BIOLOGICAL REACTIONS 361 



K The growth on potatoes is important. For several days (at 

 incubation temperature) after inoculation there is apparently no 

 growth. If looked at obliquely, the surface appears wet, and if 

 it 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 appearance. These 

 characteristic appearances are only seen when a fresh potato 

 with an acid reaction lias 

 been used. 



In bouillon incubated 

 at 37 C. for twenty-four 

 hours there is simply 

 a uniform turbidity. 

 Cover-glass preparations 

 made from such some- 

 times show filamentous 

 forms of considerable 

 length without apparent 

 segmentation. 



CoHf/ifions of (,'rntrtl,, 



ttc. The optimum tem- 



I't-nit mv of the typhoid "^^BBBB^^" 



bacillus is about 37 C., 



tlmntrh it flkn flourish^* FlG - HO. Colonies of the typhoid bacillus 



(one superficial and three deep) in a gelatin 



well at the room tern- plate. Three days' growth at room tem- 

 perature. It will not perature. x!5. 

 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 have 

 usually been found to be dead after three weeks (Frankland). 



Biological Reactions. Very important means of identi- 

 fying the typhoid bacillus are found in testing its capa- 

 cities for growth on certain special media. This facilitates 

 its being differentiated from the b. coli and the other 

 members of the coli-typhoid group. The following results will 

 be best appreciated if considered in relation to what is said 

 regarding these other organisms, as the reactions of the typhoid 

 bacilli in differentiating media are largely negative. (See 

 Table, p. 394.) 



The testa with sugars are important. The typhoid bacillus 

 produces acid without gas in maltose, laevulose, glucose, and 



