Volume 3, Part 1, Slice 2 Part 4 (1/2)

No sharp line can be drawn between pathogenic and non-pathogenic Schizomycetes, and some of the most marked steps in the progress of our modern knowledge of these organisms depend on the discovery that their pathogenicity or virulence can be modified--diminished or increased--by definite treatment, and, in the natural course of epidemics, by alterations in the environment. Similarly we are unable to divide Schizomycetes sharply into parasites and saprophytes, since it is well proved that a number of species--facultative parasites--can become one or the other according to circ.u.mstances. These facts, and the further knowledge that many bacteria never observed as parasites, or as pathogenic forms, produce toxins or poisons as the result of their decompositions and fermentations of organic substances, have led to important results in the applications of bacteriology to medicine.

[Ill.u.s.tration: FIG. 20.--The ginger-beer plant.

A. One of the brain-like gelatinous ma.s.ses into which the mature ”plant”

condenses.

B. The bacterium with and without its gelatinous sheaths (cf. fig. 19).

C. Typical filaments and rodlets in the slimy sheaths.

D. Stages of growth of a sheathed filament--a at 9 A.M., b at 3 P.M., c at 9 P.M., d at 11 A.M. next day, e at 3 P.M., f at 9 P.M., g at 10.30 A.M.

next day, h at 24 hours later. (H. M. W.)]

[Sidenote: Bacteriosis in plants.]

Bacterial diseases in the higher plants have been described, but the subject requires careful treatment, since several points suggest doubts as to the organism described being the cause of the disease referred to their agency. Until recently it was urged that the acid contents of plants explained their immunity from bacterial diseases, but it is now known that many bacteria can flourish in acid media. Another objection was that even if bacteria obtained access through the stomata, they could not penetrate the cell-walls bounding the intercellular s.p.a.ces, but certain anaerobic forms are known to ferment cellulose, and others possess the power of penetrating the cell-walls of living cells, as the bacteria of Leguminosae first described by Marshall Ward in 1887, and confirmed by Miss Dawson in 1898. On the other hand a long list of plant-diseases has been of late years attributed to bacterial action. Some, _e.g._ the Sereh disease of the sugar-cane, the slime fluxes of oaks and other trees, are not only very doubtful cases, in which other organisms such as yeasts and fungi play their parts, but it may be regarded as extremely improbable that the bacteria are the primary agents at all; they are doubtless saprophytic forms which have gained access to rotting tissues injured by other agents.

Saprophytic bacteria can readily make their way down the dead hypha of an invading fungus, or into the punctures made by insects, and Aphides have been credited with the bacterial infection of carnations, though more recent researches by Woods go to show the correctness of his conclusion that Aphides alone are responsible for the carnation disease. On the other hand, recent investigation has brought to light cases in which bacteria are certainly the primary agents in diseases of plants. The princ.i.p.al features are the stoppage of the vessels and consequent wilting of the shoots; as a rule the cut vessels on transverse sections of the shoots appear brown and choked with a dark yellowish slime in which bacteria may be detected, _e.g._ cabbages, cuc.u.mbers, potatoes, &c. In the carnation disease and in certain diseases of tobacco and other plants the seat of bacterial action appears to be the parenchyma, and it may be that Aphides or other piercing insects infect the plants, much as insects convey pollen from plant to plant, or (though in a different way) as mosquitoes infect man with malaria. If the recent work on the cabbage disease may be accepted, the bacteria make their entry at the water pores at the margins of the leaf, and thence via the glandular cells to the tracheids. Little is known of the mode of action of bacteria on these plants, but it may be a.s.sumed with great confidence that they excrete enzymes and poisons (toxins), which diffuse into the cells and kill them, and that the effects are in principle the same as those of parasitic fungi. Support is found for this opinion in Beyerinck's discovery that the juices of tobacco plants affected with the disease known as ”leaf mosaic,” will induce this disease after filtration through porcelain.

