Sur

vol exci p. 147: Klein, L., Ber. d. deutschen bot. Gesellsch. (1889), | have since been made, but the routine methods in bacteriological Bd vi; and Cent. f. Bukt und Par. (1889), Bd. vi. Classification: procedure still employed are in great part those given by Koch. Marshall Ward, "On the Characters or Marks employed for classify. By 1876 the anthrax bacillus had been obtained in pure culture Ann. of Bot., 1892, vol. vi.; Lehmann ing the Schizomycetes," and Neumann, Atlas and Essentials of Bacteriology; also the works by Koch, and some other pathogenic bacteria had been observed of Migula and Fischer already cited. Myxobacteriaceae: Berkeley, in the tissues, but it was in the decade 1880-1890 that the most important discoveries were made in this field. Thus the Introd to Cryptogamic Botany (1857), p. 313; Thaxter, "A New organisms of suppuration, tubercle, glanders, diphtheria, typhoid 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. fever, cholera, tetanus, and others were identified, and their (1904), p. 405: Baur, "Myxobakterienstudien," Arch. f. Protisten- relationship to the individual diseases established. In the last Myxobacteria," Jour. of decade of the 19th century the chief discoveries were of the kunde, Bd. v. (1904), p. 92; Smith, bacillus of influenza (1892), of the bacillus of plague (1894) and Balary, 1901, p. 69; Quehl, Cent. f. Bakt. xvi. (1896), p. 9. Growth: Marshall Ward, "On the Biology of B. ramosus," Proc. R. Soc. vol. ii. p. 1 (1895). Fermentation, &c.: Warington, The Chemical of the bacillus of dysentery (1898). Immunity against diseases Action of some Micro-organisms (London, 1888); Winogradsky, caused by bacteria has been the subject of systematic research *Recherches sur les organismes de la nitrification," Ann. de l'Inst. from 1880 onwards. In producing active immunity by the Past, 1890, pp. 213, 257, 760, 1891, pp. 92 and 577: W. S. Greenfield in Great Britain, and Pasteur, Toussaint and l'assimilation de l'azote gazeux, &c.," Compt. Rend., 12 Feb. 1894; attenuated virus, Duguid and J. S. Burdon-Sanderson and Cent. f. Bakt. "Zur Microbiologie des Nitrifikationsprozesses," Chauveau in France, were pioneers. The work of Metchnikoff, Abt. 11. Bd. ii. (1896), p. 415; " Ueber Schwefel-Bakterien," Bot. Dug., 1887. Nos. 31-37: Beitr. zur Morph. u. Phys. der Bakterien, dating from about 1884, has proved of high importance, his Hi (1888); "Ueber Eisenbakterien," Bot. Zeitg., 1888, p. 261; Ueber den Einfluss der organischen Substanzen theory of phagocytosis (vide infra) having given a great stimulus to research, and having also contributed to important advances. and Omeliansky, auf die Arbeit der nitrifizierenden Organismen," Cent. f. Bakt. Abt. The modes by which bacteria produce their effects also became II. Bd. v. (1896); Schorler," Beitr. zur Kenntniss der Eisenbaka subject of study, and attention was naturally turned to their terien," Cent. f. Bakt. Abt. II. Bd. xii. (1904), p. 681; Marshall toxic products. The earlier work, notably that of L. Brieger, Ward, On the Tubercular Swellings on the Roots of Vicia Faba,' "Unters. über Phil. Trans., 1877. p. 539; Hellriegel and Wilfarth, die Stickstoff nahrung der Gramineen u. Leguminosen," Beit. Zeit. d. chiefly concerned ptomaines (vide infra), but no great advance Vereins für die Rubenzuckerindustrie (Berlin, 1888); Nobbe and resulted. A new field of inquiry was, however, opened up when, Hiltner, Landw. Versuchsstationen (1899). Bd. 51. p. 241, and Bd. 52, by filtration a bacterium-free toxic fluid was obtained which 455; Mazé, Annales de l'Institut Pasteur, t. 11, p. 44, and t. 12, of diphtheria by P. P. E. Roux and A. Yersin (1888), and in the 1 (1897); Prazmowski, Land. Versuchsstationen, Bd. 37 (1890), produced the important symptoms of the disease-in the case case of tetanus a little later by various observers. Research P 161, Ed. 38 (1891), p. 5; Frank, Landw. Jahrb. Bd. 17 (1888), e 441: Omelianski, "Sur la fermentation de la cellulose," Compt. was thus directed towards ascertaining the nature of the toxic Rend., 4 Nov. 1895; van Senus, Beitr. zur Kenntn. der Cellulose"Sur la fermentation de la bodies in such a fluid, and Brieger and Fraenkel (1890) found that ahrung (Leiden, 1890); van Tieghem, they were proteids, to which they gave the name "toxalbumins." cellulose," Bull. de la soc. bot. de Fr. t.xxvi. (1879), p. 28; Beyerinck "Leber Spirillum desulphuricans, &c.," Cent. f. Bakt. Abt. II. Bd. i. (1895), p. 1; Molisch, Die Pflanze in ihren Beziehungen zum Eisen Though subsequent researches have on the whole confirmed these results, it is still a matter of dispute whether these proteids Jena, 1892). Pigment Bacteria; Ewart, "On the Evolution of are the true toxins or merely contain the toxic bodies precipitated Oxygen from Coloured Bacteria," Linn. Journ., 1897. vol. xxxiii. p123; Molisch, Die Purpurbakterien (Jena, 1907). Oxydases and Enzymes: Green, The Soluble Ferments and Fermentation (Cam-along with them. In the United Kingdom the work of Sidney bridge, 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. Ixxiv.; Macfadyen and Rowland, Proc. R. Soc. vol. Ixvi. 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. Die Fortschritte der ImmunitätsR. Soc. vol. Ixvi. p. 424; Calcar, und Spezifizetätslehre seit 1870." Progressus Rei Botanicae, Bd. I Heft 3(1907). Bacteriosis: Migula, 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. (H. M. W.; V. H. B.)

