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    "source_title": "Encyclopaedia Britannica (1926)",
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    "chunk_id": "1926:bacteriology:0e2fd294a5bd",
    "title": "BACTERIOLOGY",
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    "verified_text": "in the sections below are de- scribed the main features of the remarkable progress made in the science of bacteriology since 1910 and of the important ad- ditions to medicine and industry which have accompanied these developments. reference should also be made to the article on filter passing microbes. i. bacteria of the soil the systematic study of the bacteria of the soil has been ham- pered by two main difficulties. the first of these is the want of a rational scheme for identifying and classifying the apparently limitless number of varieties that can be isolated from soil. l6h- nis and smith claim (1914, 1921) that a large number of bacteria exhibit complicated life-cycles of changing forms. such cycles have been established and confirmed by other observers in the bis guido (1830-1016), italian physician and poli- bacchelli—bacteriology case of certain organisms, notably azefebacter chroococcum and bacillus radicicola and, if of general occurrence, they may supply a rational basis of classification. ‘the second difhiculty has been that of determining which of the bacteria isolated take an active part in the soil population. h. j. conn (1917) held that the bacteria most active in the formation of ammonia in soil belonged to types that grow with difficulty in ordinary plate cultures and that the large spore- forming rods so evident in such cultures were present in the soil mainly in a resting condition. winogradsky (1925), employing a new method for staining micro-organisms in the soil itself, observed that in unmanured soil the bacterial flora consists of colonies of short rods and cocci, usually imbedded in the colloidal matter of the soil. in addition, such soil must contain a great variety of other bacteria in numbers normally too small to be apparent by his technique, for the addition of energy-giving material causes the appearance in preponderating numbers of one or a few types which differ according to the substance added. owing to this changing quality of the bacterial flora, mislead- ing inferences are easily made from estimates of total bacterial numbers in soil based on platings of a diluted soil suspension. such estimates, however, help to indicate the influence of soil conditions on the bacterial flora as a whole and the relations between this flora and other sections of the soil population. the bacterial numbers in a field soil rise and fall at daily and even hourly intervals. ‘the work of cutler, crump and sandon (1923) shows that the daily changes in bacterial numbers are related inversely to those of the active amoebae. there seems therefore to be a fluctuating equilibrium between the bacterial and pro- tozoan sections of the soil population. the heating of the soil to 55° c., or treatment with toluene, at first reduces the bacteria, but eventually causes a marked increase in bacterial numbers, and there is evidence connecting this with the harmful effect of the treatment on the soil protozoa. thus the existence of an equilibrium between the sections of the soil population com- plicates the action of physical and chemical conditions on the soil bacteria. in arable soil, therefore, no simple relationship is usually found between the bacterial numbers and the tempera- ture or moisture. conn (1914) has even observed that the freez- ing of soil may cause a large increase in the bacteria. the reac- tion of the soil has a marked influence on certain groups such as cazotebucter and the ammonia-oxidising bacteria which are very susceptible to acidity. the action of alkali salts on soil bacteria has been studied in america (greaves, 1916) where soil alkaliisa serious problem. certain salts such as those of arsenic have been found in lew concentrations to stimulate bacterial activities, but the cause of this action is not yet understood. activities of soil bacteria—chemical changes brought about by bacteria in the soil fall naturally into two groups, those that result in a release of energy which the bacteria utilise for their vital processes, and those by which the bacteria build up the material of their cells. certain groups of soil bacteria can obtain their energy from inorganic compounds. in addition to the ammonia and nitrite oxidising groups discovered by winogradsky, and the hydro- gen oxidising organisms found by kaserer (1906), another group of such organisms has recently attracted much attention in america. these bacteria oxidise sulphur and sulphides to sul- phuric acid (waksman and joffe, 1922), and raise the acidity of their environment to a high degree. this fact has been utilised in the treatment of potato scab disease by composting with sul- phur; the scab parasite being intolerant of the acid conditions produced by the sulphur oxidation. nearly all the energy material present in soil, however, is de- rived from plant remains. there is evidence that the simpler carbohydrates are among the first plant constituents to undergo decomposition. the fermentation of pentoses by bacteria has recently been investigated by fred peterson and davenport bacteriology (1920), who isolated an organism capable of splitting xvlose and arabinose into a mixture of lactic and acetic acid. a large pro- portion of plant tissues consists of cellulose and its derivatives. it seems improbable that, in normal well-aerated soil, the an- aerobic bacteria described by omeliansky can play a leading part in cellulose decomposition. macbeth and scales (1913) isolated a number of aerobic bacteria claimed to be capable of decomposing cellulose, but their experimental evidence has been criticised. a remarkable organism was isolated by hutchinson and clayton (1919) and named sphirochaetu cytophaga. this organism com- monly exists in the shape of flexible, tapering filaments but it possesses a complex life-cycle including a spherical ‘* sporoid ” stage. it will rapidly decompose filter paper, but its growth in artificial media is quickly inhibited by more soluble or- ganic compounds such as sugars. more recently gray and chalmers (1924) isolated another organism, aficrospira agar- liquefaciens. the destruction of cellulose by this organism has been found to be stimulated by small quantities of xylose and lignin. the determination of the nitrogen and phosphate re- quired for cellulose decomposition has led to the development of a process now in use, by which straw or other plant remains treated with appropriate salts are made to decompose into a mixture having a decomposition and properties closely resem- bling farmyard manure. ammonia-producing bacteria.