forms-the zoogloea is now known to be a sort of resting condition of the Schizomycetes, the various elements being glued together, as it were, by their enormously swollen and diffluent cell-walls becoming contiguous. The zoogloea is formed by active division of single or of several mother-cells, and the progeny appear to go on secreting the cell-wall substance, which then absorbs many times its volume of water, and remains as a consistent matrix, in which the cells come to rest. The matrix c. the swollen cell-walls-in some cases consists mainly of cellulose, in others chiefly of a proteid substance; the matrix in some cases is horny and resistant, in others more like a thick solution of gum. It is intelligible from the mode of formation that foreign bodies may become entangled in the gelatinous matrix, and compound zoogloeae may arise by the apposition of several distinct forms, a common event in macerating troughs (fig. 3, A). Characteristic forms may be assumed by the young zoogloea of different species,-spherical, ovoid, reticular, filamentous, fruiticose, lamellar, &c.,-but these vary considerably as the mass increases or comes in contact with others. Older 847 910 940 105 1040 855 327 336 1030 alo FIG. 7.-Bacillus anthracis. (After Koch.) A. Bacilli mingled with blood-corpuscles from the blood of a guinea-pig; some of the bacilli dividing. B. The rodlets after three hours' culture in a drop of aqueous humour. They grow out into long leptothrix-like filaments, which become septate later, and spores are developed in the segments. zooglocae may precipitate oxide of iron in the matrix, if that metal exists in small quantities in the medium. Under favourable conditions the elements in the zoogloea again become active, and move out of the matrix, distribute themselves in the Surrounding medium, to grow and multiply as before. If the zoogloca is formed on a solid substratum it may become firm and horny; immersion in water softens it as described above. ment of growth. The growth of an ordinary bacterium consists in uniform elongation of the rodlet until its length is doubled, followed by division by a median septum, then by the simulMeasure taneous doubling in length of each daughter cell, again followed by the median division, and so on (figs. 13, 14). If the cells remain connected the resulting filament repeats these processes of elongation and subsequent division uniformly so long as the conditions are maintained, and very accurate measurements have been obtained on such a form, e.g. B. ramosus. If a rodlet in a hanging drop of nutrient gelatine is fixed under the microscope and kept at constant temperature, a curve of growth can be obtained recording the behaviour during many hours or days. The measured lengths are marked off on ordinates erected on an abscissa, along which the times are noted. The curve obtained on joining the former points then brings out a number of facts, foremost among which are (1) that as long as the conditions remain constant the doubling periods-i.e. the times taken by any portion of the filament to double its length-are constant, because each cell is equally 535 35 330 (Fraenkel), constructed from data such as in fig. 4. The abscissae FIG. 8.-Curve of growth of a filament of Bacillus ramosus represent intervals of time, the ordinates the measured lengths of the growing filament. Thus, at 2.33 P.M. the length of the filament was 6 u; at 5.45, 20 μ; at 8 P.M., 70 μ and so on. Such curves show curve), and to alterations of light (lamp) and darkness. (H. M. W.) differences of steepness according to the temperature (see temp. A very characteristic method of reproduction is that of sporeformation, and these minute reproductive bodies, which represent a resting stage of the organism, are now known in many Spores. 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 passes 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 64 which are actively swarming, the movements not being interfered with by the process (fig. 4, D). The so-called Köpfchenbacterien " 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 o-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 B may be relatively thick, and even exhibit shells or strata. 02 FIG. 10-Bacillus subtilis. (After Strasburger.) A. Zoogloca pellicle, B. Motile rodlets C. Development of spores. 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 classification based on the power of growing colonics A B 9 A.M. 10.40 C 12.30 D FIG. 9. 