[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.

The princ.i.p.al objections to this system are the following:--(1) The extraordinary difficulty in obtaining satisfactory preparations showing the cilia, and the discovery that these motile organs are not formed on all substrata, or are only developed during short periods of activity while the organism is young and vigorous, render this character almost nugatory. For instance, _B. megatherium_ and _B. subtilis_ pa.s.s in a few hours after commencement of growth from a motile stage with peritrichous cilia, into one of filamentous growth preceded by casting of the cilia. (2) By far the majority of the described species (over 1000) fall into the three genera--_Micrococcus_ (about 400), _Bacillus_ (about 200) and _Bactridium_ (about 150), so that only a quarter or so of the forms are selected out by the other genera. (3) The monotrichous and lophotrichous conditions are by no means constant even in the motile stage; thus _Pseudomonas rosea_ (Mig.) may have 1, 2 or 3 cilia at either end, and would be distributed by Fischer's cla.s.sification between _Bactrinium_ and _Bactrillum_, according to which state was observed. In Migula's scheme the attempt is made to avoid some of these difficulties, but others are introduced by his otherwise clever devices for dealing with these puzzling little organisms.

The question, What is an individual? has given rise to much difficulty, and around it many of the speculations regarding pleomorphism have centred without useful result. If a tree fall apart into its const.i.tuent cells periodically we should have the same difficulty on a larger and more complex scale. The fact that every bacterial cell in a species in most cases appears equally capable of performing all the physiological functions of the species has led most authorities, however, to regard it as the individual--a view which cannot be consistent in those cases where a simple or branched filamentous series exhibits differences between free apex and fixed base and so forth. It may be doubted whether the discussion is profitable, though it appears necessary in some cases--_e.g._ concerning pleomorphy--to adopt some definition of individual.

[Ill.u.s.tration: FIG. 12.

A. _Myxococcus digelatus_, bright red fructification occurring on dung.

B. _Polyangium primigenum_, red fructification on dog's dung.

C. _Chondromyces apiculatus_, orange fructification on antelope's dung.

D. Young fructification.

E. Single cyst germinating.

(A, B, after Quehl; C-E, after Thaxter.) From Strasburger's _Lehrbuch der Botanik_, by permission of Gustav Fischer.]

_Myxobacteriaceae._--To the two divisions of bacteria, Haplobacterinae and Trichobacterinae, must now be added a third division, Myxobacterinae. One of the first members of this group, _Chondromyces crocatus_, was described as long ago as 1857 by Berkeley, but its nature was not understood and it was ascribed to the Hyphomycetes. In 1892, however, Thaxter rediscovered it and showed its bacterial nature, founding for it and some allied forms the group Myxobacteriaceae. Another form, which he described as _Myxobacter_, was shown later to be the same as _Polyangium vitellinum_ described by Link in 1795, the exact nature of which had hitherto been in doubt. Thaxter's observations and conclusions were called in question by some botanists, but his later observations and those of Baur have established firmly the position of the group. The peculiarity of the group lies in the fact that the bacteria form plasmodium-like aggregations and build themselves up into sporogenous structures of definite form superficially similar to the cysts of the Mycetozoa (fig. 12). Most of the forms in question are found growing on the dung of herbivorous animals, but the bacteria occur not only in the alimentary ca.n.a.l of the animal but also free in the air. The Myxobacteria are most easily obtained by keeping at a temperature of 30-35 C. in the dark dung which has lain exposed to the air for at least eight days. The high temperature is favourable to the growth of the bacteria but [v.03 p.0164] inimical to that of the fungi which are so common on this substratum.

[Sidenote: Function and life of bacteria.]

The discoveries that some species of nitrifying bacteria and perhaps pigmented forms are capable of carbon-a.s.similation, that others can fix free nitrogen and that a number of decompositions. .h.i.therto unsuspected are accomplished by Schizomycetes, have put the questions of nutrition and fermentation in quite new lights. Apart from numerous fermentation processes such as rotting, the soaking of skins for tanning, the preparation of indigo and of tobacco, hay, ensilage, &c., in all of which bacterial fermentations are concerned, attention may be especially directed to the following evidence of the supreme importance of Schizomycetes in agriculture and daily life. Indeed, nothing marks the att.i.tude of modern bacteriology more clearly than the increasing attention which is being paid to useful fermentations. The vast majority of these organisms are not pathogenic, most are harmless and many are indispensable aids in natural operations important to man.

[Ill.u.s.tration: FIG. 13.--A series of phases of germination of the spore of _B. ramosus_ sown at 8.30 (to the extreme left), showing how the growth can be measured. If we place the base of the filament in each case on a base line in the order of the successive times of observation recorded, and at distances apart proportional to the intervals of time (8.30, 10.0, 10.30, 11.40, and so on) and erect the straightened-out filaments, the proportional length of each of which is here given for each period, a line joining the tips of the filaments gives the curve of growth. (H. M. W.)]

