14 THE NATURE AND IDENTIFICATION OF BACTERIA 



The nature of the metabolism of an organism is closely con- 

 nected with the aerobic or anaerobic nature of its growth 

 conditions. 



Biochemical characteristics 



This book is mainly concerned with the variety of chemical 

 changes that bacteria can produce in their environment. 

 Where these changes can be detected easily they can often 

 be used to separate individuals which appear to be alike in 

 morphological and other characteristics. This will be dealt 

 with in greater detail below. 



Serological characteristics 



When a foreign body, particularly if it is of protein nature, 

 is introduced into the blood-stream of an animal it may there 

 act as an antigen and stimulate the animal to produce 

 antibody. The serum of the animal will then contain the 

 antibody which will react specifically with the antigen. 

 Bacterial cells are antigenic and if we inject bacterial cells 

 into an animal, the serum of that animal will eventually 

 contain antibody which will react with the cells in vitro so as 

 to produce a visible result such as agglutination. Since the 

 antibody formed in response to the injection of an antigen 

 is specific for that antigen, the reaction can be used as a 

 delicate test for that antigen. The surface of the bacterial 

 cell may contain several different antigens and the composition 

 of the surface varies from one organism to another. Conse- 

 quently the serum prepared as a result of the injection of an 

 organism A will react with cells of A itself or of organisms 

 possessing the same antigen in their surface. By preparing 

 the antibody to A we can therefore determine what other 

 cells belong to the same antigenic group or, alternatively, 

 we can divide a collection of organisms into groups according 

 to their antigenic reactions. For example, the species 

 Streptococcus haemolyticus has been divided by Lancefield 

 into a number of groups, known as the Lancefield Groups 

 A, B, C, D, etc., by serological methods. By a modification 