Methods of study.

Sir Almroth Wright has shown that the use of such vaccines | tone ") to make up for proteids lost by coagulation in the may be of service even after infection has occurred, especially preparation. The reaction of the media must in cvery case be when the resulting disease is localized. In this case a general carefully attended to, a neutral or slightly alkaline reaction reaction is stimulated by the vaccine which may aid in the being, as a rule, most suitable; for delicate work it may be destruction of the invading organisms. In regulating the necessary to standardize the reaction by titration methods. administration of such vaccines he has introduced the method The media from the store-flasks are placed in glass test-tubes of observing the opsonic index, to which reference is made or small flasks, protected from contamination by cotton wool below. of the discoveries of new organisms the most important plugs, and are sterilized by heat. For most purposes the solid is that of the Spirochaete pallida in syphilis by Schaudinn and media are to be preferred, since bacterial growth appears as a Hoffmann in 1905; and although proof that it is the cause of discrete mass and accidental contamination can be readily the disease is not absolute, the facts that have been established recognized. Cultures are made by transferring by means of a constitute very strong presumptive evidence in favour of this sterile platinum wire a little of the material containing the being the case. It may be noted, however, that it is still bacteria to the medium. The tubes, after being thus inoculated, doubtful whether this organism is to be placed amongst the are kept at suitable temperatures, usually either at 37° C., the bacteria or amongst the protozoa.

temperature of the body, or at about 20° C., a warm summer The methods employed in studying the relation of bacteria temperature, until growth appears. For maintaining a constant to disease are in principle comparatively simple, but considerable temperature incubators with regulating apparatus are used.

