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 vol. cxci. p. 147; Klein, L., ''Ber. d. deutschen bot. Gesellsch.'' (1889), Bd. vii.; and ''Cent. f. Bakt. und Par''. (1889), Bd. vi. Classification: Marshall Ward, “On the Characters or Marks employed for classifying 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. xxxvii. (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’assimilation 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; Hellriegel and Wilfarth, “Unters. über die Stickstoffnahrung der Gramineen u. Leguminosen,” ''Beit. Zeit. d. Vereins für die Rübenzuckerindustrie (Berlin, 1888); Nobbe and Hiltner, Landw. Versuchsstationen'' (1899), Bd. 51, p. 241, and Bd. 52, p. 455; Mazé, Annales de l’Institut Pasteur, t. II, p. 44, and t. 12, p. 1 (1897); Prazmowski, ''Land. Versuchsstationen'', Bd. 37 (1890), p. 161, Bd. 38 (1891), p. 5; Frank, ''Landw. Jahrb.'' Bd. 17 (1888), p. 441; Omelianski, “Sur la fermentation de la cellulose,” ''Compt. Rend., 4 Nov. 1895; van Senus, Beitr. zur Kenntn. der Cellulosegährung (Leiden, 1890); van Tieghem, “Sur la fermentation de la cellulose,” Bull. de la soc. bot. de Fr.'' t. xxvi. (1879), p. 28; Beyerinck “Ueber Spirillum desulphuricans, &c.,” ''Cent. f. Bakt.'' Abt. II. Bd. i. (1895), p. 1; Molisch, Die Pflanze in ihren Beziehungen zum Eisen (Jena, 1892). Pigment Bacteria: Ewart, “On the Evolution of Oxygen from Coloured Bacteria,” ''Linn. Journ.'', 1897, vol. xxxiii. p. 123; Molisch, Die Purpurbakterien (Jena, 1907). Oxydases and Enzymes: Green, The Soluble Ferments and Fermentation (Cambridge, 1899). Action of Light, &c.: Marshall Ward, “The Action of Light on Bacteria,” ''Phil. Trans.'', 1893, p. 961, and literature. Resistance to Cold, &c.: Ravenel, ''Med. News, 1899, vol. lxxiv.; Macfadyen and Rowland, Proc. R. Soc.'' vol. lxvi. pp. 180, 339, and 488; Farmer, “Observations on the Effect of Desiccation of Albumin upon its Coagulability,” ibid. p. 329. Pathogenic Bacteria: Baumgarten, Pathologische Mykologie (1890); Kolle and Wassermann, Handbuch der pathogenen Mikroorganismen (1902–1904); and numerous special works in medical literature. Immunity: Ehrlich, “On Immunity with Special Reference to Cell-life,” ''Proc. R. Soc.'' vol. lxvi. p. 424; Calcar, “Die Fortschritte der Immunitäts- und Spezifizetätslehre seit 1870,” Progressus Rei Botanicae, Bd. I. Heft 3 (1907). Bacteriosis: Migula, l.c. p. 322, has collected the literature; see also Sorauer, Handbuch der Pflanzenkrankheiten, I. (1905), pp. 18-93, for later literature. Symbiosis: Marshall Ward, “Symbiosis,” ''Ann. of Bot.'' vol. xiii. p. 549, and literature.

The action of bacteria as pathogenic agents is in great part merely an instance of their general action as producers of chemical change, yet bacteriology as a whole has become so extensive, and has so important a bearing on subjects widely different from one another, that division of it has become essential. The science will accordingly be treated in this section from the pathological standpoint only. It will be considered under the three following heads, viz. (1) the methods employed in the study; (2) the modes of action of bacteria and the effects produced by them; and (3) the facts and theories with regard to immunity against bacterial disease.

