THE IDENTIFICATION OF MICRO-ORGANISMS 361 



ance, aud is employed as a primary criterion for differentiating between the mem- 

 bers of the streptococcal group. The effect on growth of adding blood, serum, 

 glucose, nitrates, and bile salts to the medium is important, since it is often of 

 differential value. The formation of pigment should be studied on various media 

 and at different temperatures ; as a rule it will be found that pigment is best 

 formed on the surface of a solid medium at a temperature of 25-30° C. In liquid 

 media, or in the depth of solid media, and at temperatures above 35° C, pigment 

 is formed less abundantly ; under strict anaerobic conditions it is formed only by 

 exceptional organisms, such as F. melamnogenicus. 



A study of the salient metabolic functions of an organism as a rule adds con- 

 siderably to the information derived from the previous methods of examination. 

 Oxygen and temperature requirements, and pigment formation, especially are of 

 great classificatory value, being frequently used for the difTerentiation of species. 

 F. Fermentation Reactions and Other Biochemical Properties. — Under this 

 heading we include a study of the fermentative action on certain carbohydrates 

 and alcohols (colloquially spoken of as " sugars ") ; of the proteolytic powers, 

 especially the digestion of gelatin, egg, and serum ; of the fat-splitting powers ; 

 of the power to reduce certain dyes such as methylene blue and litmus, or certain 

 salts such as nitrate and tellurite ; of the production of catalase ; the production 

 of indole from peptone ; the formation of NH3 and H,S ; the final hydrogen-ion 

 concentration in glucose broth ; and the power to utilize certain salts such as 

 tartrates and citrates. 



As a rule the fermentation of sugars is observed qualitatively, the formation 

 of acid being rendered evident by the inclusion in the medium of an indicator, and 

 the liberation of gas by an inverted Durham tube, or by a special fermentation 

 tube. For the testing of the other biochemical properties, certain fairly stereo- 

 typed methods have been evolved, which it is unnecessary to describe here. 



In bacterial differentiation the biochemical reactions are often of the greatest 

 importance ; in many groups of organisms the classification is made on the basis 

 of sugar fermentation, of proteolytic power, or of both tests taken together. The 

 sugar tests especially afford a means of bringing out the finer distinctions between 

 closely allied organisms. The oxidation of tartrates and citrates is employed in 

 the differentiation of the coliform group of bacteria. 



G. Antigenic Structure. — For identifying bacteria, the serological reactions 

 most frequently employed are agglutination and complement fixation. Provided 

 that adequate controls are used, these reactions — particularly agglutination — 

 afford the most rapid and reliable method of identifying a given bacterium. For 

 the identification to be complete the organism should be agglutinated to titre by a 

 serum prepared against the organism which it is supposed to resemble, and should 

 absorb all the agglutinins from that serum ; moreover the type organism should 

 be agglutinated to titre by a serum prepared against the unknown organism, and 

 should likewise remove all agglutinins from it. That is to say, there should be 

 complete cross-agglutination and cross-absorption between the two sera and the 

 two organisms. In certain groups of bacteria, the serological method is found to 

 be much the quickest and most satisfactory way of distinguishing between the 

 different members, and it is therefore extensively used for rapid identification. 

 In other groups the agglutination method is not of much help, either because there 

 are numerous varieties within the species, or because the organisms are auto- 

 agglutinable, or for some other reason. Apart from affording a rapid means of 



