38 MICROBIOLOGY 



IDENTIFICATION OF SPECIES. 



The identification of species. The student of bacteri- 

 ology soon learns that there are a large number of species. 

 of bacteria and that in external nature they exist together 

 in greater or less numbers. With organisms so small and of 

 such simple structure the differentiation of one species from 

 another often becomes a difficult task. The work that has 

 been done in identifying species of bacteria shows that the 

 family and genera are determined by the morphology of 

 the organism, and its species by its cultural manifestations, 

 biochemic properties or pathogenesis. Thus in identifying 

 a culture of Bacillus coli the family Bacteriaceae to which it 

 belongs is determined because morphologically it is rod- 

 shaped. The genus Bacillus to which it belongs is ascer- 

 tained from its motility and peritrichial flagella. The species 

 is determined from the character of its growth in and upon 

 culture media, such as bouillon, gelatin, and agar, its power 

 to ferment the sugars, to coagulate milk, etc. It is ordi- 

 narily not difficult to determine from the microscopic exami- 

 nation the family and genus, but to determine the species 

 is often a more difficult task, unless the organism possesses 

 some distinctive biological or pathogenic property. Bac- 

 teriologists, however, are not fully agreed as to what con- 

 stitutes specific characters or to what extent variation in the 

 cultural or physiological properties may be accepted. The 

 very close resemblance in the growth of many bacteria Be- 

 longing to the same genus has made it difficult in many 

 cases to identify the species. 



Formerly many bacteria were given specific names but 

 the nature of their growth on a very few media was recorded. 

 There are therefore many species on record whose identify- 

 ing descriptions are restricted to their growth on gelatin, 

 agar and in bouillon. More than that, it is very difficult in 

 many cases to determine to which of several species an 

 organism belongs because of the close resemblance if not 

 identity of the brief descriptions given. This condition, which 