The demonstration by Pasteur that definite diseases could be produced by bacteria, proved a great stimulus to research in the etiology of infective conditions, and the result was a rapid advance in human knowledge. An all-important factor in this remarkable progress was the introduction by Koch of solid culture media, of the “plate-method,” &c., an account of which he published in 1881. By means of these the modes of cultivation, and especially of separation, of bacteria were greatly simplified. Various modifications have since been made, but the routine methods in bacteriological procedure still employed are in great part those given by Koch. By 1876 the anthrax bacillus had been obtained in pure culture by Koch, and some other pathogenic bacteria had been observed in the tissues, but it was in the decade 1880–1890 that the most important discoveries were made in this field. Thus the organisms of suppuration, tubercle, glanders, diphtheria, typhoid fever, cholera, tetanus, and others were identified, and their relationship to the individual diseases established. In the last decade of the 19th century the chief discoveries were of the bacillus of influenza (1892), of the bacillus of plague (1894) and of the bacillus of dysentery (1898). Immunity against diseases caused by bacteria has been the subject of systematic research from 1880 onwards. In producing active immunity by the attenuated virus, Duguid and J. S. Burdon-Sanderson and W. S. Greenfield in Great Britain, and Pasteur, Toussaint and Chauveau in France, were pioneers. The work of Metchnikoff, dating from about 1884, has proved of high importance, his theory of phagocytosis (vide infra) having given a great stimulus to research, and having also contributed to important advances. The modes by which bacteria produce their effects also became a subject of study, and attention was naturally turned to their toxic products. The earlier work, notably that of L. Brieger, chiefly concerned ptomaines (vide infra), but no great advance resulted. A new field of inquiry was, however, opened up when, by filtration a bacterium-free toxic fluid was obtained which produced the important symptoms of the disease—in the case of diphtheria by P. P. E. Roux and A. Yersin (1888), and in the case of tetanus a little later by various observers. Research was thus directed towards ascertaining the nature of the toxic bodies in such a fluid, and Brieger and Fraenkel (1890) found that they were proteids, to which they gave the name “toxalbumins.” Though subsequent researches have on the whole confirmed these results, it is still a matter of dispute whether these proteids are the true toxins or merely contain the toxic bodies precipitated along with them. In the United Kingdom the work of Sidney Martin, in the separation of toxic substances from the bodies of those who have died from certain diseases, is also worthy of mention. Immunity against toxins also became a subject of investigation, and the result was the discovery of the antitoxic action of the serum of animals immunized against tetanus toxin by E. Behring and Kitazato (1890), and by Tizzoni and Cattani. A similar result was also obtained in the case of diphtheria. The facts with regard to passive immunity were thus established and were put to practical application by the introduction of diphtheria antitoxin as a therapeutic agent in 1894. The technique of serum preparation has become since that time greatly elaborated and improved, the work of P. Ehrlich in this respect being specially noteworthy. The laws of passive immunity were shown to hold also in the case of immunity against living organisms by R. Pfeiffer (1894), and various anti-bacterial sera have been introduced. Of these the anti-streptococcic serum of A. Marmorek (1895) is one of the best known. The principles of protective inoculation have been developed and practically applied on a large scale, notably by W. M. W. Haffkine in the case of cholera (1893) and plague (1896), and more recently by Wright and Semple in the case of typhoid fever. One other discovery of great importance may be mentioned, viz. the agglutinative action of the serum of a patient suffering from a bacterial disease, first described in the case of typhoid fever independently by Widal and by Grünbaum in 1896, though led up to by the work of Pfeiffer, Gruber and Durham and others. Thus a new aid was added to medical science, viz. serum diagnosis of disease. The last decade of the 19th century will stand out in the history of medical science as the period in which serum therapeutics and serum diagnosis had their birth.

In recent years the relations of toxin and antitoxin, still obscure, have been the subject of much study and controversy. It was formerly supposed that the injection of attenuated cultures or dead organisms—vaccines in the widest sense—was only of service in producing immunity as a preventive measure against the corresponding organism, but the work of