THE DIPHTHERIA BACILLUS 327 



intermediate in virulence, but this is by no means a 

 universal rule. Westbrook x 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 has been suggested that a positive microscopical 

 finding, unless the organism is proved to be virulent by 

 inoculation, is unreliable. Such a procedure would be 

 impossible for the innumerable cases examined nowadays, 

 and the statement is misleading. If the bacilli present 

 in a culture from a suspicious throat possess all the 

 characters of the diphtheria bacillus, inoculation experi- 

 ments are not needed, and if the bacilli were avirulent, the 

 experiment would prove little, for the bacilli cultivated 

 from different parts of the 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 

 experiment may help, but would be of little value if a 

 negative result were obtained. It is essential in the 

 microscopical examination for diphtheria to use a good 

 lens, proper illumination and sufficient amplification, 

 not less than 800-1,000 diameters. 



Pathogenicity . 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 suscep- 

 tible, while mice and rats are immune. In man the 

 respiratory tract is usually affected, though the con- 

 junctiva and other mucous membranes, as of the vagina 



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