—a variety of bacteria can obtain energy by decomposing the nitrogen compounds in the soil with the release of ammonia. the work of doryland (1916) and others has emphasised the fact that the ammonia is only a by-product in the economy of the bacteria. the common ammonia-producing bacteria can also derive their energy from carbohydrates and, when such compounds are present in excess, they can assimilate the ammonia and nitrate already present in their surroundings. it is for this reason that unrotted straw ploughed into the soil causes a temporary loss of nitrate and consequent lowering of fertility. nitrogen-fixing bacteria—where non-nitrogenous sources of energy occur in sufficient excess, the groups of bacteria that assimilate free nitrogen may come into prominence. thus wino- gradsky (1924) found that the addition of mannitol to soil caused the predominance of the nitrogen-fixing bacteria azotobacter or clostridium pasteurianum. a large number of soluble organic substances can be utilised as sources of energy for nitrogen fixa- tion, and the products of the decomposition of cellulose by spirochoela cytophaga can also be utilised by azotobacter (hutch- inson and clayton, 1gro). several new organisms capable of assimilating nitrogen have been described (bondorff, 1918; fulmer and fred, 1917) but there is no evidence that their importance compares with that of azolobacter or closiridium pasteurianum. the relative activity of the latter organism is difficult to estimate. it is a strict an- aerobe but there 1s evidence that it may live in association with aerobic organisms that produce a local deficiency of oxygen around it (omelianski and solunskov,1916). attention is still directed to the nitrogen-fixing bacteria living in beneficial asso- ciation with higher plants. the best known case of this is that of the organism inhabiting the root cells of leguminous plants. this organism, bacillus radicicola (beijerinck) has been shown to possess a complex life-cycle through which it passes in culture (bewley and tutchinson, 192c), in the soil (thornton and gangu- lee, 1924),and within the tissues of the host plant (wallin, 1922). the cycle includes an actively motile stage during which the bacteria migrate through the soil. infection usually takes place through a root hair and the bacteria induce the multiplication of the cells of the root cortex which they subsequently infect. in normal plants, vascular strands from the centre of the root grow out into the tissues of the nodules, and it is believed that carbohydrate energy material, necessary for nitrogen fixation, is brought along them to the bacteria. under abnormal conditions, nodules can be formed, in which these strands are absent or de- fective, and in such nodules practically no nitrogen is fixed but the bacteria become parasitic on the host cells, deriving their food by attacking the protoplasm (brenchley and thornton, 299 1925). the benefit that leguminous plants normally derive from the presence of bacillus radicicola has led to many attempts at artificial inoculation by treating the seed with a suspension of bacterial culture. it has been shown that physiological varieties of the organism exist, each capable of infecting but a limited group of host plants and also distinguishable by serological methods (klimmer and kruger, 1914). the practice of seed inoculation has proved beneficial where a leguminous crop has been introduced into a district in which the soil is deficient in the correct variety of the organism. bactertological tests of soil fertility—many attempts have been made to estimate the fertility of a soil by bacteriological tests. no close correlation can be found between estimates of total bacterial numbers and soil productiveness. attempts to gauge the biological activity of soil flora have usually been made by estimating the amount of ammonia or nitrate or the increase in total nitrogen when the soil is kept under laboratory condi- tions or added to nutrient solutions. discordant results have been obtained, probably owing to the unnatural conditions to which the bacterial flora is thus subjected. biological tests have been used with greater success to reveal definite factors in a soil that limit bacterial activity. thus christensen (1915, 1923) has devised bacteriological tests whereby unfavourable acidity and deficiency in available phosphate can be detected. for references to soil bacteriology the following journals should be consulted: jour. agric. sci.; jour. agric. research (washington); soil science (new brunswick, n.j.); proc. roy. soc., ser. b.: jour. of bact. (baltimore); annals applied biology (cambridge); nature; zentralblatt f. bact. etc. (jena); reports of u.s.a. agric. exp. sta- tions, etc. (hget ii. bacteriology in industry cellulose-decomposing bacteria.—light has been thrown on the activity of these organisms in the destruction of cellulose- containing textiles which become exposed to damp. both cotton, flax, jute, wood pulp, and the textiles made from these fibres have long been known to decay, to become tender when stored under damp conditions. the financial losses thereby involved are very great and for american raw cotton alone have been esti- mated to amount to many millions of dollars annually. aerobic cellulose-decomposing bacteria are responsible for a very large percentage of these ravages. the study of the stages by which the bacterial decomposition of the cellulose molecule proceeds has thrown light on many ob- servations which had hitherto been obscure. thanks chiefly to pringsheim’s researches it has been established that cellulose (¢.v.), on being acted upon by the enzymes of certain bacteria, yields cellobiose and glucose as intermediate decomposition products. both of these sugars can be utilised as food substances, not only by a large number of bacteria which are unable to attack cellu- jose, but by herbivorous animals and