[Sidenote: Symbiosis.]

In addition to such cases as the kephir and ginger-beer plants (figs. 19, 20), where anaerobic bacteria are a.s.sociated with yeasts, several interesting examples of symbiosis among bacteria are now known. _Bacillus chauvaei_ ferments cane-sugar solutions in such a way that normal butyric arid, inactive lactic acid, carbon dioxide, and hydrogen result; _Micrococcus acidi-paralactici_, on the other hand, ferments such solutions to optically active paralactic acid. Nencki showed, however, that if both these organisms occur together, the resulting products contain large quant.i.ties of normal butyl alcohol, a substance neither bacterium can produce alone. Other observers have brought forward other cases. Thus neither _B. coli_ nor the _B. denitrificans_ of Burri and Stutzer can reduce nitrates, but if acting together they so completely undo the structure of sodium nitrate that the nitrogen pa.s.ses off in the free state.

Van Senus showed that the concurrence of two bacteria is necessary before his _B. amylobacter_ can ferment cellulose, and the case of mud bacteria which evolve sulphuretted hydrogen below which is utilized by sulphur bacteria above has already been quoted, as also that of Winogradsky's _Clostridium [v.03 p.0170] pasteurianum_, which is anaerobic, and can fix nitrogen only if protected from oxygen by aerobic species. It is very probable that numerous symbiotic fermentations in the soil are due to this co-operation of oxygen-protecting species with anaerobic ones, _e.g._ _Teta.n.u.s_.

[Ill.u.s.tration: FIG. 21.--A plate-culture colony of a species of _Bacillus--Proteus_ (Hauser)--on the fifth day. The flame-like processes and outliers are composed of writhing filaments, and the contours are continually changing while the colony moves as a whole. Slightly magnified.

(H. M. W.)]

[Sidenote: Activity of bacteria.]

Astonishment has been frequently expressed at the powerful activities of bacteria--their rapid growth and dissemination, the extensive and profound decompositions and fermentations induced by them, the resistance of their spores to dessication, heat, &c.--but it is worth while to ask how far these properties are really remarkable when all the data for comparison with other organisms are considered. In the first place, the extremely small size and isolation of the vegetative cells place the protoplasmic contents in peculiarly favourable circ.u.mstances for action, and we may safely conclude that, weight for weight and molecule for molecule, the protoplasm of bacteria is brought into contact with the environment at far more points and over a far larger surface than is that of higher organisms, whether--as in plants--it is distributed in thin layers round the sap-vacuoles, or--as in animals--is bathed in fluids brought by special mechanisms to irrigate it. Not only so, the isolation of the cells facilitates the exchange of liquids and gases, the pa.s.sage in of food materials and out of enzymes and products of metabolism, and thus each unit of protoplasm obtains opportunities of immediate action, the results of which are removed with equal rapidity, not attainable in more complex multi-cellular organisms. To put the matter in another way, if we could imagine all the living cells of a large oak or of a horse, having given up the specializations of function impressed on them during evolution and simply carrying out the fundamental functions of nutrition, growth, and multiplication which mark the generalized activities of the bacterial cell, and at the same time rendered as accessible to the environment by isolation and consequent extension of surface, we should doubtless find them exerting changes in the fermentable fluids necessary to their life similar to those exerted by an equal ma.s.s of bacteria, and that in proportion to their approximation in size to the latter. Ciliary movements, which undoubtedly contribute in bringing the surface into contact with larger supplies of oxygen and other fluids in unity of time, are not so rapid or so extensive when compared with other standards than the apparent dimensions of the microscopic field. The microscope magnifies the distance traversed as well as the organism, and although a bacterium which covers 9-10 cm. or more in 15 minutes--say 0.1 mm. or 100 per second--appears to be darting across the field with great velocity, because its own small size--say 5 1 --comes into comparison, it should be borne in mind that if a mouse 2 in.