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. A, Bacillus anthra- The spore swells and elongates and the cis. (After de Bary.) contents grow forth to a cell like that which Two of the long fila- produced it, in some cases clearly breaking ments (B, fig. 10), in which are through the membrane, the remains of spores being developed. The which may be seen attached to the young specimen was culti; germinal rodlet (figs. 5, 9 and 11); in other vated in broth, and cases the surrounding membrane of the spores are drawn a little too small spore swells and dissolves. The germinal they should be of the cell then grows forth into the forms typical same diameter trans- for the particular Schizomycete concerned. versely as the seg- The conditions for spore-formation differ. ments. spores; 2-5, succes B. Bacillus sub- Anaerobic species usually require little tilis. (After de oxygen, but aerobic species a free supply. Bary.) 1, fragments Each species has an optimum temperature of filaments with ripe and many are known to require very special sive stages in the food-media. The systematic interference germination of the with these conditions has enabled bacteriospores, the remains logists to induce the development of soof the spore at- called asporogenous races, in which the tached to the ger 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. minal rodlets. The difficult subject of the classification of bacteria dates The difficulties presented by such minute and simple organisms as the Schizomycetes are due partly to the few" characters which they possess and partly to the dangers of error in manipulating them; it is anything but an easy matter either to trace the whole development of a single form or to recognize with certainty any one stage in the development unless the others are known. This being the case, and having regard to the minuteness and ubiquity of these organisms, we should be very careful in accepting evidence as to the continuity or otherwise of any two forms which falls short of direct and uninterrupted observation. The outcome of all these considerations is that, while recognizing that the "genera" and "species" as defined by Cohn must be recast, we are not warranted in uniting any forms the continuity of which has not been directly 4 P.M. 8 P.M. (Fraenkel), in the order and at the times given, in a hanging drop FIG. 11.-Stages in the development of spores of Bacillus ramosus culture, under a very high power. The process begins with the formation of brilliant granules (A, B); these increase, and the brilliant substance gradually balls together (C) and forms the spores (D), one in each segment, which soon acquire a membrane and ripen (E). (H. M. W.) 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 observed; or, at any rate, the strictest rules should be followed in accepting the evidence adduced to render the union of any forms probable. and the influence of external factors on its growth. Even when (b) Filaments showing slow pendulous and creeping used in conjunction with purely morphological characters, these movements, and with no distinct sheath:physiological properties are too variable to aid us in the disBeggiatoa (Trev.), with sulphur particles. crimination of species and genera, and are apt to break down at The principal objections to this system are the following:-(1) critical periods. Among the more characteristic of these schemes The extraordinary difficulty in obtaining satisfactory preparations adopted at various times may be mentioned those of Miquel showing the cilia, and the discovery that these motile organs are (1891), Eisenberg (1891), and Lehmann and Neumann (1897). periods of activity while the organism is young and vigorous, render not formed on all substrata, or are only developed during short Although much progress has been made in determining the value this character almost nugatory. For instance, B. megatherium and constancy of morphological characters, we are still in need and B. subtilis pass in a few hours after commencement of growth of a sufficiently comprehensive and easily applied scheme of growth preceded by casting of the cilia. (2) By far the majority of from a motile stage with peritrichous cilia, into one of filamentous classification, partly owing to the existence in the literature of the described species (over 1000) fall into the three genera-Microimperfectly described forms the life-history of which is not yet coccus (about 400), Bacillus (about 200) and Bactridium (about known, or the microscopic characters of which have not been 150), so that only a quarter or so of the forms are selected out by examined with sufficient accuracy and thoroughness. The the other genera. (3) The monotrichous and lophotrichous condi tions are by no means constant even in the motile stage; thus principal attempts at morphological classifications recently Pseudomonas rosea (Mig.) may have 1, 2 or 3 cilia at either end, brought forward are those of de Toni and Trevisan (1889), and would be distributed by Fischer's classification between Bac Fischer (1897) and Migula (1897). Of these systems, which trinium and Bactrillum, according to which state was observed. alone are available in any practical scheme of classifi-In Migula's scheme the attempt is made to avoid some of these difficulties, but others are introduced by his otherwise clever devices cation, the two most important and most modern are for dealing with these puzzling little organisms. 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 classification based on these and other morphological characters, and differing essentially from any preceding one. This scheme may be tabulated as follows: Fischer's scheme. 1. ORDER-Haplobacterinae. Vegetative body unicellular; 1. Family-COCCACEAE. Vegetative cells spheroidal. (b) Sub-family-HOMOCOCCACEAE. Division planes regular (a) Sub-family-BACILLEAE. Sporogenous rodlets cylindric, trichous). trichous). 