by man. by the aid of these sugars, nitrogen-fixing bacteria, living symbiotically with cellulose-destroying forms, can fix atmospheric nitrogen and render it available for plant growth. the liberation of glucose. from cellulose in the animal intestine also enables herbivorous animals to derive the same nutritive value from cellulose, as they would from a corresponding quantity of starch. | the final decomposition products of the bacterial decay of cellulose have been shown to comprise fatty acids, methane, hydrogen and ethyl alcohol. quite recently efforts have been made to utilise cellulose-decomposing bacteria for the produc- tion of alcohol (q.v.) for industrial purposes. this work, how- ever, is still in its infancy and it remains to be shown whether the very large quantities of cellulose containing plant debris which accumulate in nature can, in this way, be turned to useful ac- count. the same may be said of the sporadic efforts made to produce methane and hydrogen by the fermentation of cellulose. hemicelluloses—that ethyl alcohol can be produced indus- trially by the bacterial fermentation of hemicelluloses, another constituent of many plant debris, now appears to be established. the application of this process should be of considerable inter- est in many localities where liquid fuels are expensive. on the whole the bacterial activities which involve the decomposition 300 of hemicelluloses bid fair to become an important subject. the preparation of silage, for instance, has now definitely been shown to be associated with the bacterial fermentation of hemicellu- loses contained in the plant tissues subjected to ensilaging. ‘the same may be claimed to have been finally established during recent years for the spontancous combustion of hay. whether the fermentation process which tobacco undergoes during ma- turing belongs to this category of bacterial activities is likely, but not yet definitely proved. pectin.—the bacterial fermentation processes by which yet another constituent of vegetable tissues, their pectin, is decom- posed has received much attention during the past decade and several new retting processes for the preparation of fibres such as flax and hemp have been evolved. the most important of these are the ochmann process, conducted in running water at low temperatures and in the presence of oxygen, the rossi process, carried out at blood temperature in the presence of oxygen, and the carbone process, worked at blood temperature in the absence of oxygen. starch—in the study of the bacterial fermentations of starch two lines of research deserve special mention. the first of these, the conversion of starch containing plant tissues into a mixture of butyl alcohol and acetone, derived its impetus from the re- quirements of the world war. during the period of the war the demand for acetone far exceeded the available supplies, and new sources for the production of this solvent had to be devised. the foresight of the british govt. made it possible to evolve a fermentation process in which certain types of bacteria, related to the anaerobic retting bacteria, were utilised for the conversion of starch into a mixture of butyl alcohol and acetone. with the ever increasing demand for butyl alcohol this fermentation proc- ess is now well established industrially and in the united states more than 200 tons of butyl alcohol is being produced weekly. the other line of research connected with the bacterial decom- position of starch is that of panary fermentations. it is on the fermentation processes occurring in the dough during rising that successful bread-making depends, and a thorough study of the complicated biological reactions thereby involved is long overdue. serious efforts are now being made in the united states to throw light on these reactions. sugars.—in the study of the microbiological decomposition of sugars the most interesting recent progress is connected with the production of glycerine by the ordinary yeast fermentation of sugars, and with the conversion of sugars into lactic acid through bacterial action, the latter with a view to obtaining the sodium salt of lactic acid. this sodium salt yields very viscous solutions with water which, on account of their low freezing point, are superior to glycerine and lubricating oils for a great number of purposes. the production of glycerine during the ordinary alcohol fer- mentation by yeast 1s based on neuberger’s and connstein’s observation that sodium sulphite, or other alkaline salts, added to a fermenting sugar solution interfere with the normal course of the reaction and cause glycerine and acetaldehyde to be ac- cumulated at the expense of alcohol and carbon dioxide. work- ing this reaction on a technical scale, as much as 25% of glycerine, calculated on the sugar fermented, may be obtained. in touching upon the bacterial fermentation of sugars it should be recalled that considerable progress has been made during recent years in the study of the interesting fermentation process to which freshly gathered cacao beans are normally subjected. in the way this process is carried out industrially micro-organ- isms—both bacteria and yeasts—are responsible for the desired changes occurring in the bean. dairying.—though no startling revelations have been made in the field of dairy bacteriology during the last decade, progress has been steady and important. attention has been paid to the important problem of securing a more hygienic milk by processes of eliminating the bacteria already present and by preventing their entry into, and their development in, the milk. in the manufacture of butter and cheese, where the presence of certain bacteria is essential, the conditions for the development of the bacteriology desired forms have been further investigated. the additions of pure cultures of the types responsible for the required changes have gone far to ensure more reliable processes of manufacture as well as more stable products. see ii. pringsheim, die polysaccharid, (1923); a. c. thaysen and hl. j. bunker, the microbiology of cellulose, itemicelluloses, pectin and gums (oxford, 1925); journ instit. of brewing; jour. indust. and fengineer. chem. (lcaston, pa.); jour. of the franklin instit. (philadel- phia); biochem. zettsch., etc. (a.c. 75) ih. medical bacteriology and immunology many advances have been made since 1910 in our knowledge