experience and great care are necessary in applying Subsequent cultures or, as they are called, "subcultures," them and in interpreting results. In any given disease may be made by inoculating fresh tubes, and in this way growth

there are three chief steps, viz. (1) the discovery of a may be maintained often for an indefinite period. The simplest bacterium in the affected tissues by means of the microscope; case is that in which only one variety of bacterium is present, (2) the obtaining of the bacterium in pure culture; and (3) the and a “pure culture" may then be obtained at once. When, production of the discase by inoculation with a pure culture. however, several species are present together, means must be By means of microscopic examination more than one organism adopted for separating them. For this purpose various methods may sometimes be observed in the tissues, but one single organism have been devised, the most important being the plate-method by its constant presence and special relations to the tissue of Koch. In this method the bacteria are distributed in a changes can usually be selected as the probable cause of the gelatine or agar medium liquefied by heat, and the medium is disease, and attempts towards its cultivation can then be made. then poured out on sterile glass plates or in shallow glass dishes, Such microscopic examination requires the use of the finest and allowed to solidify. Each bacterium capable of growth lenses and the application of various staining methods. In these gives rise to a colony visible to the naked eye, and if the colonies latter the basic aniline dyes in solution are almost exclusively are sufficiently apart, an inoculation can be made from any one used, on account of their special affinity for the bacterial proto-to a tube of culture-medium and a pure culture obtained. Of plasm. The methods vary much in detail, though in each case course, in applying the method means must be adopted for the endeavour is to colour the bacteria as deeply, and the tissues suitably diluting the bacterial mixture. Another important as faintly, as possible. Sometimes a simple watery solution of method consists in inoculating an animal with some fluid conthe dye is sufficient, but very often the best result is obtained by taining the various bacteria. A pathogenic bacterium present increasing the staining power, e.g. by addition of weak alkali, may invade the body, and may be obtained in pure culture application of heat, &c., and by using some substance which from the internal organs. This method applies especially to acts as a mordant and tends to fix the stain to the bacteria. pathogenic bacteria whose growth on culture media is slow, e.g. Excess of stain is afterwards removed from the tissues by the the tubercle bacillus. use of decolorizing agents, such as acids of varying strength The full description of a particular bacterium implies an and concentration, alcohol, &c. Different bacteria behave very account not only of its microscopical characters, but also of differently to stains; some take them up rapidly, others slowly, its growth characters in various culture media, its biological some resist decolorization, others are easily decolorized. In properties, and the effects produced in animals by inoculation. some instances the stain can be entirely removed from the To demonstrate readily its action on various substances, cortissues, leaving the bacteria alone coloured, and the tissues cantain media have been devised. For example, various sugars, then be stained by another colour. This is the case in the lactose, glucose, saccharose, &c.—are added to test the fermenmethods for staining the tubercle bacillus and also in Gram's tative action of the bacterium on these substances; litmus is method, the essential point in which latter is the treatment with added to show changes in reaction, specially standardized media a solution of iodine before decolorizing. In Gram's method, being used for estimating such changes; peptone solution is however, only some bacteria retain the stain, while others lose it. commonly employed for testing whether or not the bacterium The tissues and fluids are treated by various histological methods, forms indol; sterilized milk is used as a culture medium to but, to speak generally, examination is made either in films determine whether or not it is curdled by the growth. Somesmeared on thin cover-glasses and allowed to dry, or in thin limes a bacterium can be readily recognized from one or two sections cut by the microtome after suitable fixation and harden-characters, but not infrequently a whole series of tests must be ing of the tissue. In the case of any bacterium discovered, made before the species is determined. As our knowledge has observation must be made in a long series of instances in order advanced it has become abundantly evident that the so called to determine its invariable presence.

pathogenic bacteria are not organisms with special features, In cultivating bacteria outside the body various media to but that each is a member of a group of organisms possessing serve as food material must be prepared and sterilized by heat. closely allied characters. From the point of view of evolution Cultiva

The general principle in their preparation is to supply we may suppose that certain races of a group of bacteria have the nutriment for bacterial growth in a form as nearly gradually acquired the power of invading the tissues of the

similar as possible to that of the natural habitat of the body and producing disease. In the acquisition of pathogenic organisms in the case of pathogenic bacteria, the natural fluids properties some of their original characters have become changed, of the body. The media are used either in a fluid or solid condi- but in many instances this has taken place only lo a slight tion, the latter being obtained by a process of coagulation, or degree, and, furthermore, some of these changes are not of a by the addition of a gelatinizing agent, and are placed in glass permanent character. It is to be noted that in the case of tubes or flasks plugged with cotton-wool. To mention examples, bacteria we can only judge of organisms being of different blood serum solidified at a suitable temperature is a highly species by the stability of the characters which distinguish suitable medium, and various media are made with extract of them, and numerous examples might be given where their meat as a basis, with the addition of gelatine or agar as solidify- characters become modified by comparatively slight change in ing agents and of non-coagulable proteids (commercial "pep-I their environment. The cultural as well as the microscopical

tion.

