244 BACTERIOLOGY. 



entirely useless for bacteriological investigations. It 

 cannot be too much emphasized that materials which 

 are not immediately used should be sent to the labora- 

 tory as quickly as possible, for in such substances as 

 feces, where enormous numbers of various kinds of 

 bacteria are present, those which we seek most, such as 

 the typhoid bacilli, frequently succumb to the delete- 

 rious products of the other bacteria present. Even 

 when abundantly present living typhoid bacilli may 

 entirely disappear from the feces in the course of even 

 twelve hours, while at other times they may remain 

 present for weeks. These differences depend on the 

 associated organisms present, the chemical constitution 

 of the feces or urine, and the conditions under which 

 the material is obtained. 



