362 METHODS OF OBTAINING PURE CULTURES 



identification of certain bacteria, the serological method is as a rule the most delicate 

 method available for bringing out the finer distinctions between closely allied 

 organisms. In this respect it is more valuable even than the biochemical tests. 

 It is often the only method available for differentiating between the sub-species 

 or varieties of a given species of organism. Of recent years increasing attention 

 has been paid to the precipitin test, which is particularly useful when homogeneous 

 suspensions of bacteria cannot be obtained for agglutination, or when, as with the 

 haemolytic streptococci, a particular antigen can be extracted from the organisms 

 and recognized quickly by a simple precipitin technique. 



H. Pathogenicity. — The pathogenicity of bacteria is usually tested on laboratory 

 animals, especially the guinea-pig, rabbit, rat, and mouse. It may be advisable 

 to introduce the organism directly into the tissues by inoculation subcutaneously, 

 intramuscularly, intraperitoneally, or intravenously ; or it may be given by the 

 mouth, or in the form of a spray, which the animal is made to inhale. Patho- 

 genicity tests are open to numerous errors, but provided these are adequately 

 guarded against, they often al?ord very important information. This is limited, 

 however, to certain groups of organisms. In the study of the purely saprophytic 

 bacteria and of certain bacteria that are harmless to laboratory animals, the patho- 

 genicity test is of no value except to establish the absence of virulence. It is 

 used chiefly in distinguishing virulent from avirulent members of the same genus 

 or species. But it is also used to distinguish between closely alhed organisms, 

 both of which are virulent to the same animal, but which produce in it lesions of 

 varying extent or localization ; or which differ in their virulence to different species 

 of animal. 



For the complete identification of a pathogenic species, it may be necessary to 

 determine whether or not it forms a soluble exotoxin ; that is to say, whether sterile 

 filtrates of cultures grown for a suitable time in suitable fluid media produce death 

 with characteristic lesions. In species which produce such exotoxins, neutralization 

 with a specific antitoxin may play an important part in identification. In the 

 description of any newly isolated pathogenic species a record of the toxicity or 

 non-toxicity of filtrates should always be included. 



It will be realized that the complete identification of an unknown organism is 

 often a lengthy proceeding. As a rule it is easy to refer it to its proper genus ; 

 this can be done by simple examination of the morphological and staining reactions, 

 aided at times by the cultural reactions. But its more exact denomination requires 

 the use of the most delicate tests at our disposal, namely the biochemical, serological, 

 and pathogenicity tests. It is advisable never to place too much weight on any 

 one test ; errors of technique or of interpretation are always liable to occur. If 

 a large series of tests is carried out, and the organism is studied by several different 

 methods, then the chances of being misled are very greatly reduced. 



The complete identification of a given organism with any known type is not, 

 of course, always possible ; within the type species there are often varieties 

 differing in minor respects from the type organism. It is very important to realize 

 this ; in any large collection of organisms of apparently the same species, there 

 will almost invariably be found a number that differ from the rest in one or more 

 of their properties ; sometimes these differences are so numerous, or a single one 

 of them may be so important, that it is necessary to revise one's classification. 



If the characters of the organism differ from those of any described species, 