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 difficulty on a larger and more complex scale. The fact that every into its constituent cells periodically we should have the same bacterial cell in a species in most cases appears equally capable of performing all the physiological functions of the species has led which cannot be consistent in those cases where a simple or branched most authorities, however, to regard it as the individual-a view 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. Myxobacteriaceae.-To the two divisions of bacteria, Haplo- established firmly the position Family SPIRILLACEAE. Vegetative cells, cylindric but II. ORDER-Trichobacterinae. Vegetative body of branched 1. Family-TRICHOBACTERIACEAE. Characters those of the Order. (a) Filaments rigid, non-motile, sheathed:-Crenothrix FIG. 12. A. Myxococcus digelatus,bright B. Polyangium primigenum, D. Young fructification 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 canal 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 (A, B. after Quehl: C-E,after Thaxter.) From Strasburger's Lehrbuch der Botanik, by permission of Gustav Fischer. stratum. Nitrogen inimical to that of the fungi which are so common on this sub-forms are termed by Fischer Metatrophic, because they require various kinds of organic materials obtained from the dead The discoveries that some species of nitrifying bacteria and remains of other organisms or from the surfaces of their bodies, perhaps pigmented forms are capable of carbon-assimilation, and can utilize and decompose them in various ways (Polytrophic ) that others can fix free nitrogen and that a number or, if monotrophic, are at least unable to work them up. The Peactile or of decompositions hitherto unsuspected are accom- true parasites-obligate parasites of de Bary—are placed by bacteria.plished by Schizomycetes, have put the questions of Fischer in a third biological group, Paratrophic bacteria, to mark nutrition and fermentation in quite new lights. Apart the importance of their mode of life in the interior of living from numerous fermentation processes such as rotting, the organisms where they live and multiply in the blood, juices soaking of skins for tanning, the preparation of indigo and of or tissues. tobacco, hay, ensilage, &c., in all of which bacterial fermenta- When we reflect that some hundreds of thousands of tons of tions are concerned, attention may be especially directed to the urea are daily deposited, which ordinary plants are unable to following evidence of the supreme importance of Schizomycetes assimilate until considerable changes have been underin agriculture and daily life. Indeed, nothing marks the attitude gone, the question is of importance, What happens in bacteria. of modern bacteriology more clearly than the increasing attention the meantime? In effect the urea first becomes which is being paid to useful fermentations. The vast majority carbonate of ammonia by a simple hydrolysis brought about by of these organisms are not pathogenic, most are harmless and 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 débris. 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 12.42. capable of forcing the inert free nitrogen to combine with other 11.40 elements into compounds assimilable by plants. This was long 8.30 asserted 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 pro tected from access of free oxygen by an envelope of aerobic 2.35 bacteria or fungi, and provided with the carbohydrates and 2.58 4.52 minerals necessary for its growth, fixes nitrogen in proportion 3.43 4.30 to the amount of sugar consumed. This interesting case of 4.12 symbiosis is equalled by yet another case. The work of numerous 4.52 5.20 observers has shown that the free nitrogen of the atmosphere 5.20 is brought into combination in the soil in the nodules filled with FIG. 13.—A series of phases of germination of the spore of B. bacteria on the roots of Leguminosae, and since these nodules ramosus sown at 8.30 (to the extreme left), showing how the growth are the morphological expression of a symbiosis between the on a base line in the order of the successive times of observation higher plant and the bacteria, there is evidently here a case recorded, and at distances apart proportional to the intervals of time similar to the last. (8.30, 10.0, 10.30, 11.40, and so on) and erect the straightened-out As regards the ammonium carbonate accumulating in the filaments, the proportional length of each of which is here given for soil from the conversion of urea and other sources, we know each period, a line joining the tips of the filaments gives the curve of from Winogradsky's researches that it undergoes oxidation in growth. (H. M. W.) two stages owing to the activity of the so-called " nitrifying " many are indispensable aids in natural operations important bacteria (an unfortunate term inasmuch as " nitrification refers merely to a particular phase of the cycle of changes Fischer has proposed that the old division into saprophytes undergone by nitrogen). It had long been known that under and parasites should be replaced by one which takes into account certain conditions large quantities of nitrate (saltpetre) are other