characters of a pathogenic organism may be closely similar to | to obtain them in an absolutely pure condition have, however, other non-pathogenic members of the same group, and it thus failed in important cases. So that when a "toxin is spoken comes to be a matter of extreme difficulty in certain cases to of, a mixture with other organic substances is usually implied. state what criterion should be used in differentiating varieties. Or the toxin may be precipitated with other organic substances, The tests which are applied for this purpose at present are chiefly purified to a certain extent by re-solution, re-precipitation, &c., and of two kinds. In the first place, such organisms may be differ- desiccated. A "dry toxin" is thus obtained, though still in an entiated by the chemical change produced by them in various impure condition. Toxic substances have also been separated by culture media, c.g. by their fermentative action on various corresponding methods from the bodies of those who have died sugars, &c., though in this case such properties may become of certain diseases, and the action of such substances on animals modified in the course of time. And in the second place, the is in some cases an important point in the pathology of the various serum reactions to be described below have been called disease. Another auxiliary method has been applied in this into requisition. It may be stated that the introduction of a department, viz. the separation of organic substances by particular bacterium into the tissues of the body leads to certain filtration under high pressure through a colloid membrane, properties appearing in the serum, which are chiefly exerted gelatine supported in the pores of a porcelain filter being usually towards this particular bacterium. Such a serum may accord- employed. It has been found, for example, that a toxin may ingly within certain limits be used for differentiating this organism pass through such a filter while an antitoxin may not. The from others closely allied to it (vide infra). methods of producing immunity are dealt with below.

The modes of cultivation described apply only to organisms which grow in presence of oxygen. Some, however the strictly anaerobic bacteria-grow only in the absence of oxygen; hence means must be adopted for excluding this gas. It is found that if the inoculation be made deep down in a solid medium, growth of an anaerobic organism will take place, especially if the medium contains some reducing agent such as glucose. Such cultures are called "deep cultures." To obtain growth of an anaerobic organism on the surface of a medium, in using the plate method, and also for cultures in fluids, the air is displaced by an indifferent gas, usually hydrogen.

Inocula tion.

In testing the effects of bacteria by inoculation the smaller rodents, rabbits, guinea-pigs, and mice, are usually employed. One great drawback in certain cases is that such animals are not susceptible to a given bacterium, or that the disease is different in character from that in the human subject. In some cases, e.g. Maltafever and relapsing fever, monkeys have been used with success, but in others, e.g. leprosy, none of the lower animals has been found to be susceptible. Discretion must therefore be exercised in interpreting negative results in the lower animals. For purposes of inoculation young vigorous cultures must be used. The bacteria are mixed with some indifferent fluid, or a fluid culture is employed. The injections are made by means of a hypodermic syringe into the subcutaneous tissue, into a vein, into one of the serous sacs, or more rarely into some special part of the body. The animal, after injection, must be kept in favourable surroundings, and any resulting symptoms noted. It may die, or may be killed at any time desired, and then a post-mortem examination is made, the conditions of the organs, &c., being observed and noted. The various tissues affected are examined microscopically and cultures made from them; in this way the structural changes and the relation of bacteria to them can be determined.