long only, travelled twenty times its own length, _i.e._ 40 in., in a second, the distance traversed in 15 minutes at that rate, viz. 1000 yards, would not appear excessive. In a similar way we must be careful, in our wonder at the marvellous rapidity of cell-division and growth of bacteria, that we do not exaggerate the significance of the phenomenon. It takes any ordinary rodlet 30-40 minutes to double its length and divide into two equal daughter cells when growth is at its best; nearer the minimum it may require 3-4 hours or even much longer. It is by no means certain that even the higher rate is greater than that exhibited by a tropical bamboo which will grow over a foot a day, or even common gra.s.ses, or asparagus, during the active period of cell-division, though the phenomenon is here complicated by the phase of extension due to intercalation of water. The enormous extension of surface also facilitates the absorption of energy from the environment, and, to take one case only, it is impossible to doubt that some source of radiant energy must be at the disposal of those prototrophic forms which decompose carbonates and a.s.similate carbonic acid in the dark and oxidize nitrogen in dry rocky regions where no organic materials are at their disposal, even could they utilize them. It is usually stated that the carbon dioxide molecule is here split by means of energy derived from the oxidation of nitrogen, but apart from the fact that none of these processes can proceed until the temperature rises to the minimum cardinal point, Engelmann's experiment shows that in the purple bacteria rays are used other than those employed by green plants, and especially ultra-red rays not seen in the spectrum, and we may probably conclude that ”dark rays”--_i.e._ rays not appearing in the visible spectrum--are absorbed and employed by these and other colourless bacteria.

The purple bacteria have thus two sources of energy, one by the oxidation of sulphur and another by the absorption of ”dark rays.” Stoney (_Scient.

Proc. R. Dub. Soc._, 1893, p. 154) has suggested yet another source of energy, in the bombardment of these minute ma.s.ses by the molecules of the environment, the velocity of which is sufficient to drive them well into the organism, and carry energy in of which they can avail themselves.

[Ill.u.s.tration: FIG. 22.--Portions of a colony such as that in fig. 21, highly magnified, showing the kinds of changes brought about in a few minutes, from A to B, and B to C, by the growth and ciliary movements of the filaments. The arrows show the direction of motion. (H. M. W.)]

AUTHORITIES.--General: Fischer, _The Structure and Functions of Bacteria_ (Oxford, 1900, 2nd ed.), German (Jena, 1903); Migula, _System der Bakterien_ (Jena, 1897); and in Engler and Prantl, _Die naturlichen Pflanzenfamilien_, I. Th. 1 Abt. a; Lafar, _Technical Mycology_ (vol. i.

London, 1898); Mace, _Traite pratique de bakteriologie_ (5th ed. 1904).

Fossil bacteria: Renault, ”Recherches sur les Bacteriacees fossiles,” _Ann.

des Sc. Nat._, 1896, p. 275. Bacteria in Water: Frankland and Marshall Ward. ”Reports on the Bacteriology of Water,” _Proc. R. Soc._, vol. li. p.

183, vol. liii. p. 245, vol. lvi. p. 1; Marshall Ward, ”On the Biology of _B. ramosus_,” _Proc. R. Soc._, vol. lviii. p. 1; and papers on Bacteria of the river Thames in _Ann. of Bot._ vol. xii. pp. 59 and 287, and vol. xiii.

p. 197. Cell-membrane, &c.: Butschli, _Weitere Ausfuhrungen uber den Bau der Cyanophyceen und Bakterien_ (Leipzig, 1896); Fischer, _Unters. uber den Bau der Cyanophyceen und Bakterien_ (Jena, 1897); Rowland, ”Observations upon the Structure of Bacteria,” _Trans. Jenner Inst.i.tute_, 2nd ser. 1899, p. 143, with literature. Cilia: Fischer, ”Unters. uber Bakterien,”

_Pringsh. Jahrb._ vol. xxvii.; also the works of Migula and Fischer already cited. Nucleus: Wager in _Ann. Bot._ vol. ix. p. 659; also Migula and Fischer, _l.c._; Vejdovsky, ”uber den Kern der Bakterien und seine Teilung,” _Cent. f. Bakt._ Abt. II. Bd. xi. (1904) p. 481; _ibid._ ”Cytologisches uber die Bakterien der Prager Wa.s.serleitung,” _Cent. f.

