90 MEDICAL BACTERIOLOGY 



ably a pseudodiphtheria bacillus is found associated with an infection of the 

 nose or throat. Experienced clinical bacteriologists can usually do this by ob- 

 serving the morphology and staining of microscopic preparations together with 

 the character of growth on Loeffler's blood-serum medium. It is one of the 

 most difficult problems that confronts the beginner. 



While there is much confusion and disagreement on the subject it seems 

 most probable that we have to deal with organisms that naturally divide them- 

 selves into three groups: 



First, true diphtheria bacilli which have temporarily lost some of their 

 faculties. 



Second, several distinct species not diphtheria bacilli neither pathogenic 

 or toxin producers, including Hoffman's bacillus and the xerosis bacillus. 



Third, a numerous heterogeneous group of non-pathogenic saprophytes 

 more or less similar to and always distinguishable from typical diphtheria 

 bacilli. This third group is almost ubiquitous. 