Fischer has proposed that the old division into saprophytes and parasites should be replaced by one which takes into account other peculiarities in the mode of nutrition of bacteria. The nitrifying, nitrogen-fixing, sulphur- and iron-bacteria he regards as monotrophic, _i.e._ as able to carry on one particular series of fermentations or decompositions only, and since they require no organic food materials, or at least are able to work up nitrogen or carbon from inorganic sources, he regards them as primitive forms in this respect and terms them _Prototrophic_. They may be looked upon as the nearest existing representatives of the primary forms of life which first obtained the power of working up non-living into living materials, and as playing a correspondingly important _role_ in the evolution of life on our globe. The vast majority of bacteria, on the other hand, which are ordinarily termed saprophytes, are _saprogenic_, _i.e._ bring organic material to the putrefactive state--or _saprophilous_, _i.e._ live best in such putrefying materials--or become _zymogenic_, _i.e._ their metabolic products may induce blood-poisoning or other toxic effects (facultative parasites) though they are not true parasites. These forms are termed by Fischer _Metatrophic_, because they require various kinds of organic materials obtained from the dead remains of other organisms or from the surfaces of their bodies, and can utilize and decompose them in various ways (_Polytrophic_) or, if monotrophic, are at least unable to work them up. The true parasites--obligate parasites of de Bary--are placed by Fischer in a third biological group, _Paratrophic_ bacteria, to mark the importance of their mode of life in the interior of living organisms where they live and multiply in the blood, juices or tissues.

[Sidenote: Nitrogen bacteria.]

When we reflect that some hundreds of thousands of tons of urea are daily deposited, which ordinary plants are unable to a.s.similate until considerable changes have been undergone, the question is of importance, What happens in the meantime? In effect the urea first becomes carbonate of ammonia by a simple hydrolysis brought about by bacteria, more and more definitely known since Pasteur, van Tieghem and Cohn first described them.

Lea and Miquel further proved that the hydrolysis is due to an enzyme--urase--separable with difficulty from the bacteria concerned. Many forms in rivers, soil, manure heaps, &c., are capable of bringing about this change to ammonium carbonate, and much of the loss of volatile ammonia on farms is preventible if the facts are apprehended. The excreta of urea alone thus afford to the soil enormous stores of nitrogen combined in a form which can be rendered available by bacteria, and there are in addition the supplies brought down in rain from the atmosphere, and those due to other living debris. The researches of later years have demonstrated that a still more inexhaustible supply of nitrogen is made available by the nitrogen-fixing bacteria of the soil. There are in all cultivated soils forms of bacteria which are capable of forcing the inert free nitrogen to combine with other elements into compounds a.s.similable by plants. This was long a.s.serted as probable before Winogradsky showed that the conclusions of M. P. E. Berthelot, A. Laurent and others were right, and that _Clostridium pasteurianum_, for instance, if protected from access of free oxygen by an envelope of aerobic bacteria or fungi, and provided with the carbohydrates and minerals necessary for its growth, fixes nitrogen in proportion to the amount of sugar consumed. This interesting case of symbiosis is equalled by yet another case. The work of numerous observers has shown that the free nitrogen of the atmosphere is brought into combination in the soil in the nodules filled with bacteria on the roots of Leguminosae, and since these nodules are the morphological expression of a symbiosis between the higher plant and the bacteria, there is evidently here a case similar to the last.

As regards the ammonium carbonate acc.u.mulating in the soil from the conversion of urea and other sources, we know from Winogradsky's researches that it undergoes oxidation in two stages owing to the activity of the so-called "nitrifying" bacteria (an unfortunate term inasmuch as "nitrification" refers merely to a particular phase of the cycle of changes undergone by nitrogen). It had long been known that under certain conditions large quant.i.ties of nitrate (saltpetre) are formed on exposed heaps of manure, &c., and it was supposed that direct oxidation of the ammonia, facilitated by the presence of porous bodies, brought this to pa.s.s. But research showed that this process of nitrification is dependent on temperature, aeration and moisture, as is life, and that while nitre-beds can infect one another, the process is stopped by sterilization.