Bakt._ Abt. II. Bd. xv. (1905); Mencl, ”Nachtrage zu den Strukturverhaltnissen von Bakterium gammari” in _Archiv f. Protistenkunde_, Bd. viii. (1907), p. 257. Spores, &c.: Marshall Ward, ”On the Biology of _B. ramosus_,” _Proc. R. Soc._, 1895, vol. lviii. p. 1; Sturgis, ”A Soil Bacillus of the type of de Bary's _B. megatherium_,” _Phil. Trans._ [v.03 p.0171] vol. cxci. p. 147; Klein, L., _Ber. d. deutschen bot. Gesellsch._ (1889), Bd. vii.; and _Cent. f. Bakt. und Par._ (1889), Bd. vi.

Cla.s.sification: Marshall Ward, ”On the Characters or Marks employed for cla.s.sifying the Schizomycetes,” _Ann. of Bot._, 1892, vol. vi.; Lehmann and Neumann, _Atlas and Essentials of Bacteriology_; also the works of Migula and Fischer already cited. Myxobacteriaceae: Berkeley, _Introd. to Cryptogamic Botany_ (1857), p. 313; Thaxter, ”A New Order of Schizomycetes,” _Bot. Gaz._ vol. xvii. (1892), p. 389; and ”Further Observations on the Myxobacteriaceae,” _ibid._ vol. xxiii. (1897), p. 395, and ”Notes on the Myxobacteriaceae,” _ibid._ vol. x.x.xvii. (1904), p. 405; Baur, ”Myxobakterienstudien,” _Arch. f. Protistenkunde_, Bd. v. (1904), p.

92; Smith, ”Myxobacteria,” _Jour. of Botany_, 1901, p. 69; Quehl, _Cent. f.

Bakt._ xvi. (1896), p. 9. Growth: Marshall Ward, ”On the Biology of _B.

ramosus_,” _Proc. R. Soc._ vol. lviii. p. 1 (1895). Fermentation, &c.: Warington, _The Chemical Action of some Micro-organisms_ (London, 1888); Winogradsky, ”Recherches sur les organismes de la nitrification,” _Ann. de l'Inst. Past._, 1890, pp. 213, 257, 760, 1891, pp. 92 and 577; ”Sur l'a.s.similation de l'azote gazeux, &c.,” _Compt. Rend._, 12 Feb. 1894; ”Zur Microbiologie des Nitrifikationsprozesses,” _Cent. f. Bakt._ Abt. II. Bd.

ii. (1896), p. 415; ”Ueber Schwefel-Bakterien,” _Bot. Zeitg._, 1887, Nos.

31-37; _Beitr. zur Morph. u. Phys. der Bakterien_, H. 1 (1888); ”Ueber Eisenbakterien,” _Bot. Zeitg._, 1888, p. 261; and Omeliansky, ”Ueber den Einfluss der organischen Substanzen auf die Arbeit der nitrifizierenden Organismen,” _Cent. f. Bakt._ Abt. II. Bd. v. (1896); Schorler, ”Beitr. zur Kenntniss der Eisenbakterien,” _Cent. f. Bakt._ Abt. II. Bd. xii. (1904), p. 681; Marshall Ward, ”On the Tubercular Swellings on the Roots of Vicia Faba,” _Phil. Trans._, 1877, p. 539; h.e.l.lriegel and Wilfarth, ”Unters. uber die Stickstoffnahrung der Gramineen u. Leguminosen,” _Beit. Zeit. d.