1290 MANUAL OF DETERMINATIVE BACTERIOLOGY 



group, the small forms appear in appropriate preparations as small bipolar 

 stained bacilli. The transformation of the bacilli to round bodies of 

 variable size often occurs in bacterial cultures and is not specific for the 

 pleuropneumonia group. Furthermore, it has been observed in several 

 species of bacteria that they reproduce in the large round forms in a manner 

 similar to that observed in the pleuropneumonia group. Thus the form 

 variation and reproductive processes observed in the pleuropneumonia 

 group are not specific to this gi'oup. They represent general bacterial 

 properties and should be included in the definition of the true bacteria. 



According to these considerations, the organisms belonging to the 

 pleuropneumonia group are small, Gram-negative bacilli often showing 

 bipolar staining and their distinctive characteristic is the tendency to 

 swell up into round forms and multiply by the reproduction of l^acilli in the 

 round forms. Their habitat is in the mucous membranes of animals and 

 man and many of them are pathogenic. They are exacting in their media 

 requirements and usually require fresh animal serum for their growth. 

 These properties indicate a close similarity to the species now included in 

 the genera Pasteurella and Haemophilus. The pleuropneumonia group 

 might well be classified in the same or a closely related family. It is un- 

 certain whether the strains isolated from earth and sewage should be classi- 

 fied with the strains isolated from animals and men. The soil and sewage 

 strains are less soft, stain more easily and gi-ow abundantlj'^ without animal 

 serum. The strains isolated from bacterial cultures are most probably 

 variant forms of the bacteria and should be classified with the parent 

 organisms. 



The viruses of psittacosis and lymphogranuloma present similarities to 

 the pleuropneumonia group both in morphology and in their methods of 

 reproduction. This gives added weight to the thought that the pleuro- 

 pneumonia group represents an intermediary stage in the evolution of the 

 small. Gram-negative bacteria of the mucous membranes into the filterable 

 viruses. 



