerobic species a free supply. Each species has an optimum temperature and many are known to require very special food-media. The systematic interference with these conditions has enabled bacteriologists to induce the development of so-called asporogenous races, in which the formation of spores is indefinitely postponed, changes in vigour, virulence and other properties being also involved, in some cases at any rate. The addition of minute traces of acids, poisons, &c., leads to this change in some forms; high temperature has also been used successfully.

[Ill.u.s.tration: FIG. 11.--Stages in the development of spores of _Bacillus ramosus_ (Fraenkel), in the order and at the times given, in a hanging drop culture, under a very high power. The process begins with the formation of brilliant granules (A, B); these increase, and the brilliant substance gradually b.a.l.l.s together (C) and forms the spores (D), one in each segment, which soon acquire a membrane and ripen (E). (H. M. W.)]

[Sidenote: Cla.s.sification.]

The difficult subject of the cla.s.sification[4] of bacteria dates from the year 1872, when Cohn published his system, which was extended in 1875; this scheme has in fact dominated the study of bacteria ever since. Zopf in 1885 proposed a scheme based on the acceptance of extreme views of pleomorphism; his system, however, was extraordinarily impracticable and was recognized by him as provisional only. Systems have also been brought forward based on the formation of arthrospores and endospores, but as explained above this is eminently unsatisfactory, as arthrospores are not true spores and both kinds of reproductive bodies are found in one and the same form. Numerous attempts have been made to construct schemes of cla.s.sification based on the power of growing colonies to liquefy gelatine, to secrete coloured pigments, to ferment certain media with evolution of carbon dioxide or other gases, or to induce pathological conditions in animals. None of these systems, which are chiefly due to the medical bacteriologists, has maintained its position, owing to the difficulty of applying the characters and to the fact that such properties are physiological and liable to great fluctuations in culture, because a given organism may vary greatly in such respects according to its degree of vitality at the time, its age, the mode of nutrition [v.03 p.0163] and the influence of external factors on its growth. Even when used in conjunction with purely morphological characters, these physiological properties are too variable to aid us in the discrimination of species and genera, and are apt to break down at critical periods. Among the more characteristic of these schemes adopted at various times may be mentioned those of Miquel (1891), Eisenberg (1891), and Lehmann and Neumann (1897). Although much progress has been made in determining the value and constancy of morphological characters, we are still in need of a sufficiently comprehensive and easily applied scheme of cla.s.sification, partly owing to the existence in the literature of imperfectly described forms the life-history of which is not yet known, or the microscopic characters of which have not been examined with sufficient accuracy and thoroughness. [Sidenote: Fischer's Scheme.] The princ.i.p.al attempts at morphological cla.s.sifications recently brought forward are those of de Toni and Trevisan (1889), Fischer (1897) and Migula (1897). Of these systems, which alone are available in any practical scheme of cla.s.sification, the two most important and most modern are those of Fischer and Migula. The extended investigations of the former on the number and distribution of cilia (see fig. 1) led him to propose a scheme of cla.s.sification based on these and other morphological characters, and differing essentially from any preceding one. This scheme may be tabulated as follows:--

I. ORDER--HAPLOBACTERINAE. Vegetative body unicellular; spheroidal, cylindrical or spirally twisted; isolated or connected in filamentous or other growth series.

1. _Family_--COCCACEAE. Vegetative cells spheroidal.

(a) Sub-family--ALLOCOCCACEAE. Division in all or any planes, colonies indefinite in shape and size, of cells in short chains, irregular clumps, pairs or isolated:-- _Micrococcus_ (Cohn), cells non-motile; _Planococcus_ (Migula), cells motile.

(b) Sub-family--h.o.m.oCOCCACEAE. Division planes regular and definite:--_Sarcina_ (Goods.), cells non-motile; growth and division in three successive planes at right angles, resulting in packet-like groups; _Planosarcina_ (Migula), as before, but motile; _Pediococcus_ (Lindner), division planes at right angles in two successive planes, and cells in tablets of four or more; _Streptococcus_ (Billr.), divisions in one plane only, resulting in chains of cells.

2. _Family_--BACILLACEAE. Vegetative cells cylindric (rodlets), ellipsoid or ovoid, and straight. Division planes always perpendicular to the long axis.

(a) Sub-family--BACILLEAE. Sporogenous rodlets cylindric, not altered in shape:--_Bacillus_ (Cohn), non-motile; _Bactrinium_ (Fischer), motile, with one polar flagellum (monotrichous); _Bactrillum_ (Fischer), motile, with a terminal tuft of cilia (lophotrichous); _Bactridium_ (Fischer), motile, with cilia all over the surface (peritrichous).

(b) Sub-family--CLOSTRIDIEAE. Sporogenous rodlets, spindle-shaped:--_Clostridium_ (Prazm.), motile (peritrichous).

(c) Sub-family--PLECTRIDIEAE. Sporogenous rodlets, drumstick-shaped:--_Plectridium_ (Fischer), motile (peritrichous).

3. _Family_--SPIRILLACEAE. Vegetative cells, cylindric but curved more or less spirally. Divisions perpendicular to the long axis:--_Vibrio_ (Muller-Loffler), comma-shaped, motile, monotrichous; _Spirillum_ (Ehrenb.), more strongly curved in open spirals, motile, lophotrichous; _Spirochaete_ (Ehrenb.), spirally coiled in numerous close turns, motile, but apparently owing to flexile movements, as no cilia are found.

II. ORDER--TRICHOBACTERINAE. Vegetative body of branched or unbranched cell-filaments, the segments of which separate as swarm-cells (_Gonidia_).

1. _Family_--TRICHOBACTERIACEAE. Characters those of the Order.

(a) Filaments rigid, non-motile, sheathed:--_Crenothrix_ (Cohn), filaments unbranched and devoid of sulphur particles; _Thiothrix_ (Winogr.), as before, but with sulphur particles; _Cladothrix_ (Cohn), filaments branched in a pseudo-dichotomous manner.

(b) Filaments showing slow pendulous and creeping movements, and with no distinct sheath:--_Beggiatoa_ (Trev.), with sulphur particles.