1024 BACTERIA IN AIR, SOIL, WATER, AND MILK 



intestines of domestic animals. According to Poujol, B. coli does 

 not even always point to fecal contamination, since this author was 

 able to find the bacillus in the water of a number of wells where 

 no possible contamination of any sort could be traced. Prescott 18 

 explains this, as well as similar cases, by the fact that organisms 

 of the colon group may occasionally be parasitic upon plants. 



The opinions of hygienists are widely at variance as to the value 

 of the colon test. While the discovery of isolated bacilli of the colon 

 group may therefore be of little value, it is nevertheless safe to 

 follow the opinion of Houston, 17 who states that the discovery of 

 B. coli in considerable numbers invariably points to sewage pollu- 

 tion, and that the absolute absence of B. coli is, as a rule, reliable 

 evidence of purity. 



Rosenau states that a ground water should be condemned even 

 if only a few colon bacilli are found, for, as he put it, ' * these bacteria 

 have no business in a soil-filtered and properly protected well or 

 spring-water." Surface waters, however, may easily contain a few 

 colon bacilli without necessarily having been exposed to contamina- 

 tion by human forces. The limit of safety, Rosenau states, is one 

 colon bacillus per c.c. If more are present the water should be 

 regarded as suspicious. If more than 10 per c.c. are found the 

 water must be regarded as dangerous and unqualifiedly condemned. 



Presumptive Colon Tests. For this purpose, a large number of 

 methods have been devised. In examining sewage or other polluted 

 waters in which the number of colon bacilli is comparatively large, 

 the direct use of lactose litmus agar plates yields excellent results. 

 The method advised by the American Public Health Association is 

 as follows : 



"Add the quantities of water or sewage to be tested to fermentation tubes 

 holding at least forty cubic centimeters Q lactose bile, 18 incubate at 37 C. 

 and note the production of gas. The standard time for observing gas produc- 

 tion is forty-eight hours. Small numbers of somewhat attenuated B. coli 

 may require three days to form gas. Attenuated B. coli does not represent 

 recent contamination and all B. coli not attenuated grows readily in lactose 

 bile. No other organism except B. Welchii gives such a test in lactose bile. 

 B. Welchii is of rather rare occurrence in water, is of fecal origin, is almost 

 invariably accompanied by B. coli, and the sanitary significance is the same. 



"Prescott, Science, xv, 1903. 



17 Houston, Kep. Medical Officer, Local Gov. Board, London, 1900. 



18 Prescott, Science, xvi, 1902. 