of bacteria as causative agents of disease in man and animals and in the science of immunology. improvements in technique dark ground illumination—much use has been made of this method of studying bacteria, whether in the live or fixed state. its chief practical applications have been in the demonstration of slender objects such as the spirochaete pallida in material from syphilitic sores (see venereal diseases), the detec- tion of motility and the enumeration of bacteria in emulsions such as vaccines. to obtain the dark ground effect, the micro- scope requires to be fitted with a special form of condenser, of which there are now many models, and a powerful source of illumination is necessary. the rays from the source of light after reflection at the substage mirror can enter only the peripheral portion of the condenser and, owing to their obliquity, are then either wholly reflected from the upper surface of the cover-glass or, according to the type of condenser, refracted into space be- yond the range of the objective lens. should, however, small objects come in the way, such as bacilli possessing a different refractility from that of the fluid in which they are suspended, the rays are dispersed by them, and are able to enter the objec- tive. bacilli then appear as white spots on a dark background. with an ordinary condenser a very similar effect is produced by mixing material containing delicate organisms with india ink, or collargol or congo-red, and spreading in a thin film on a slide. delicate organisms in the material can then be readily picked up from the surrounding dark field. recently the photography of organisms beyond the range of ordinary microscopic vision has been attempted with the aid of ultraviolet light, but the method is as yet on trial. (see microscopy.) enumeration of bacteria.—besides the dark ground illumina- tion method used in conjunction with the well-known thoma- zeiss ruled slide chamber, in which bacteria can be counted directly, turbidity measurements give very accurate estimations, the turbidity of the bacterial emulsion, suitably diluted, being compared with that of a series of tubes containing varying con- centrations of barium sulphate previously standardised to repre- sent a definite bacterial content of the particular organism under investigation. isolation and cultivation of single bactertal cells ——for the cultivation of “ pure line ”’ strains of micro-organisms it is essen- tial to start from a single microbe. in barber’s method (1904) a single bacillus in an appropriately diluted emulsion is picked up into an extremely fine capillary pipette working on a special holder under microscopic control. it is really founded on the earlier method of van schouten (1899), who employed a fine glass loop for the same purpose. barber’s method has been also greatly developed in general cytological studies and it is now possible to perform surgical operations on single cells. in burri’s method, dilutions of the culture are mixed with india ink and drops of these are laid on gelatine or agar plates. under the mi- croscope these drops are examined, and if one is found containing a single bacillus it is marked and allowed to grow into colony form, or the single organism may be removed directly to a nu- trient medium. a recent method, elaborated by barnard, depends on the fact that ultraviolet light destroys bacteria. a single bacillus is marked down, and between it and the entering rays a minute droplet of mercury is placed under microscopic control. the bacteriology bacillus is thus protected, while the surrounding organisms are killed. it can then be dealt with culturally. methods of culture-—numerous improvements in cultural methods have been introduced of late years, such as the addition of egg to a nutrient basis of agar, or the use of solid, inspissated mixed white and yolk of egg, especially for the growth of tuber- cle bacilli. a medium composed of broth and serum, covered with liquid paraffin, and containing a piece of sterile, unheated rabbit’s kidney was introduced by noguchi for the growth of spirochaetes, and has been recommended for other organisms of difficult culture. attention has been directed of late to the im- portance of making culture medium of exactly the right degree of alkalinity, and the method now used for this purpose is to test and standardise the media at a constant temperature by means of delicate colour indicators, e.g., phenol-sulphone-phthaleine. a hydrogen-ion concentration of 7-6 has been found the most suitable for general bacteriological purposes. anaerobiosis —anaerobic culture methods have been improved by the introduction of various patterns of jars to contain petri dishes or test-tubes; the object of these is to facilitate the re- moval of all, or a part, of the oxygen with an air pump, and re- place it with hydrogen or other indifferent gas. the removal of the oxygen is also attained by using the catalytic action of spongy platinum (or palladium) to produce slow combination of oxygen with hydrogen. after removing the greater part of the air with a pump and replacing it with hydrogen, the jar is closed and the catalyst contained in a small wire-gauze cage heated by means of an electric current. the remaining oxygen then combines quietly with hydrogen and this action keeps the platinum hot as long as it is required, 7.e., until the oxygen is used up. fermentations produced by bacteria chemical processes involved in fermentation.