VITALITY. 271 



In gelatin stab cultures the principal growth occurs on the 

 surface of the medium. Along the track of the needle 

 development is extremely slight. The surface growth re- 

 sembles the superficial colonies on the plate. The gelatin is 

 not liquefied. 



Stroke cultures on agar-agar and blood-serum are not char- 

 acteristic. A thin transparent grayish coating is formed 

 along the line of inoculation. Bouillon is clouded, with the 

 occasional formation of a delicate surface membrane. 



The growth on potato is quite characteristic. It is usually 

 referred to as the " invisible growth.^ 7 Even after two or 

 three days there is no apparent growth on the surface of the 

 potato, but when the platinum needle is slowly and gently 

 passed over the surface it meets with resistance, due to a 

 heavy moist but absolutely colorless film which has formed. 

 By reflected light the growth is made visible. Occasionally 

 the growth is of an ochre color, or even brown with a green- 

 ish tinge. This growth resembles that of the colon bacillus 

 on potato. 



The typhoid bacillus flourishes in milk without changing 

 the medium in any way. At times it may produce a very 

 slight amount of acid, although as a rule it is not acid-produc- 

 ing. Neither does it cause fermentation. It does not pro- 

 duce aromatics. These characteristics serve to identify the 

 typhoid bacillus, and to differentiate it from the colon bacil- 

 lus, which resembles it in every other respect. 



Vitality : One of the peculiarities of this organism is its 

 resisting power. It will remain alive in distilled water for 

 three months. In ordinary water the bacilli disappear within 

 a week or two because of the vigorous growth of the other 

 bacteria contained in the water. In quiescent pools of water 

 the typhoid bacillus will remain alive for a month. When 

 contained in water for any length of time, its appearance is so 

 changed that recognition is impossible. It has been found to 

 retain its vitality in milk for five weeks. Transplanted into 

 the upper layers of the soil, it remains alive for nearly six 

 months. When nutrient bouillon is poured over the soil the 

 germ will retain its vitality for twelve months. In feces it 



