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and the influence of external factors on its growth. Even when

(6) 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 dis

Beggiatoa (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 not formed on all substrata, or are only developed during short

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 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 from a motile stage with peritrichous cilia, into one of filamentous

(2) By far the majority of classiscation, partly ing 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 Fischer's

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 investiga- The question, What is an individual ? has given rise to much tions of the former on the number and distribution of cilia (sce difficulty, and around it many of the speculations regarding plco

If a tree fall apart fig. 1) led him to propose a scheme of classification based on these morphism have centred without useful result.

into its constituent cells periodically we should have the same and other morphological characters, and differing essentially difficulty on a larger and more complex scale. The fact that every from any preceding one. This scheme may be tabulated as bacterial cell in a species in most cases appears equally capable of follows:

performing all the physiological functions of the species has led 1. Order-Haplobacterinae. Vegetative body unicellular; which cannot be consistent in those cases where a simple or branched

most authorities, however, to regard it as the individual-a view spberoidal, cylindrical or spirally twisted; isolated or con- filamentous series exhibits differences between free apex and fixed Dected in filamentous or other growth series.

base and so forth. It may be doubted whether the discussion is 1. Family-COCCACEAE. Vegetative cells spheroidal.

profitable, though it appears necessary in some cases-6.8. con() Sub-family-ALLOCOCCACEAE. Division in all or any cerning pleomorphy-to adopt some definition of individual.

planes, colonies indefinite in shape and size, of cells
in short chains, irregular clumps, pairs or isolated is bacterinae and Trichobacterinae, must now be added a third

Myxobacteriaceae.—To the two divisions of bacteria, Haplo-
Micrococcus (Cohn), cells non-motile; Planococcus
(Migula), cells motile.

division, Myxobacterinae. One (6) Sub-family-HOMOCOCCACEAE. Division planes regular of the first members of this

and definite:--Sarcina (Goods.), cells non-motile;
growth and division in three successive planes at

group, Chondromyces crocatus,
right angles, resulting in packet-like groups; Plano-

was described as long ago as sarcina Migula), as before, but motile; Þediococcus 1857 by Berkeley, but its nature successive planes, and cells in tablets of four or more: ascribed to the Hyphomycetes. (Lindner). division planes at right angles in two was not understood and it was Sire plococcus (Billr.). divisions in one plane only, In 1892, however, Thaxter re

resulting in chains of cells. 2. Family-BACILLACEAE. Vegetative cells cylindric (rodlets), discovered it and showed its

ellipsoid or ovoid, and straight. Division planes always bacterial nature, founding for it perpendicular to the long axis.

and some allied forms the group (@) Sub-lamily-BACILLEAE. Sporogenous rodlets cylindric. | Myxobacteriaceae. Another form,

not altered in shape :-Bacillus (Cohn), non-motile: which he described as Myxobacter,
Bactrinium (Fischer), motile, with one polar flagellum was shown later to be the same
(monotrichous); Bactrillum (Fischer), motile, with a
terminal tust of cilia (lophotrichous); Baclridium

as Polyangium vitcllinum de-
(Fischer), motile, with cilia all over the surface scribed by Link in 1795, the
(peritrichous).

exact nature of which had (b) Sub-family--CLOSTRIDIEAE. Sporogenous rodlets, hitherto been in doubt. Thaxter's

spindle-shaped :-Clostridium (Prazm.), motile (peri- observations and conclusions were

trichous).
(s) Sub-family-PLECTRIDIEAB. Sporogenous rodlets, drum called in question by some

stick-shaped :-Plectridium (Fischer), motile (peri. botanists, but his later observa-
trichous).

tions and those of Baur have 3. Family-SPIRILLACEAE. Vegetative cells, cylindric but established firmly the position

FIG. 12. curved more or less spirally. Divisions perpendicular of the group. The peculiarity A. Myxococcus digelarus,bright to the long axis:-Vibrio (Müller-Löffler), comma- of the group lies in the fact that

red fructification occurring

on dung shaped, motile, monotrichous; Spirillum (Ehrenb.), the bacteria form plasmodium

