Vital Resistance 495 



skilled and the examination thorough. If, however, the 

 surface of the medium be touched with a platinum wire, it 

 is found that its entire surface is covered with a rather 

 thick, invisible layer of a sticky vegetation which the 

 microscope shows to be made up of bacilli. This is de- 

 scribed as the invisible growth. Unfortunately, it is not a 

 constant characteristic, for occasionally a typhoid bacillus 

 will show a distinct yellowish or brownish color. The 

 typical growth seems to take place only when the reaction 

 of the potato is acid. 



Bouillon. In bouillon the only change produced by 

 the growth of the bacillus is a diffuse cloudiness. 



Milk. In milk a very slight and slow acidity is pro- 

 duced. The milk is not coagulated. 



Vital Resistance. The organisms grow well at all 

 ordinary temperatures. The thermal death-point is given 

 by Sternberg as 60 C. According to Klemperer and Levy,* 

 the bacilli can remain vital for three months in distilled 

 water, though in ordinary water the commoner and more 

 vigorous saprophytes outgrow them and cause their dis- 

 appearance in a few days. When buried in the upper 

 layers of the soil, the bacilli retain their vitality for nearly 

 six months. Robertson f found that when planted in soil 

 and occasionally fed by pouring bouillon upon the surface, 

 the typhoid bacillus maintained its vitality for twelve 

 months. He suggests that it may do the same in the soil 

 about leaky drains. 



Cold has little effect upon typhoid bacilli, for some can 

 withstand freezing and thawing several times. Observing 

 that epidemics of typhoid fever had never been traced to 

 polluted ice, Sedgwick and Winslow J made some investiga- 

 tions to determine what quantitative reduction might be 

 brought about by freezing, and accordingly experimentally 

 froze a large number of samples of water intentionally in- 

 fected with large numbers of typhoid bacilli from different 

 sources. It was found that the typhoid bacilli disappeared 

 in proportion to the length of time the water was frozen, 

 and that the reduction averaged 99 per cent, in two weeks. 

 The last two or three germs per thousand appeared very 

 resistant and sometimes remained alive after twelve weeks. 



* "Clinical Bacteriology." 

 f'Brit. Med. Jour.," Jan. 8, 1898. 



J"Jour. Boston Soc. of Med. Sci.," vol. iv, No. 7, p. 181, March 

 20, 1900. 



