324 TYPHOID FEVER. 



agar. Their characters, as seen under a low power of the 

 microscope, also correspond. 



In stroke-cultures on agar there is a bluish-grey film of 

 growth, with fairly regular margins, but without any character- 

 istic features. This film is loosely attached to the surface, and 

 can be easily scraped off. 



The growth on potatoes is most important For several days 

 (at ordinary temperature) after inoculation there is apparently 

 no growth. If looked at obliquely, the surface appears wet, and 

 if the surface is scraped with the platinum loop, a glistening 

 track is left ; a cover-glass preparation shows numerous bacilli. 

 Later, however, a slight pellicle with a dull, somewhat velvety 

 surface, may appear, and this may even assume a brown appear- 

 ance. These characteristic appearances are only seen when a 

 fresh potato with an acid reaction has been used. In America, 

 at least, the so-called invisible growth upon potato which was for- 

 merly looked upon as the most important means of recognition 

 has proven to be a very unreliable test. For, on potatoes from 

 some sections of the country, a growth quite like that of B. coli 

 is more often the rule than the exception. This can possibly be 

 due either to the' variety of the potato, or to some variation in 

 its composition dependent upon the character of the soil in which 

 it grows, acidity apparently having nothing to do with the phe- 

 nomena of visible or invisible growths. 



In bouillon incubated at 37 C. for twenty-four hours, there 

 is simply a uniform turbidity. Cover-glass preparations made 

 from such sometimes show filamentous forms of considerable 

 length, without apparent segmentation. 



In litmus milk a slight degree of acidity is produced, causing 

 the milk to assume a lilac colour ; more rarely, in some instances 

 this acidity diminishes and is lost, being replaced by a strong 

 degree of alkalinity. No coagulation of casein occurs. 



Conditions of Growth, etc. The optimum temperature of 

 the typhoid bacillus is about 37 C., though it also flourishes 

 well at the room temperature. It will not grow below 9 C. or 

 above 42 C. Growth takes place in anaerobic as well as in 

 aerobic conditions. Its powers of resistance correspond with 

 those of most non-sporing bacteria. It is killed by exposure 

 for half an hour at 60 C., or for two or three minutes at 

 100 C. Typhoid bacilli kept in distilled or ordinary tap 