B. Polyangrun primigenum, more strongly curved in open spirals, motile, lopho- like aggregations and build red fructification on dog's trichous; Spirochaele (Ehrenb.), spirally coiled in themselves up into sporogenous dung numerous close turns, motile, but apparently owing to structures of definite form super

C. Chondromyces apiculatus, flexile movements, as no cilia are found.

ficially similar to the cysts of the

orange fructification II. ORDER-Trichobacterinae. Vegetative body of branched Mycetozoa (fig. 12). Most of

antelope's dung.

D. Young fructification or unbranched cell-filaments, the segments of which separate the forms in question are found E. Single cyst germinating as swarm-cells (Gonidia).

growing on the dung of herbi1. Family-TRICHOBACTERIACEAE, Characters those of the

(A, B. after Quehl: C-E after Tharvorous animals, but the bacteria

ter.) From Strasburger's Lehrbuch der Order.

occur not only in the alimentary Botanik, by permission of Gustav

Fischer (a) Filaments rigid, non-motile, sheathed:-Crenothrix canal of the animal but also

(Cohn), filaments unbranched and devoid of frce in the air. The Myxobacteria are most easily obtained by sulphur particles; Thiothrix (Winogr.), as before, keeping at a temperature of 30-35° C. in the dark dung which but with sulphur particles; Cladothrix (Cohn), fila- has lain exposed to the air for at least eight days. The high ments branched in a pseudo-dichotomous manner. temperature is favourable to the growth of the bacteria but

E

on Function and life of bacteria.

2.4?.

inimical to that of the fungi which are so common on this sub- forms are termed by Fischer Metatrophic, because they require stratum.

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 of decompositions hitherto unsuspected are accom- true parasites-obligate parasites of de Bary-are placed by 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 under

Nitrogen in agriculture and daily life. Indeed, nothing marks the attitude gone, the question is of importance, What happens in

bacteris 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 11.40

capable of forcing the inert free nitrogen to combine with other

elements into compounds assimilable by plants. This was long 8.30

asserted as probable before Winogradsky showed that the con

clusions of M. P. E. Berthelot, A. Laurent and others were 10.0!

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 can be measured. 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 growth. (H. M. W.)

from Winogradsky's researches that it undergoes oxidation in

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 ord rily organic nitrogen, owing to its refusal to nitrily on gelatine or termed saprophytes, are sa progenic, i.e. bring organic material other nitrogenous media. Winogradsky's investigations resulted to the putrefactive state-or sa prophilous, 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 Nilrose-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

10.30*2.13

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to man.

osae.

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 armonium, 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 That the Leguminosae (a group of plants including peas, beans, as splitting the carbon dioxide molecule, --in green plants it is vetches, lupins, &c.) play a special part in agriculture was known the energy of the solar rays which does this. Since the supply even to the ancients and was mentioned by Pliny Bacteria cí free oxygen is dependent on the activity of green plants the (Historia Naturalis, viii.). These plants will not only and process is indirectly dependent on energy derived from the sun, grow on poor sandy soil without any addition of nitro- Legumia. but it is none the less an astounding one and outside the limits genous manure, but they actually enrich the soil on of our previous generalizations. It has been suggested that urea which they are grown. Hence leguminous plants are essential in is formed by polymerization of ammonium carbonate, and formic all rotation of crops. By analysis it was shown by Schulz-Lupitz aldehyde is synthesized from CO, and OH,. The Nitro-bacteria in 1881 that the way in which these plants enrich the soil is by are smaller, finer and quite different from the nitroso-bacteria, increasing the nitrogen-content. Soil which had been cultivated and are incapable of attacking and utilizing ammonium carbonate. for many years as pasture was sown with lupins for fifteen years When the latter have oxidized ammonia to nitrite, however, in succession; an analysis then showed that the soil contained the former step in and oxidize it still further to nitric acid. more than three times as much nitrogen as at the beginning of It is probable that important consequences of these actions the experiment. The only possible source for this increase was result from the presence of nitrifying bacteria in rotten stone, 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 A B

