CHARACTERS OF CULTURE 



363 



cultures there is a thin .bluish-white .film, but it does not spread 

 to such an extent as in the case of the surface growth of a stab 

 culture (Fig. 108, B). In gelatin plates also the superficial and 

 deep colonies present corresponding differences ; on gelatin the 

 surface colonies are rather more transparent than those on agar. 

 Their characters, as seen under a low power of the microscope, 

 also correspond. If a gelatin tube be inoculated and incubated 

 at 37° C, a uniform turbidity is produced. 



In stroke cultures on at/ar there is "a bluish-grey film of 

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

 istic features. This film is moist, loosely attached to the sur- 

 face, and can be easily 

 scraped off. 



The growth on pota- 

 toes is important. For 

 several days (at incuba- 

 tion temperature) after 

 inoculation there is ap- 

 parently no growth. If 

 looked at obliquely, the 

 surface appears wet, and 

 if it is scraped with. the 

 platinum loop, a glisten- 

 ing track is left : a 

 cover-glass preparation 

 shows numerous bacilli. 

 Later, however, a slight 

 pellicle with a dull, 

 somewhat velvety sur- 

 face may appear, and 

 this may even assume a 

 brown appearance. These characteristic appearances are only 

 seen when a fresh potato with an acid reaction has been used. 



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. 



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. 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 in ordinary tap water 



Fig. 109. — Colonies of the typhoid bacillus 

 (one superficial and three deep) on a gelatin 

 plate. Three days' growth at room tem- 

 perature, x 15. 



