8 MANUAL OF MICROBIOLOGICAL METHODS 



study should be made to learn whether they indicate morphologic varia- 

 tion, dissociation, or merely contamination. In making special staining 

 tests, like the gram stain, several determinations should be made on 

 separate transfers of the culture and at different ages, because there are 

 species that vary in their staining reactions and such variation cannot be 

 detected by single determinations. As a check on the technic, a known 

 positive and a known negative culture should be included in the study. 

 For example, when making a gram stain, it is good practice to place on 

 the slide, beside the culture under study, a smear containing a mixture of 

 a known gram-positive and a known gram-negative organism (which 

 differ markedly in morphology). Then it is possible to observe if the 

 expected results are obtained with the known cultures and thus to have 

 some degree of control on the technic. 



Identification. After recording the characteristics of an organism, the 

 next step is identification, if possible, with a previously described species. 

 This should never be attempted until at least six representative strains 

 of the unknown organism isolated from more than one source, if possible, 

 have been studied. No rules can be given for identifying the culture. 

 Descriptions of bacteria are scattered so widely through the literature 

 and vary so greatly in their form that identification is often extremely 

 different. Bergey's ''Manual of Determinative Bacteriology" is a great 

 help, but it is usually necessary to go back to original descriptions and 

 often to secure transfers of authentic strains before certain identification 

 can be made. Difficult as this procedure is, no one is justified in naming 

 a new species of bacteria until a comprehensive search through the litera- 

 ture of species already described has been made. Frequently it is neces- 

 sary to refer in some publication to a previously described species on the 

 basis of such an identification as this. In this case it is important to 

 state in the publication whether or not an authentic strain of the species 

 has been obtained for comparison; if so, from where obtained; if not, 

 what published description of the species was followed in making the 

 identification. As to a name to use for such a species, one may follow 

 the original author's nomenclature or may give it the name employed in 

 some modern system (e.g., Bergey). Whatever name is chosen, no con- 

 fusion will result if it is accompanied by the name of the original author 

 of the specific name and by that of the one making the combination of 

 generic and specific names. Thus, whether one says ^^ Bacillus coli 

 Migula" or ^'Escherichia coli (Migula) Castellani and Chalmers," it is 

 entirely clear what species is intended. 



Naming a new species. When it proves impossible to identify a cul- 

 ture with any species described in the literature, it is often desirable to 

 publish a description of it as a new species. When publishing such a 

 description, there are five important points to be kept in mind : 



