352 BACTERIOLOGY. 
from cases of suspected diphtheria they may lead to an 
incorrect diagnosis. The Neisser staining method is of 
value here, but, unfortunately, the absence of the stained 
bodies is not a sufficient ground to exclude the possi- 
bility of their being true diphtheria bacilli. There are 
also some varieties which resemble the short pseudo- 
bacilli in form and staining, but which produce acid in 
glucose bouillon. These bacilli are found occasionally 
in all countries where search has been made for them. 
It may be added here that no facts have come to light 
which indicate that bacilli which do not produce diph- 
theria toxin in animals ever produce it in man. It 
must also be borne in mind, however, that such proof 
is necessarily very difficult to obtain. 
Mixed Infection in Diphtheria. Virulent diphtheria 
bacilli, however, are not the only bacteria present in 
human diphtheria. Various cocci, more particularly 
streptococci, staphylococci, and pneumococci, are almost 
always found associated with Léffler bacilli in diph- 
theria, playing an important part in the disease and 
leading often to serious complications (sepsis and bron- 
chopneumonia). Indeed, the prognosis in a case of 
diphtheria is now judged to be graver, other things 
being equal, according to the degree in which other 
pathogenic bacteria influence the course of the disease. 
These cases of so-called mixed infection in diphtheria 
have within recent years attracted considerable atten- 
tion, and have been the subject of a number of animal 
experiments. Though the results of these investiga- 
tions so far have been somewhat indefinite, they would 
seem to indicate that when other bacteria are associated 
with the diphtheria bacilli they mutually assist one 
another in their attacks upon the mucous membrane, 