peculiarities in the mode of nutrition of bacteria. The formed on exposed heaps of manure, &c., and it was supposed nitrifying, nitrogen-fixing, sulphur- and iron-bacteria he regards that direct oxidation of the ammonia, facilitated by the presence as monotrophic, i.e. as able to carry on one particular series of of porous bodies, brought this to pass. But research showed fermentations or decompositions only, and since they require that this process of nitrification is dependent on temperature, no organic food materials, or at least are able to work up nitrogen aeration and moisture, as is life, and that while nitre-beds can or carbon from inorganic sources, he regards them as primitive infect one another, the process is stopped by sterilization. forms in this respect and terms them Prototrophic. They may R. Warington, J. T. Schloessing, C. A. Müntz and others had be looked upon as the nearest existing representatives of the proved that nitrification was promoted by some organism, when primary forms of life which first obtained the power of working Winogradsky hit on the happy idea of isolating the organism up non-living into living materials, and as playing a correspond - by using gelatinous silica, and so avoiding the difficulties which ingly important role in the evolution of life on our globe. The Warington had shown to exist with the organism in presence of vast majority of bacteria, on the other hand, which are ordinarily organic nitrogen, owing to its refusal to nitrify on gelatine or termed saprophytes, are saprogenic, i.e. bring organic material other nitrogenous media. Winogradsky's investigations resulted to the putrefactive state-or saprophilous, i.e. live best in such in the discovery that two kinds of bacteria are concerned in putrefying materials--or become zymogenic, i.e. their metabolic nitrification; one of these, which he terms the Nitroso-bacteria, products may induce blood-poisoning or other toxic effects is only capable of bringing about the oxidation of the ammonia (facultative parasites) though they are not true parasites. These I to nitrous acid, and the astonishing result was obtained that 10.0 1.47 to man. this can be done, in the dark, by bacteria to which only pure | the globe generally. The ammonia may be oxidized to nitrites mineral salts-e.g. carbonates sulphates and chlorides of and nitrates, and then pass into the higher plants and be worked ammonium, sodium and magnesium-were added. In other up into proteids, and so be handed on to animals, eventually to words these bacteria can build up organic matter from purely be broken down by bacterial action again to ammonia; or the mineral sources by assimilating carbon from carbon dioxide in nitrates may be degraded to nitrites and even to free nitrogen or the dark and by obtaining their nitrogen from ammonia. The ammonia, which escapes. 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, and OH. 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, E 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.) decaying bricks, &c., where all the conditions are realized for preparing primitive soil, the breaking up of the mineral constituents being a secondary matter. That "soil" is thus prepared on barren rocks and mountain peaks may be concluded with some certainty. 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 Dupetit, 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 osae. 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 Bacteria (Historia Naturalis, viii.). These plants will not only and grow on poor sandy soil without any addition of nitro- Legumingenous manure, but they actually enrich the soil on which they are grown. Hence leguminous plants are essential in all rotation of crops. By analysis 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 analysis 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. Hellriegel 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. The work of recent investigators has made clear the whole process. In ordinary arable soil there exist motile rod-like bacteria, Bacterium radicicola. These enter the root-hairs of leguminous plants, and passing down the hair in the form of a long, slimy (zoogloea) thread, penetrate the tissues of the root. As a result the tissues become hypertrophied, producing the well-known nodule. In the cells of the nodule the bacteria multiply and develop, drawing material from their host. Many of the bacteria exhibit curious involution forms ("bacteroids"), which are finally broken cell from the epidermis of root of Pea with "infection thread " (zoogloea) down and their products pushing its way through the cellabsorbed by the plant. walls. (After Prazmowski.) The nitrogen of the air is free end of a root-hair of Pea; at the absorbed by the nodules, right are particles of earth and on the left a mass of bacteria. Inside being built up into the the hair the bacteria are pushing bacterial cell and later their way up in a thin stream. handed on to the host- (From Fischer's Vorlesungen über Bakterien.) plant. It appears from the observations of Mazé that the bacterium can even absorb free nitrogen when grown in cultures 4, b, FIG. 15.-Invasion of leguminous roots by bacteria. |