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 E

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 G

F

the result of infection (if the soil extract was boiled before addi

tion to the sand no nodules were produced), and their presence FIG. 14.-Stages in the formation of a colony of a variety of enabled the plant to absorb the free nitrogen of the air. Bacillus (Proleus) vulgaris (Hauser), observed in a hanging drop: At 11 A.x a rodlet appeared (A); at 4 P.m. it had grown and divided

The work of recent investigators has made clear the whole and broken up into eight

rodlets (B); C shows further develop-process. In ordinary arable soil there exist motile rod-like ment at 8 P.M., D at 9.30 P.M.-all under a high power. At E, F, bacteria, Bacterium and G further stages are drawn, as seen under much lower power. radicicola. These enter (H. M. W.)

the root-hairs of legudecaying bricks, &c., where all the conditions are realized for minous plants, and passpreparing primitive soil

, the breaking up of the mineral con- ing down the hair in the stituents being a secondary matter. That “soil " is thus pre- form of a long, slimy pared on barren rocks and mountain peaks may be concluded (zoogloea) thread, penewith some certainty.

trate the tissues of the In addition to the bacterial actions which result in the oxidiza- root. As a result the tion of ammonia to nitrous acid, and of the latter to nitric acid, tissues become hyperthe reversal of such processes is also brought about by numerous trophied, producing the bacteria in the soil, rivers, &c. Warington showed some time well-known nodule. In ago that many species are able to reduce nitrates to nitrites, the cells of the nodule and such reduction is now known to occur very widely in nature. the bacteria multiply The researches of Gayon and Dupetit, Giltay and Aberson and and develop, drawing others have shown, moreover, that bacteria exist which carry material from their host. such reduction still further, so that ammonia or even free nitrogen Many of the bacteria exmay escape. The importance of these results is evident in ex- hibit curious involution FIG. 15.- Invasion of leguminous plaining an old puzzle in agriculture, viz. that it is a wasteful forms (“ bacteroids "),

roots by bacteria. process to put nitrates and manure together on the land. Fresh which are finally broken e, cell from the epidermis of root of Pea

with " infection thread " (zoogloea) manure abounds in de-nitrifying bacteria, and these organisms down and their products

pushing its way through the cellnot only reduce the nitrates to nitrites, even setting free nitrogen absorbed by the plant. walls. (After Prazmowski.) and ammonia, but their effect extends to the undoing of the The nitrogen of the air is 6, free end of a root-hair of Pea; at the work of what nitrifying bacteria may be present also, with great absorbed by the nodules,

right are particles of earth and on loss. The combined nitrogen of dead organisms, broken down being built up into the

the left a mass of bacteria. Inside

the hair the bacteria are pushing to ammonia by putrefactive bacteria, the ammonia of urea and bacterial cell and later their way up in a thin stream. the results of the fixation of free nitrogen, together with traces handed on to the host- (From Fischer's Vorlesungen über Bakterien.) of nitrogen salts due to meteoric activity, are thus seen to plant. It appears from the observations of Mazé that the undergo various vicissitudes in the soil, rivers and surface of bacterium can even absorb free nitrogen when grown in cultures

[graphic]
[graphic]

outside the plant. We have here a very interesting case of sym- unable to attack the cellulose itself. There exist in the mud of biosis as mentioned above. The green plant, however, always marshes, rivers and cloacae, &c., however, other anaerobic keeps the upper hand, restricting the development of the bacteria bacteria which decompose cellulose, probably hydrolysing it to the nodules and later absorbing them for its own use. It should first and then splitting the products into carbon dioxide and be mentioned that different genera require different races of the marsh gas. When calcium sulphate is present, the nascent bacterium for the production of nodules.

