426 BACTERIOLOGY. 



pus -producing 1 germs are included in the scope of a water 

 examination. 



Typhoid bacillus. The typhoid bacillus when once intro- 

 duced into a water-supply does not necessarily remain in 

 such water for a considerable length of time. The results 

 of -many experiments made by different observers would in- 

 dicate that the typhoid bacillus when introduced into or- 

 dinary water may remain alive for about a week. This is 

 about the limit when they are placed in very pure water kept 

 at a low temperature, i. e. , under conditions very unfavor- 

 able to the vitality of the org-anism. This applies to natural 

 as well as distilled water, whether sterilized or unsterilized. 



When the temperature is above 10, and especially, when 

 the water contains an appreciable quantity of organic mat- 

 ter, the typhoid bacilli may persist for a much longer per- 

 iod. The organisms under these conditions may actually 

 increase in number and retain their vitality for weeks and 

 months. Hence an impure water, as indicated by a chem- 

 ical analysis, is to be regarded as suspicious because it may 

 favor the growth and persistence of typhoid germs when 

 once introduced. 



The recognition of the typhoid bacillus in water is by 

 no means an easy matter. Usually an examination is called 

 for after an outbreak of the disease has taken place. That 

 is to say, the period of incubation, which extends from 1 to 

 2 weeks, is allowed to pass before a search is instituted for 

 the organism. This time is sufficient, under conditions in- 

 dicated above, for the typhoid germ to disappear, and, con- 

 sequently, the examination in such cases will be negative. 

 Moreover, even in water which in the beginning was 

 favorable to the growth of this organism, the detection is 

 still difficult. At the time of the examination, the germs 

 may already be dying out and decreasing in number. The 

 small volume of water which is necessarily taken for analy- 

 sis may, or may not, be a perfect sample of the whole. 