—bacteria are in many cases very selective in their chemical action on such substances as the sugars, higher alcohols and hydroxy acids, and produce from them a great variety of products, these reactions being largely used for diagnostic purposes. the trend of modern work 01 this subject is to show that specific enzymes are probably necessiry for the first stage in the attack on substances of difler- ent chemical or stereochemical constitution, but that the imme- diate products of this attack are frequently of the same character. these then become changed under the influence of a system of enzymes which is common to a considerable number of different organisms, yielding final products of a similar nature, but in differing proportions. in some cases additional enzymes are present, which modify the result, yielding special products spe- cific to a particular organism. thus, b. colt acts both on mannitol and glucose, probably by the aid of two distinct enzymes, but yields the same final prod- ucts from both substances, in different proportions, correspond- ing to the difference in composition of the two compounds. on the other hand b. lactis aerogenes, which attacks the same two compounds and produces from them all the products yielded by b. coli, in addition converts a part of one of the intermediate compounds (probably acetaldehyde) into butylene glycol, which is not produced by b. coli and the formation of which is presum- ably due to the influence of a specific enzyme. the exact way in which the primary products of the first attack on the molecule are worked up may be greatly influenced by the conditions to which the organism is subjected, and, thus, the proportions in which the final products ‘are formed may vary considerably. pasteur’s dictum that “‘ fermentation is life without air ” still maintains its validity. it is now interpreted in the sense that the changes summed up in the term fermentation proceed with evolution of energy, part of which can be utilised by the organism. in aerobic organisms this replaces the energy which, in aerobic life, is gained by the oxidation of food materials. many bacteria (facultative anaerobes) can carry out their life cycle anaerobically provided that they are supplied with material which they can decompose with evolution of energy and with the production of an intermediate substance which can be assimi- lated by the organism (e.g., b. colt in presence of glucose). in 301 many cases this can be accomplished by providing a pair of sub- stances, one of which can be oxidised (or reduced) at the expense of the other in the presence of the organism with evolution of energy and the production of an assimilable intermediate com- pound. thus b. coli cannot grow anaerobically in peptone water in presence of either a lactate or a nitrate, but grows readily when both are present, the nitrate being reduced and the lactate oxi- dised to some substance (probably pyruvic acid) which is assim- ilable by the organism. the nitrate can be replaced by fumarate which is reduced to succinate. value of sugar fermentation tests in differential diag- nosis.—a very important, and usually very reliable, method of distinguishing between allied races and species of bacteria is that founded on their fermentative action (production of acid or acid and gas) on various carbohydrates and alcohols. in addition to carbohydrates and alcohols, the salts of the organic acids have been used for a similar purpose (citrates, tartrates, etc.). the sugar reactions are not invariably constant for the same kind, or even for the same strain, of bacteria but they are perhaps as constant as any other set of characters. in the diagnosis of a bacterial species, as in the case of other living things, it is often necessary to consider several characters and not to place reliance on a single characteristic. variation of bacteria under the head of variation of bacteria three kinds of charge are understood. (1) i'luctuation which implies the temporary change due to alterations of environment which do not persist when the special circumstances are removed. (2) persistent va- riation which is hereditary and, whatever its cause, is not directly dependent for its continuance on the special circumstances, and (3) so-called cyclical variation by which bacteria are supposed to undergo successive changes in cycles, during which they as- sume very different forms and characters and may even become indistinguishable from the higher fungi presenting apparently sexual forms. the latter kind of change, however, is not gener- ally accepted as occurring in any of the well-known pathogenic bacteria, in spite of the fact that it has been described since the earliest days of bacteriology. fluctuation may be briefly dis- missed by saying that wide variation in form and size are well known to occur in most groups and the physiological and mor- phological characters may be temporarily suppressed by special conditions. hereditary variation.—true persistent hereditary variation has been only gradually accepted by bacteriologists since the identity of changed forms with the original strain has rightly been challenged till full proof was available. these variations, which have been called mutations, since they are hereditary and often appear with some suddenness, may affect many of the structural and functional characters of bacteria. within limits, size and shape of the individual bacteria and their colonies are subject to great changes, and variation in regard to fermentation of carbohydrates undoubtedly occurs, though more rarcly. many mutants are best obtained by selecting colonies of different appearance. it not infrequently happens also that colonies which are several days old show, on their surfaces, small projections or papillae, which consist of bacteria which have acquired new properties. some of the most remarkable variations affect the aggluti- nability of the cultures by salt solutions and specific serum. a commonly found mutant in old cultures is one which is spon- tarieously agglutinable in salt solution (0-85°%) while it has at the same time lost some of its agglutinability by the specific serum for the parent strain and also acquired new properties of serum agglutinability. another variant occurring in the proteus and coli-typhoid group, the o form of weil, has lost its heat- labile agglutinogen, responsible for flocculent agglutination, and at the same time ceased to be actively motile. loss or change in pigment-forming characters is also well known. an important character which is liable to variation is virulence. the loss and the regaining of virulence may, to some extent, be controlled, and the change may persist in the strain. 302 single cell vartants.