R. Warington, J. T. Schloessing, C. A. Muntz and others had proved that nitrification was promoted by some organism, when Winogradsky hit on the happy idea of isolating the organism by using gelatinous silica, and so avoiding the difficulties which Warington had shown to exist with the organism in presence of organic nitrogen, owing to its refusal to nitrify on gelatine or other nitrogenous media. Winogradsky's investigations resulted in the discovery that two kinds of bacteria are concerned in nitrification; one of these, which he terms the _Nitroso-bacteria_, is only capable of bringing about the oxidation of the ammonia to nitrous acid, and the astonishing result was obtained that [v.03 p.0165] this can be done, in the dark, by bacteria to which only pure mineral salts--_e.g._ carbonates, sulphates and chlorides of ammonium, sodium and magnesium--were added. In other words these bacteria can build up organic matter from purely mineral sources by a.s.similating carbon from carbon dioxide in the dark and by obtaining their nitrogen from ammonia. The energy liberated during the oxidation of the nitrogen is regarded as splitting the carbon dioxide molecule,--in green plants it is the energy of the solar rays which does this. Since the supply of free oxygen is dependent on the activity of green plants the process is indirectly dependent on energy derived from the sun, but it is none the less an astounding one and outside the limits of our previous generalizations. It has been suggested that urea is formed by polymerization of ammonium carbonate, and formic aldehyde is synthesized from CO_2 and OH_2. The _Nitro-bacteria_ are smaller, finer and quite different from the nitroso-bacteria, and are incapable of attacking and utilizing ammonium carbonate. When the latter have oxidized ammonia to nitrite, however, the former step in and oxidize it still further to nitric acid. It is probable that important consequences of these actions result from the presence of nitrifying bacteria in rotten stone, decaying bricks, &c., where all the conditions are realized for preparing primitive soil, the breaking up of the mineral const.i.tuents being a secondary matter. That "soil" is thus prepared on barren rocks and mountain peaks may be concluded with some certainty.

[Ill.u.s.tration: FIG. 14.--Stages in the formation of a colony of a variety of _Bacillus (Proteus) vulgaris_ (Hauser), observed in a hanging drop. At 11 A.M. a rodlet appeared (A); at 4 P.M. it had grown and divided and broken up into eight rodlets (B); C shows further development at 8 P.M., D at 9.30 P.M.--all under a high power. At E, F, and G further stages are drawn, as seen under much lower power. (H. M. W.)]

In addition to the bacterial actions which result in the oxidization of ammonia to nitrous acid, and of the latter to nitric acid, the reversal of such processes is also brought about by numerous bacteria in the soil, rivers, &c. Warington showed some time ago that many species are able to reduce nitrates to nitrites, and such reduction is now known to occur very widely in nature. The researches of Gayon and Dupet.i.t, Giltay and Aberson and others have shown, moreover, that bacteria exist which carry such reduction still further, so that ammonia or even free nitrogen may escape.

The importance of these results is evident in explaining an old puzzle in agriculture, viz. that it is a wasteful process to put nitrates and manure together on the land. Fresh manure abounds in de-nitrifying bacteria, and these organisms not only reduce the nitrates to nitrites, even setting free nitrogen and ammonia, but their effect extends to the undoing of the work of what nitrifying bacteria may be present also, with great loss. The combined nitrogen of dead organisms, broken down to ammonia by putrefactive bacteria, the ammonia of urea and the results of the fixation of free nitrogen, together with traces of nitrogen salts due to meteoric activity, are thus seen to undergo various vicissitudes in the soil, rivers and surface of the globe generally. The ammonia may be oxidized to nitrites and nitrates, and then pa.s.s into the higher plants and be worked up into proteids, and so be handed on to animals, eventually to be broken down by bacterial action again to ammonia; or the nitrates may be degraded to nitrites and even to free nitrogen or ammonia, which escapes.

[Sidenote: Bacteria and Leguminosae.]

That the Leguminosae (a group of plants including peas, beans, vetches, lupins, &c.) play a special part in agriculture was known even to the ancients and was mentioned by Pliny (_Historia Naturalis_, viii). These plants will not only grow on poor sandy soil without any addition of nitrogenous manure, but they actually enrich the soil on which they are grown. Hence leguminous plants are essential in all rotation of crops. By a.n.a.lysis it was shown by Schulz-Lupitz in 1881 that the way in which these plants enrich the soil is by increasing the nitrogen-content. Soil which had been cultivated for many years as pasture was sown with lupins for fifteen years in succession; an a.n.a.lysis then showed that the soil contained more than three times as much nitrogen as at the beginning of the experiment. The only possible source for this increase was the atmospheric nitrogen. It had been, however, an axiom with botanists that the green plants were unable to use the nitrogen of the air. The apparent contradiction was explained by the experiments of H. h.e.l.lriegel and Wilfarth in 1888. They showed that, when grown on sterilized sand with the addition of mineral salts, the Leguminosae were no more able to use the atmospheric nitrogen than other plants such as oats and barley. Both kinds of plants required the addition of nitrates to the soil. But if a little water in which arable soil had been shaken up was added to the sand, then the leguminous plants flourished in the absence of nitrates and showed an increase in nitrogenous material. They had clearly made use of the nitrogen of the air. When these plants were examined they had small swellings or nodules on their roots, while those grown in sterile sand without soil-extract had no nodules. Now these peculiar nodules are a _normal_ characteristic of the roots of leguminous plants grown in ordinary soil.

The experiments above mentioned made clear for the first time the nature and activity of these nodules. They are clearly the result of infection (if the soil extract was boiled before addition to the sand no nodules were produced), and their presence enabled the plant to absorb the free nitrogen of the air.

[Ill.u.s.tration: FIG. 15.--Invasion of leguminous roots by bacteria.