52 
BACTERIOLOGY. 
Four types of pneumococci are recognized, called 
Types I, II, III and IV. The first three are distinct, 
clear cut types, but Type IV is composed of a number 
of pneumococci, and so far it has been impossible to 
separate them one from another. The method of de¬ 
termining the type of pneumococcus present in any 
given case of pneumonia is briefly as follows:— 
From the sputum a pure culture of the pneumo¬ 
coccus is obtained either by inoculating mice which are 
very susceptible to pneumococcus infection, and culti¬ 
vating the pneumococcus from the mouse peritoneum 
or by inoculating special media (Avery) with the 
sputum. With the pure culture of the pneumococcus 
agglutination tests are made mixing the growth with 
equal amounts of each of the three types of immune 
sera. If one of the sera agglutinates or clumps the 
pneumococci the test is positive for that particular type. 
If the pneumococcus culture is not agglutinated by any 
one of the immune sera it belongs in Type IV. 
Precipitin tests may also be made. The precipi¬ 
tating substance is contained in the peritoneal wash¬ 
ings of mice or in the culture fluid. These are cen- 
trifugalized to render them perfectly clear and they 
are mixed with the immune sera. If precipitation oc¬ 
curs with any one type of serum the pneumococcus 
belongs to that type. 
Type I pneumococcus is responsible for the largest 
number of pneumonia cases, while Type II is the most 
fatal. Type IV is frequently found in the throats of 
healthy people and is the least virulent of all. 