methane induces the formation of calcium carbonate, sulphuretted The important part that these bacteria play in agriculture led hydrogen and water. We have thus an explanation of the to the introduction in Germany of a commercial product (the so- occurrence of marsh gas and sulphuretted hydrogen in bogs, called “ nitragin ") consisting of a pure culture of the bacteria, and it is highly probable that the existence of these gases in the which is to be sprayed over the soil or applied to the seeds before intestines of herbivorous animals is due to similar putrefactive sowing. This material was found at first to have a very uncertain changes in the undigested cellulose remains. effect, but later experiments in America, and the use of a modified Cohn long ago showed that certain glistening particles observed

preparation in Eng- in the cells of Beggiatoa consist of sulphur, and Winogradsky
land, under the direc- and Beyerinck have shown that a whole series of
tion of Professor sulphur bacteria of the genera Thiothrix, Chromatium,

Sulphur

bacteris Bottomley, have had Spirillum, Monas, &c., exist, and play important successful results; it parts in the circulation of this element in nature, e.g. in marshes, is possible that in the estuaries, sulphur springs, &c. When cellulose bacteria set free future a preparation of

this sort will be widely 6

used.

Theapparentspecialization of these bacteria to the leguminous plants has always been a very striking fact, for similar bacterial nodules are known only in two or three cases outside this particular group. However, Professor Bottomley announced at

the
meeting of the British
Association for the

Advancement of
FIG. 16.

Science in 1907 that Q, root nodule of the lupin, nat. size. he had succeeded in (From Woromv.)

Fig. 17:- A plate-culture of a bacillus which had been exposed b, longitudinal section through root and breaking down this for a period of four hours behind a zinc stencil-plate, in which

the letters C and B were cut.

specialization and by a nodule.

The light had to traverse a screen g, fibro-vascular bundle.

suitable treatment had

of water before passing through the C, and one of aesculin (which w, bacterial tissue. (After Woromv.)

filters out the blue and violet rays) before passing the B. The plate

caused bacteria from c, cell from bacterial tissues showing leguminous nodules to C-shaped area were all killed, whereas they developed elsewhere

was then incubated, and, as the figure shows, the bacteria on the nucleus and protoplasm filled with bacteria.

insect other plants on the plate (traces of the B are just visible to the right) and covered d, bacteria from nodule of lupin, normal such as cereals

, tomato, it with an opaque growth. (H. M. W.) undegenerate form.

rose, with a marked marsh gas, the nascent gas reduces sulphates-e.g. gypsume and s, bacteroids from Vicia villosa effect on their growth. with liberation of SH2, and it is found that the sulphur bacteria

and Lupinus albus. (After Morck.) If these results are con- thrive under such conditions by oxidizing the SH, and storing (From Fischer's Vorlesungen über Bakterien.) firmed and the treat- the sulphur in their own protoplasm. If the SH, runs short

ment can be worked they oxidize the sulphur again to sulphuric acid, which combines commercially, the importance to agriculture of the discovery with any calcium carbonate present and forms sulphate again. cannot be overestimated; each plant will provide, like the Similarly nascent methane may reduce iron salts, and the black bean and vetch, its own nitrogenous manure, and larger crops mud in which these bacteria often occur owes its colour to the will be produced at a decreased cost.

FeS formed. Beyerinck and Jegunow have shown that some Another important advance is in our knowledge of the part partially anaerobic sulphur bacteria can only exist in strata played by bacteria in the circulation of carbon in nature. The at a certain depth below the level of quiet waters where SH, is Cellulose

enormous masses of cellulose deposited annually on being set free below by the bacterial decompositions of vegetable bacteria. the earth's surface are, as we know, principally the mud and rises to meet the atmospheric oxygen coming down