The fact that in anthrax, one of the first diseases to be fully studied, numerous bacilli are present in the blood of infected animals, gave origin to the idea that the organisms Bacteria might produce their effect by using up the oxygen as agents of the blood. Such action is now known to be quite a of disease. subsidiary matter. And although effects may sometimes be produced in a mechanical manner by bacteria plugging capillaries of important organs, e.g. brain and kidneys, it may now be stated as an accepted fact that all the important results of bacteria in the tissues are due to poisonous bodies or toxins formed by them. Here, just as in the general subject of fermentation, we must inquire whether the bacteria form the substances in question directly or by means of non-living ferments or enzymes. With regard to toxin formation the following general statements may be made. In certain instances, e.g. in the case of the tetanus and diphtheria bacilli, the production of soluble toxins can be readily demonstrated by filtering a culture in bouillon germ-free by means of a porcelain filter, and then injecting some of the filtrate into an animal. In this way the characteristic features of the disease can be reproduced. Such toxins being set free in the culture medium are often known as extracellular. In many cases, however, the filtrate, when injected, produces comparatively little effect, whilst toxic action is observed when the bacteria in a dead condition are used; this is the case with the organisms of tubercle, cholera, typhoid and many others. The toxins are here manifestly contained within the bodies of the bacteria, i.e. are intracellular, though they may become free on disintegration of the bacteria. The action of these intracellular toxins has in many instances nothing characteristic, but is merely in the direction of producing fever and interfering with the vital processes of the body generally, these disturbances often going on to a fatal result. In other words, the toxins of different Though the causal relationship of a bacterium to a disease bacteria are closely similar in their results on the body and the may be completely established by the methods given, another features of the corresponding diseases are largely regulated by very important part of bacteriology is concerned with the poisons the vital properties of the bacteria, their distribution in the of toxins formed by bacteria. These toxins may become free tissues, &c. The distinction between the two varieties of toxins, in the culture fluid, and the living bacteria may then be got rid though convenient. must not be pushed too far, as we know of by filtering the fluid through a filter of unglazed porcelain, little regarding their mode of formation. Although the formation whose pores are sufficiently small to retain them. The passage of toxins with characteristic action can be shown by the above of the fluid is readily effected by negative pressure methods, yet in some cases little or no toxic action can be demonSeparation of toxins. produced by an ordinary water exhaust-pump. The strated. This, for example, is the case with the anthrax effects of the filtrate are then tested by the methods bacillus; although the effect of this organism in the living body used in pharmacology. In other instances the toxins are re-indicates the production of toxins which diffuse for a distance tained to a large extent within the bacteria, and in this case the dead bacteria are injected as a suspension in fluid. Methods have been introduced for the purpose of breaking up the bodies of bacteria and setting free the intracellular toxins. For this purpose Koch ground up tubercle bacilli in an agate mortar and treated them with distilled water until practically no deposit remained. Rowland and Macfadyen for the same purpose introduced the method of grinding the bacilli in liquid air. At this temperature the bacterial bodies are extremely brittle, and are thus readily broken up. The study of the nature of toxins requires, of course, the various methods of organic chemistry. Attempts

around the bacteria. This and similar facts have suggested that some toxins are only produced in the living body. A considerable amount of work has been done in connexion with this subject, and many observers have found that fluids taken from the living body in which the organisms have been growing, contain toxic substances, to which the name of aggressins has been applied. Fluid containing these aggressins greatly increases the toxic effect of the corresponding bacteria, and may produce death at an earlier stage than ever occurs with the bacteria alone. They also appear to have in certain cases a paralysing action on the cells which act as phagocytes. The

Nature of toxios.

work on this subject is highly suggestive, and opens up new | diphtheria Sidney Martin obtained toxic albumoses in the spleen, possibilities with regard to the investigation of bacterial action which he considered were due to the digestive action of an enzyme within the body. Not only are the general symptoms of poisoning formed by the bacillus in the membrane and absorbed into the in bacterial disease due to toxic substances, but also the tissue circulation. According to this view, then, a part at least of the changes, many of them of inflammatory nature, in the neighbour-directly toxic substance is produced in the living body by enzymes hood of the bacteria. Thus, to mention examples, diphtheria present in the so-called toxin obiained from the bacterial culture. toxin produces inflammatory oedema which may be followed Recent researches go to show that enzymes play a greater part by necrosis; dead tubercle bacilli give rise to a tubercle-like in fermentation by living ferments than was formerly supposed, nodule, &c. Furthermore, a bacillus may give rise to more than and by analogy it is likely that they are also concerned in the one toxic body, either as stages in one process of change or as processes of disease. But this has not been proved, and hitherto distinct products. Thus paralysis following diphtheria is in all no enzyme has been separated from a pathogenic bacterium capprobability due to a different toxin from that which causes the ablc of forming, by digestive or other action, the toxic bodies from acute symptoms of poisoning or possibly to a modification of it proteids outside the body. It is also to be noted that, as in the sometimes formed in specially large amount. It is interesting case of poisons of known constitution, each toxin has a minimum to note that in the case of the closely analogous example of snake lethal dose which is proportionate to the weight of the animal venoms, there may be separated from a single venom a number and which can be ascertained with a fair degree of accuracy. of toxic bodies which have a selective action on different animal