—variants in different directions have been shown to arise from single cell cultures. the hereditary variants may persist indefinitely, especially if care and time are spent in obtaining them pure by repeatedly making plate cul- tures and selecting isolated colonies. in other cases reversion cannot be avoided. tht bacteriophage this represents a newly observed phenomenon, first described by f. w. twort (1915) and f. d’herelle (1918). the former ob- served a lytic change accompanied by translucency of some colonies in certain cultures of staphylococcus and found that this lysis could be communicated to other normal colonies by an ex- tract of the affected culture which had been passed through a berkefeld filter, and was itself free from bacteria. d’herelle, starting with a berkefeld filtrate from the faeces of dysentery convalescents, was able by adding a drop of this to a culture of b. dysenteriae (shiga) to produce lysis of the bacteria. a drop of the resulting solution after filtration could be used to repeat the phenomenon in a fresh culture, and in this way a renewal of the active principle could be brought about indefinitely. this prin- ciple d’herelle called the bacteriophage (bucteriophagum in- festinale), and he maintains that it is an ultra-microscopic, living organism, which is parasitic in the bacteria and reproduces itself. the solution containing the principle is often active in a dilution of 1 in 100 million, and 5 c.c. of broth containing bacteriophage in this dilution, if inoculated with the sensitive culture, will re- produce in 24 hours in the incubator a clear solution containing the bacteriophage in the original strength, which again ts still active when similarly diluted. if a tube of suitable diluted phage, inoculated with b. dvsen- teriae, is incubated for three hours, and a loop of it then inocu- lated on agar, confluent growth of the bacteria may be obtained with a number of small, round, clear areas where no growth of bacteria has occurred. these spaces d’herelle called ddches vierges and considered them to represent colonies of the bacterio- phage which had devoured the bacteria. other bacteria are attacked besides b. dysenteriae, e.g., b. typhosus, b. paratvpho- sus and b. colt. most strains of bacteriophage are specific for certain kinds of bacteria, or even for certain strains of these species. thus some varieties of bacteriophage will act on “rough ” variants of b. dysenteriae and on some strains of b. coli, but not on the corresponding normal or ‘‘ smooth ” cultures. after the action of the bacteriophage on a broth culture and the resulting clearing of the original turbidity, it is usual for some bacteria to remain alive which, when cultured on agar, are found to be considerably altered in the appearance of their col- onies. their agglutinability by salts is also changed. these colonies are often resistant to the bacteriophage. there 1s no doubt that bacteriophage may be present in small quantities in cultures, but remain unnoticed, and it has been suggested that many variations, occurring apparently spontancously, are in reality brought about by the presence of unrecognised bacterio- phage. d’herelle has advocated the use of bacteriophage in the prevention and treatment of infective disease, but its value for this purpose is not widely recognised. the question of the nature of the bacteriophage has stimu- lated a very large amount of research on account of its theoret- ical importance. if the more generally accepted view ascribing the properties of the bacteriophage to a non-living enzyme or catalyst were proved, the fact that it appears to infect and mul- tiply in a culture of bacteria might by analogy throw light on some infective diseases associated with ultramicroscopic viruses affecting the higher forms of life. the characteristics which make it less probable that the bacteriophage is a living entity are its ready filtrability through a porcelain filter, its resistance to heat (60° to 65° c.), its resistance to acetone and chloroform and its inability to lyse dead bacteria or to multiply in the ab- sence of a living bacterial culture. acute infectious jaundice or weil’s disease —this iseea se: which came into some prominence as a war disease, mpecully on the western front, was found by inada and ido con) to be due to a motile spirochaetal organism leptospira icterohaemor- bacteriology rhagiae. in certain localities it appears to be a parasite in the kidneys of wild rats, and infection of food and water by the urine of such rats is the most likely mode of transmission. a similar organism has recently been found to be the cause of a disease in young dogs called the yellows. yellow fever —noguchi since torg has brought forward excel- lent evidence that another /eptospira (l. ictereides) is responsi- ble for yellow fever (g.v.), and the employment of therapeutic serum, prepared by immunising horses with this /eptospira, has very favourably influenced the course of the disease. also good reports have been received of the use of prophylactic vaccines in lowering the incidence. tularaemiu.—mccoy (1911) in the course of examining ground squirrels in california for evidence of plague infection discovered another plague-like disease in these rodents which was not due to b. pestis. in the following year, he and chapin isolated the specific organism and it was given the name of b. tularense from tulare co. in california where the epidemic in the ground squirrels was prevalent. during the years preceding the demonstration of the organism, cases of severe lymphangitis in man apparently following insect bites had been observed in the state of utah. since the discovery of the parasite quite a number of human cases of infection have occurred in some of the amer- ican states. the disease is rarely fatal but a very protracted convalescence is the rule. the mode of infection in the field is almost certainly by the bite of the horse-fly (chrysops discalis) which has fed on infected ground squirrels or jack rabbits. in the laboratory, also, workers engaged on research into this dis- ease have been attacked, e.g., at the washington hygiene lab- oratories and at the lister institute, london. the mode of transmission in the laboratory is as yet unknown, but probably the organism gains entrance by the respiratory tract as in pneu- monic plague. outside the united states of amcrica this disease of rodents has not, so far, been reported. melioidosis —this disease was first described in 1913 by whitmore as a “ glanders-like disease ” occurring in rangoon, and indeed the clinical relationship to glanders, both in man and in horses, is very close. ‘their morbid anatomy also is very simi- lar. it runs a more acute course, however, than glanders, and is usually fatal. the causative organism b. w/ifmort is prima- rily a parasite of rodents and as stanton and fletcher have found, it possesses a close serological relationship to b. malict, the cause of glanders, but differs from it in its growth on nutrient media and in the fact of its motility. rat-bite fever —this is a prolonged, febrile disease, of which inflammation of the lymphatics and a well-marked rash on the skin, together with wasting, are prominent symptoms. the course of the disease is very irregular and the fever intermittent. a spirochaete has been described and is generally accepted as the causal micro-organism. the disease occurs in japan, india and other parts of asia, and cases have been recorded in europe, including great britain and america. | gas gangrene.