322 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



Distribution. Several distinct types, races or varieties 

 of pneumococcus have been described. Some of these are 

 not uncommon in the normal mouth and throat. Others 

 are usually associated with disease production or are pres- 

 ent in the mouths and throats of those associated with those 

 who are diseased. Within the same type there are appar- 

 ently considerable variations in virulence. Probably in no 

 disease do we have a better illustration of the fact that pro- 

 duction of disease is due to lack of immunity on the part of 

 the individual attacked or to high virulence on the part of 

 the attacking organism. In other words, the ability to pro- 

 duce disease always depends upon the relative virulence of 

 the organism and the resisting power of the individual. 



Morphology. The Diplococcus pneumonia usually oc- 

 curs in pairs, more rarely in chains with few elements. The 

 cells are sometimes spherical, but are most frequently some- 

 what flattened at the point of contact and with the opposite 

 side somewhat elongated and pointed. This lance-shaped 

 appearance has given rise to the name Diplococcus lanceo- 

 latus. Capsules may usually be readily demonstrated in 

 sputum or in body fluids. The organism stains readily and 

 is Gram-positive. 



Cultural Characters. The pneumococcus is most readily 

 isolated upon blood agar, that is, agar which has been 

 melted and has had added to it a small amount of fresh 

 blood which has been drawn under aseptic precautions to 

 insure its sterility. If culture is desired from sputum 

 where other organisms are apt to be present, it is usually 

 necessary first to inoculate a suitable laboratory animal, 

 such as a mouse. The colonies are never luxuriant, the 

 organisms developing as discrete, transparent, dewdroplike 

 colonies upon the surface. Broth is clouded and milk may 

 be somewhat acidified. 



Physiology. The pneumococcus grows but little or not 

 at all at temperatures lower than blood heat. Acids are 