The action of toxins is little understood. It consists in all tissues.

probability of disturbance, by means of the chemical affinities Regarding the chemical nature of toxins less is known than of the toxin, of the highly complicated molecules of living cells. regarding their physiological action. Though an enormous This disturbance results in disintegration to a varying degree,

amount of work has been done on the subject, no and may produce changes visible on microscopic examination. important bacterial toxin has as yet been obtained In other cases such changes cannot be detected, and the only

in a pure condition, and, though many of them are evidence of their occurrence may be the associated symptoms. probably of proteid nature, even this cannot be asserted with The very important work of Ehrlich on diphtheria toxin shows absolute certainty. Brieger, in his earlier work, found that that in the molecule of toxin there are at least two chief atom alkaloids were formed by bacteria in a variety of conditions, and groups--one, the "haptophorous," by which the toxin molecule that some of them were poisonous. These alkaloids he called is attached to the cell protoplasm; and the other the " toxo. plomaines. The methods used in the investigations were, however, phorous," which has a ferment-like action on the living molecule, open to objection, and it is now recognized that although organic producing a disturbance which results in the toxic symptoms. bases may sometimes be formed, and may be toxic, the important On this theory, susceptibility to a toxin will imply both a chemical toxins are not of that nature. A later research by Brieger along affinity of certain-tissues for the toxin molecule and also sensitivewith Fraenkel pointed to the extracellular toxins of diphtheria, ness to its actions; and, furthermore, non-susceptibility may tetanus and other diseases being of proteid nature, and various result from the absence of either of these two properties. other observers have arrived at a like conclusion. The general A bacterial infection when analysed is seen to be of the nature result of such research has been to show that the toxic bodies are, of an intoxication. There is, however, another all-important like proteids, precipitable by alcohol and various salts; they are factor concerned, viz. the multiplication of the living soluble in water, are somewhat casily dialysable, and are rela- organisms in the tissues; this is essential to, and tively unstable both to light and heat. Attempts to get a pure regulates, the supply of toxins. It is important that toxin by repeated precipitation and solution have resulted in the these two essential factors should be kept clearly in view, since production of a whitish amorphous powder with highly toxic the means of defence against any disease may depend upon the properties. Such a powder gives a proteid reaction, and is no power either of neutralizing toxins or of killing the organisms doubt largely composed of albumoses, hence the name fox-producing them. It is to be noted that there is no fixed relation albumoses has been applied. The question has, however, been between toxin production and bacterial multiplication in the raised whether the toxin is really itself a proteid, or whether it is body, some of the organisms most active as toxin producers not merely carried down with the precipitate. Brieger and Boer, having comparatively little power of invading the tissues. by precipitation with certain salts, notably of zinc, obtained a We shall now consider how bacteria may behave when they body which was toxic but gave no reaction of any form of proteid. have gained entrance to the body, what effects may be produced, There is of course the possibility in this case that the toxin was a and what circumstances may modify the disease in any proteid, but was in so small amount that it escaped detection particular case. The extreme instance of bacterial duction et These facts show the great difficulty of the problem, which is invasion is found in some of the septicaemias in the disease. probably insoluble by present methods of analysis, the only lower animals, e.g. anthrax septicaemia in guinea-pigs, test, in fact, for the existence of a toxin is its physiological effect. pneumococcus septicaemia in rabbits. In such diseases the It may also be mentioned that many toxins have now been bacteria, when introduced into the subcutaneous tissue, rapidly obtained by growing the particular organism in a proteid-free gain entrance to the blood stream and multiply freely in it, and medium, a fact which shows that if the toxin is a proteid it may by means of their toxins cause symptoms of general poisoning. be formed synthetically by the bacterium as well as by modifica- A widespread toxic action is indicated by the lesions found tion of proteid already present. With regard to the nature of cloudy swelling, which may be followed by fatty degeneration, intracellular toxins, there is even greater difficulty in the investi in internal organs, capillary haemorrhages, &c. In septicaemia gation and still less is known. Many of them, probably also of in the human subject, often due to streptococci, the process is proteid nature, are much more resistant to heat; thus the intra- similar, but the organisms are found especially in the capillaries cellular toxins of the tubercle bacillus retain certain of their of the internal organs and may not be detectable in the peripheral effects even after exposure to 100° C. Like the extracellular circulation during life. In another class of discases, the organisms toxins they may be of remarkable potency; for example, fever first produce some well-marked local lesion, from which secondary is produced in the human subject by the injection into the extension takes place by the lymph or blood stream to other parts blood of an extremely minute quantity of dead typhoid bacilli. of the body, where corresponding lesions are formed. In this way