result of chlorophyll action on the carbon dioxide of from above, and that this zone of physiological activity rises the atmosphere decomposed by energy derived from the sun; and falls with the variations of partial pressure of the gases due and although we know little as yet concerning the magnitude of to the rate of evolution of the SHg. In the deeper parts of this other processes of carbon-assimilation-e.g. by nitrifying bacteria zone the bacteria absorb the SH., and, as they rise, oxidize it -it is probably comparatively small. Such cellulose is gradually and store up the sulphur; then ascending into planes more reconverted into water and carbon dioxide, but for some time highly oxygenated, oxidize the sulphur to SOs. These bacteria nothing positive was known as to the agents which thus break therefore employ SH, as their respiratory substance, much as up the paper, rags, straw, leaves and wood, &c., accumulating higher plants employ carbohydrates—instead of liberating in cesspools, forests, marshes and elsewhere in such abundance. energy as heat by the respiratory combustion of sugars, they The work of van Tieghem, van Senus, Fribes, Omeliansky and do it by oxidizing hydrogen sulphide. Beyerinck has shown others has now shown that while certain anaerobic bacteria that Spirillum desul phuricans, a definite anaerobic form, attacks decompose the substance of the middle lamella-chiefly pectin and reduces sulphates, thus undoing the work of the sulphur compounds-and thus bring about the isolation of the cellulose bacteria as certain de-nitrifying bacteria reverse the operations fibres when, for instance, flax is steeped or “retted,” they are l of nitro-bacteria. Here again, therefore, we have sulphur, taken

[graphic]
[graphic]
[graphic]

bacteria.

into the higher plants as sulphates, built up into proteids, decom- | oxygen, while others do not, may have bearings on the facultative posed by putrefactive bacteria and yielding SH, which the anaerobism of these organisms. sulphur bacteria oxidize; the resulting sulphur is then again A branch of bacteriology which offers numerous problems of oxidized to SO, and again combined with calcium to gypsum, importance is that which deals with the organisms so common the cycle being thus complete.

in milk, butter and cheese. Milk is a medium not Chalybeate waters, pools in marshes near ironstone, &c., only admirably suited to the growth of bacteria, but,

Dalry abound in bacteria, some of which belong to the remarkable as a matter of fact, always contaminated with these

genera Crenothrix, Clodothrix and Leplothrix, and organisms in the ordinary course of supply. F. Lafar has stated Lroa bacteria.

contain ferric oxide, i.e. rust, in their cell-walls. that 20% of the cows in Germany suffer from tuberculosis,

This iron deposit is not merely mechanical but is due which also affected 17.7% of the cattle slaughtered in Copento the physiological activity of the organism which, according hagen between 1891 and 1893, and that one in every thirteen to Winogradsky, liberates energy by oxidizing ferrous and ferric samples of milk examined in Paris, and one in every nineteen in oxide in its protoplasm—a view not accepted by H. Molisch. Washington, contained tubercle bacilli

. Hence the desirability The iron must be in certain soluble conditions, however, and the of sterilizing milk used for domestic purposes becomes imperative. soluble bicarbonate of the protoxide of chalybeate springs seems most favourable; the hydrocarbonate absorbed by the cells is oxidized, probably thus

2FeCO:+30H+0 = Fez (OH).+2CO3. The ferric hydroxide accumulates in the sheath, and gradually passes into the more insoluble ferric oxide. These actions are of extreme importance in nature, as their continuation results in the enormous deposits of bog-iron ore, ochre, and-since Molisch has shown that the iron can be replaced by manganese in some bacteria-of manganese ores.

