272 A MANUAL OF BACTERIOLOGY 



of the diphtheria bacillus is the most, and the short form 

 the least, virulent, the medium being intermediate, but 

 this is by no means a universal rule. Westbrook 1 has 

 divided all forms of the diphtheria bacillus into three 

 groups, distinguished by their staining reactions with 

 methylene blue. Those with deeply staining granules he 

 calls " granular forms" those with transverse bands 

 " barred forms" and those staining evenly " solid forms" 

 Each group is further divided into seven types according 

 to shape and size, the types being designated by the letters 

 A to G and being progressively smaller from A to G. 



It is sometimes stated that a microscopical examination, 

 unless controlled by inoculation of the isolated bacteria, is 

 unreliable. Such a statement is extremely misleading. 

 If the bacilli which have been cultivated from a suspicious 

 throat possess all the characters of diphtheria bacilli, 

 inoculation experiments are not needed, and if they were 

 performed with a negative result (i.e. the bacteria are not 

 virulent) would prove little, for the bacilli from different 

 parts of a culture from a throat often possess different 

 degrees of virulence. Occasionally, it is true, even the 

 expert may be in doubt about a particular bacillus, but 

 such cases are the exception. Here an inoculation experi- 

 ment may help, but would be of no value if a negative 

 result were obtained. It is absolutely essential in the 

 microscopical examination for diphtheria to use a good 

 lens, proper illumination, and sufficient amplification, not 

 less than 800-1000 diameters. 



Paihogenicity . The diphtheria bacillus is pathogenic 

 for man, the horse, ox, rabbit, guinea-pig, cat, chicken, 

 pigeon, and finches, all of which are more or less susceptible, 

 while mice and rats are immune. In man the respiratory 

 tract is usually affected, though the conjunctiva and other 

 mucous membranes, as of the vagina and stomach, and 



1 Rep. Minnesota State Board of Health, 1899-1900. 