We cannot as yet speak definitely with regard to the part secondary abscesses, secondary tubercle glanders and nodules, played by enzymes in these toxic processes. Certain toxins &c., result; in typhoid fever there is secondary invasion of the Enzymes. resemble enzymes as regards their conditions of pre-mesenteric glands, and clumps of bacilli are also found in internal

cipitation and relative instability, and the fact that in organs, especially the spleen, though there may be little tissue most cases a considerable period intervenes between the time of change around them. In all such cases there is seen a selective injection and the occurrence of symptoms has been adduced in character in the distribution of the lesions, some organs being in support of the view that enzymes are present. In the case of any disease much more liable to infection than others. In still

Bacterial lofectioa.

The pro

Tissue

another class of diseases the bacteria are restricted to some par- | believed to be the causes of disease are now recognized as playing ticular part of the body, and the symptoms are due to toxins their part in predisposing to the action of the true causal agent, which are absorbed from it. Thus in cholera the bacteria are viz. the bacterium. In health the blood and internal tissues practically confined to the intestine, in diphtheria to the region are bacterium-free; after death they offer a most suitable of the false membrane, in tetanus to some wound. In the last- pabulum for various bacteria; but between these two extremes mentioned disease even the local multiplication depends upon the lie states of varying liability to infection. It is also probable presence of other bacteria, as the tetanus bacillus has practically that in a state of health organisms do gain entrance to the blood no power of multiplying in the healthy tissues when introduced from time to time and are rapidly killed off. The circumstances alone. which alter the virulence of bacteria will be referred to again in The effects produced by bacteria may be considered under connexion with immunity, but it may be stated here that, as the following heads: (1) tissue changes produced in the vicinity a general rule, the virulence of an organism towards an animal of the bacteria, either at the primary or secondary is increased by sojourn in the tissues of that animal. The increase changes. foci; (2) tissue changes produced at a distance by of virulence becomes especially marked when the organism is absorption of their toxins; (3) symptoms. The inoculated from animal to animal in series, the method of changes in the vicinity of bacteria are to be regarded partly passage. This is chiefly to be regarded as an adaptation to as the direct result of the action of toxins on living cells, and surroundings, though the fact that the less virulent members partly as indicating a reaction on the part of the tissues. (Many of the bacterial species will be liable to be killed off also plays such changes are usually grouped together under the heading a part. Conversely, the virulence tends to diminish on cultivation of "inflammation" of varying degree-acute, subacute and on artificial media outside the body, especially in circumstances chronic.) Degeneration and death of cells, haemorrhages, serous little favourable to growth. and fibrinous exudations, leucocyte emigration, proliferation of connective tissue and other cells, may be mentioned as some of the fundamental changes. Acute inflammation of various types, suppuration, granulation-tissue formation, &c., represent some of the complex resulting processes. The changes produced at a distance by distribution of toxins may be very manifoldcloudy swelling and fatty degeneration, serous effusions, capillary haemorrhages, various degenerations of muscle, hyaline degenera-in nature. That an animal possesses natural immunity can only tion of small blood-vessels, and, in certain chronic diseases, waxy degeneration, all of which may be widespread, are examples of the effects of toxins, rapid or slow in action. Again, in certain cases the toxin has a special affinity for certain tissues. Thus in diphtheria changes in both nerve cells and nerve fibres have been found, and in tetanus minute alterations in the nucleus and protoplasm of nerve cells.