Considerable advances in our knowledge of the various chromogenic bacteria have been made by the studies of Beyerinck, Pigmeat

Lankester, Engelmann, Ewart and others, and have bacteria.

assumed exceptional importance owing to the discovery

that Bacleriopurpurin the red colouring matter contained in certain sulphur bacteria-absorbs certain rays of solar energy, and enables the organism to utilize the energy for its own life-purposes. Engelmann showed, for instance, that these red-purple bacteria collect in the ultra-red, and to a less extent in the orange and green, in bands which agree with the absorption spectrum of the extracted colouring matter. Not only so, but

FIG. 18.–A similar preparation to fig. 17, except that two slit-like the evident parallelism between this absorption of light and openings of equal length allowed the light to pass, and that the light that by the chlorophyll of green plants, is completed by the was that of the electric arc passed through a quartz prism and casting demonstration that oxygen is set free by these bacteria-i.e. a powerful spectrum on the plate. The upper slit was covered with by means of radiant energy trapped by their colour-screens the glass, the lower with quartz. The bacteria were killed over the living cells are in both cases enabled to do work, such as the sponds to the line F (green end of the blue), and the beginning of the

clear areas shown. The left-hand boundary of the clear area correreduction of highly oxidized compounds.

ultra-violet was at the extreme right of the upper (short) area. The The most recent observations of Molisch seem to show that lower area of bactericidal action extends much farther to the right, bacteria possessing bacteriopurpurin exhibit a new type of does glass. The red-yellow-green to the left of F were without effect.

because the quartz allows more ultra-violet rays to pass than assimilation—the assimilation of organic material under the (H. M. W.) influence of light. In the case of these red-purple bacteria the colouring matter is contained in the protoplasm of the cell, but No milk is free from bacteria, because the external orifices of the in most chromogenic bacteria it occurs as excreted pigment on milk-ducts always contain them, but the forms present in the and between the cells, or is formed by their action in the medium. normal fluid are principally those which induce such changes Ewart has confirmed the principal conclusions concerning these as the souring or “ turning ” so frequently observed in standing purple, and also the so-called chlorophyll bacteria (B. viride, milk (these were examined by Lord Lister as long ago as 1873B. chlorinum, &c.), the results going to show that these are, as 1877, though several other species are now known), and those many authorities have held, merely minute algae. The pigment which bring about the various changes and fermentations in itself may be soluble in water, as is the case with the blue-green butter and cheese made from it. The presence of foreign germs, fluorescent body formed by B. pyocyaneus, B. fluorescens and which may gain the upper hand and totally destroy the flavours a whole group of fluorescent bacteria. Neelson found that the of butter and cheese, has led to the search for those particular pigment of B. cyanogenus gives a band in the yellow and strong forms to which the approved properties are due. A definite lines at E and F in the solar spectrum-an absorption spectrum bacillus to which the peculiarly fine flavour of certain butters is almost identical with that of triphenyl-rosaniline. In the case due, is said to be largely employed in purc cultures in American of the scarlet and crimson red pigments of B. prodigiosus, B. dairies, and in Denmark certain butters are said to keep fresh ruber, &c., the violet of B. violacens, B. janthinus, &c., the red- much longer owing to the usc of pure cultures and the treatment purple of the sulphur bacteria, and indeed most bacterial pig-employed to suppress the forms which cause rancidity. Quite ments, solution in water does not occur, though alcohol extracts distinct is the search for the germs which cause undesirable the colour readily. Finally, there are a few forms which yield changes, or " diseases "; and great strides have been made in their colour to neither alcohol nor water, e.g. the yellow Micro discovering the bacteria concerned in rendering milk “ropy," coccus cercus-flaous and the B. berolinensis. Much work is still butter “oily” and “rancid,” &c. Chcese in its numerous necessary before we can estimate the importance of these pig-forms contains myriads of bacteria, and some of these are now ments. Their spectra are only imperfectly known in a few known to be concerned in the various processes of ripening cases, and the bearing of the absorption on the life-history is and other changes affecting the product, and although little is still a mystery. In many cases the colour-production is de known as to the exact part played by any species, practical pendent on certain definite conditions—temperature, presence applications of the discoveries of the decade 1890-1900 have of oxygen, nature of the food-medium, &c. Ewart's important been made, e.g. Edam cheese. The Japanese have cheeses discovery that some of these lipochrome pigments occlude resulting from the bacterial fermentation of boiled Soja beans.

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