—during the world war the occurrence of large numbers of infections of wounds led to a close study of the bacteria responsible for them. the anaerobic bacilli in particu- lar were investigated anew, and much confusion in their classi- fication and nomenclature cleared up. to three species espe- cially, b. perfringens (b. welchii), vibrion septique (b. ocdematis maligni) and b. ocdematiens, most of the cases of gas gangrene were found to be due. from these three species, moreover, toxins and antitoxins were prepared which were found to be of service clinically. unfortunately, these discoveries were not made till too late to be of use in the treatment of the large number of cases prevalent during the first years of the war. (acp. med. research committee, spec. rep. no. 39.) progress in immunology antibodies generally —the study of the antibodies in the blood serum of animals which have been inoculated with bac- terial suspensions or toxins, has proved of great value in many directions. in the case of antitoxin, apart from its great thera- peutic value in diseases such as diphtheria (see infectious bacteriology fevers), it is now possible to form some estimate of the suscep- tibility or immunity of an individual by determining the amount of antitoxin in the blood (schick test in diphtheria). the other well-known antibodies, agglutinins, precipitins, bacteriolysins and opsonins, do not give an accurate measure of immunity. these latter are, however, used as important aids (1) in eluci- dating the processes concerned in disease and its cure, (2) in making a diagnosis of the presence of a disease, (3) in identifying a given bacterium by the use of a serum evoked in the animal body by inoculation of the particular species of bacterium con- cerned, or indeed, for differentiating bacteria that are very closely allied. agglutination.—the agglutination of bacteria by a specific serum appears to be a process of the same kind as the reaction which occurs when a diluted solution of protein, ¢.g., serum protein or egg albumin, is added to a serum obtained from an animal which has been injected with this protein. in the case of bacteria, the phenomenon is less simple, since there is no doubt that many bacteria contain more than one substance, perhaps many, capable of exciting the production of agglutinins, and consequently, more than one agglutinin is present in the serum of the inoculated animal. the extent of the variation, and difference in the composition, of these agglutinogenic substances is not yet fully known, but their existence and diversity are of importance in identifying or differentiating allied bacteria. when a serum has been prepared that will agglutinate a given kind of bacterium it is often found that it will also agglutinate other kinds of bacteria of the same group to a lesser degree. this fact suggests that these different bacteria have some factor that is common to them all in their composition. one method of comparing antigenically different species of bacteria is to prepare separate agglutinating scra for each kind and to test the degree of agglutination of each emulsion with each serum. agglutinating serum with the emulsion, to remove the clumps by the centrifuge after a few hours, and then to test the super- natant fluid with a fresh emulsion to sce whether the property of clumping has been removed or diminished for the same and other strains. this method (absorption of agglutinins) has been extensively used in attempts to classify the antigenic constitu- ents of allied bacterial races. by this means many at first sight identical strains have been divided into groups and subgroups, e.g., the salmonella group by schiitze and bruce white, the meningococcal group by gordon and the dysentery group by andrewes. these groups are sometimes separable by fermenta- tion and other tests and are said to remain quite constant in culture, maintaining their finer ditferences. perhaps sufficient time has hardly elapsed, or sufficient study been given, to this very ditlicult subject to decide to what extent the lesser distinc- tions are maintained in a state of nature or under varying con- ditions of culture. the agglutinogens present in a given culture, which are usually considcred, though probably not quite accurately, to be identi- cal with the agglutinable substances, can be differentiated by other means. for example, there are agg slutinogens which resist a temperature of 100° c. for 5 to 30 min. and others which can no longer be detected after this treatment. there is some evidence that the heat-labile antigens are, at least in part, asso- ciated with the flagella, since non-motile varieties of motile or- ganisms are often deficient in the heat-labile antigen. ‘these two classes of antigen are related to distinct agglutinins in serum. another means of distinguishing antigens has been brought to light by chemical processes of extraction and purification (by ether, alcohol, etc.) and it has been found that certain partial components of a carbohydrate (prenmoroccus, avery and itei- delberger) or lipoid (b. t#berculosis, zinnser) nature, play a part in these reactions, though probably in every case only when united with protein material. discordant facts which require harmonising with any accepted system of classification may be mentioned. (1) some pure strains may lose or gain agglutinability in culture without apparent a more refined method of analysis is to treat the | 393 change in other respects. (2) some variants may have lost their agglutinability with the serum made from the parent strain, but may agglutinate with serum made from the variant which leaves the parent untouched. (3) cultures which seem alike culturally and as regards pathogenesis, e.g., staphylococct, meningococct, gonococci, etc., may