By immunity is meant non-susceptibility to a given disease, or to experimental inoculation with a given bacterium or toxin. The term must be used in a relative: sense, and account Immunity. must always be taken of the conditions present. An animal may be readily susceptible to a disease on experimental inoculation, and yet rarely or never suffer from it naturally, because the necessary conditions of infection are not supplied be shown on exposing it to such conditions, this being usually most satisfactorily done in direct experiment. Further, there are various degrees of immunity, and in this connexion conditions of local or general diminished vitality play an important part in increasing the susceptibility. Animals naturally susceptible may acquire immunity, on the one hand by successfully passing through an attack of the disease, or, on the other hand, by various methods of inoculation. Two chief varieties of artificial immunity are now generally recognized, differing chiefly accorda reaction or series of reactions is produced in the body of the animal, usually by injections of bacteria or their products. The second-passive immunity-is produced by the transference of a quantity of the serum of an animal actively immunized to a fresh animal; the term is applied because there is brought into play no active change in the tissues of the second animal. The methods of active immunity have been practically applied in preventive inoculation against disease; those of passive immunity have given us serum therapeutics. The chief facts with regard to each may now be stated.

The lesions mentioned are in many instances necessarily accompanied by functional disturbances or clinical symptoms, Symptoms. varying according to site, and to the nature and degreeing to the mode of production. In the first-active immunity

of the affection. In addition, however, there occur in bacterial diseases symptoms to which the correlated structural changes have not yet been demonstrated. Amongst these the most important is fever with increased protein metabolism, attended with disturbances of the circulatory and respiratory systems. Nervous symptoms, somnolence, coma, spasms, convulsions and paralysis are of common occurrence. All such phenomena, however, are likewise due to the disturbance of the molecular constitution of living cells. Alterations in metabolism are found to be associated with some of these, but with others no corresponding physical change can be demonstrated. The action of toxins on various glands, producing diminished or increased functional activity, has a close analogy to that of certain drugs. In short, if we place aside the outstanding exception of tumour growth, we may say that practically all the important phenomena met with in disease may be experimentally produced by the injection of bacteria or of their toxins.

Suscepti

bility.

The result of the entrance of a virulent bacterium into the tissues of an animal is not a disease with hard and fast characters, but varies greatly with circumstances. With regard to the subject of infection the chief factor is susceptibility; with regard to the bacterium virulence is allimportant. Susceptibility, as is well recognized, varies much under natural conditions in different species, in different races of the same species, and amongst individuals of the same race. It also varies with the period of life, young subjects being more susceptible to certain diseases, e.g. diphtheria, than adults. Further, there is the very important factor of acquired susceptibility. It has been experimentally shown that conditions such as fatigue, starvation, exposure to cold, &c., lower the general resisting powers and increase the susceptibility to bacterial infection. So also the local powers of resistance may be lowered by injury or depressed vitality. In this way conditions formerly

1. Active Immunity.-The key to the artificial establishment of active immunity is given by the fact long established that recovery from an attack of certain infective diseases is accompanied by protection for varying periods of time against a subsequent attack. Hence follows the idea of producing a modified attack of the disease as a means of preventiona principle which had been previously applied in inoculation against smallpox. Immunity, however, probably results from certain substances introduced into the system during the disease rather than from the disease itself; for by properly adjusted doses of the poison (in the widest sense), immunity may result without any symptoms of the disease occurring. Of the chief methods used in producing active immunity the first is by inoculation with bacteria whose virulence has been diminished, i.e. with an "attenuated virus." Many of the earlier methods of attenuation were devised in the case of the anthrax bacillus, an organism which is, however, somewhat exceptional as regards the relative stability of its virulence.. Many such methods consist, to speak generally, in growing the organism outside the body under somewhat unsuitable conditions, e.g. at higher temperatures than the optimum, in the presence of weak antiseptics, &c. The virulence of many organisms, however, becomes diminished when they are grown on the ordinary artificial media, and the diminution is sometimes accelerated by passing a current