3808 BACTERIOLOGY. 
forty cases, came to the following conclusions: 1. The 
presence of a large number of bacteria in a satisfactory 
and thoroughly washed specimen of sputum indicates 
that these bacteria probably play an active part in the 
disease. . 
2. The presence of a small number of bacteria in 
such sputum does not necessarily indicate that they are 
not active in that case, for they may penetrate more or 
less deeply into the lung tissue, and produce patho- 
logical changes without being thrown off in large num- 
bers with the exudate. It is probable, however, that, 
as a rule, the smaller the number found the less the 
degree of mixed infection. 
3. Cases of clinically secondary infection frequently 
give pure cultures of some one organism which appeared ° 
to be capable of causing the symptoms. 
4, In the majority of severe cases of clinically 
mixed infection many organisms have been found which 
usually have belonged to several different species or 
varieties. 
5. In the majority of cases of clinically non-mixed 
infection very few organisms have been found. 
6. Only bacteria which might cause pathological 
changes were present. 
7. Very few of the organisms found were virulent 
in rabbits, even though coming from severe cases of 
mixed infection. 
The virulence for laboratory animals of bacteria ob- 
tained from the sputum is, therefore, no indication of 
their virulence for man, because of the impossibility of 
reproducing in such animals the exact condition of sus- 
ceptibility present in human infection. 
General Rules in Microscopical Examination of Sputum. 