be readily differentiated by agglutination, and subdivided by absorption technique. (4) strains which appear identical serologically may difler markedly in pathogenic properties, e.g., af. melitensis and b. abortus. (5) strains may change in virulence and in fermentation characters, and yet be unaltered serologically. para-agglutination and heteroagglutina- tion are similar phenomena which signify the agglutination of di- verse strains by serum obtained from an animal which is suffering from an apparently unrelated infection, e.g., the agglutination of b. proteus xig by the serum of typhus fever patients (felix and weil). the facts which have emerged during the recent extensive study of agglutination have given rise to the hypothesis that each bacterium contains a mosaic of antigens, some of which are highly specific and peculiar to each subspecies, while some are common to the whole species or group and others appear to be entirely unrelated as regards phylogeny and occur in very di- verse bacteria (e.g., forssmann’s nonspecific antigens). it is probable that the identification and classification of bacteria by their antigens have their limits, though the resultant data are extremely valuable in some cases. complement fixation.—this is the name given to abies procedure by which the presence of antisubstances to bacteria can be recognised in the blood serum of animals. in principle it rests on the discovery by bordet and gengou that when a bac- teriolytic serum containing bacteriolysin and a suspension of the corresponding bacteria are allowed to interact in the presence of fresh serum, a constituent of the latter (alexin or complement) joins the combination of bacteria and antisubstance, and remains fixed to it, this union resulting in the death and solution of the bacteria. to render this fixation of complement obvious even where lysis of the bacteria is not visible, another system which also needs complement for its consummation is added to the mixture, and the occurrence or not of the first reaction is deter- mined by the completion or the non-completion of the second reaction. the most commonly used antigen-antibody system for the second system is a mixture of a suspension in salt solution of the red-blood corpuscles of the sheep and of a serum which is known to be lytic for them but which does not contain active complement. this latter desideratum can be assured by previously heating the haemolytic serum to 56° c. for 30 minutes. if there 1s free complement at liberty to join the red corpuscles and antiserum, the red cells break up, and the haemoglobin is set free as a clear solution in the surrounding medium. the colour and transpar- ency of the solution is a readily observed indicator of the pres- ence of free complement and, to some extent, of its amount. this by inference allows an estimate to be made of the reagents present in the first system. if no appropriate bacterial substance or no antisubstance is present, then no complement will be fixed in the first reaction, and consequently it will still be available for haemolysis. it was found by weil and his colleagues that the complement fixation in the group of bacteria which they especially studied (intestinal gram negative group) was associated with the heat stable rather than with the heat-labile antigen (agglutinogen) in the bacteria. vaccines for prophylaxis and therapy.—a bacterial vaccine is a preparation of antigen (bacteria or their products) intro- duced into an animal to excite active immunity. the use of preventive vaccines is firmly established by animal experiment (e.g., anthrax, plague, dysentery, enteric and paratyphoid fevers, etc.). so far as accurate experiments have been possible, the living bacteria, or bacteria killed at low temperatures, e.g., 52°c., have been found more efficient than emulsions treated at higher temperatures, or by strong chemical agents. the use of vaccines for treating disease rests on a very different 304 footing, since little or no satisfactory evidence from animal ex- periments is forthcoming, and the clinical results with human disease have been extremely irregular. the earlier hope that it would be possible to produce clinical improvement by graduated doses, fitted to the varying resistance to the disease as estimated by in vitro tests, has been for the most part given up, though new methods are often introduced and require critical examina- tion. the hypothesis that the strain of bacterium (homologous) actually infecting the patient is the most suitable for treatment is unsupported experimentally and is now less regarded by cli- nicians. experimental and clinical evidence have also pointed to the beneficial effects in some cases of inoculation with various foreign proteins and unrelated bacterial emulsions. probably much of the vaccine treatment of the past was of no real value and a large part of the good effects reported have been due to a non-specific element in the inoculum. national collection of type cultures —this collection of living cultures from all spheres of bacteriological activity was founded ‘in 1920 at the lister institute, london, under the auspices of the medical research council. it has been of great service to mi- crobiologists in all parts of the world, over 2,000 strains being maintained in 1925. the second edition of the catalogue ap- peared in 1925 and can be procured from h.m. stationery office, london. references.—the chief journals devoted to medical bacteriology and immunology are:—jour. pathology and bact.; brit. jour. exp. path.; jour. bact. (chicago); jour. inf. dis. (baltimore); jour. exp. medicine; jour. immun. (baltimore); zeit. f. bact., abt. 1 (jena); zlettsch, f. iyg., etc. (leipzig); archiv f. izy¢.; zettsch. f. immunitatsf., etc. (jena); ana. de uinstit. pastetr. the larger reference books are those of r. kraus and c, levaditi, handbuch der immunitdtsforschung und experimentellen therapie (jena, 1914); and w. nolle and h. hetsch, die experimentelile bakte- riologie, etc. (1922). smaller text-books are e. o. jordan, text-book of general bacteriology (philadelphia, 1915); j. a. kolmer, a practical text-book of infection, immunity and specific therapy (19185); k. muir and j. ritchie, afanual of bacteriology, 7th ed. (1919); r. t. hewlett, pathology (1922); etc. cu cre baste hee)",
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