112 BACTERIOLOGY OF WATER. AIR, MILK. ETC. 



bacillus is present in quantities of 10 c.c. or less; waters of good quali- 

 ties frequently showing the presence of the colon bacillus in quantities 

 of 100 to 500 c.c. 



It is generally accepted that if a water shows the presence of the 

 colon bacillus in quantities of i c.c. or less, it should be regarded as 

 suspicious. 



At the present time the medium that gives the least source of error 

 in carrying out the quantitative presumptive tests is the bile lactose. 

 It is made by adding i%, of lactose to ox bile, and fermentation tubes 

 of the media showing gas may be considered as very probably con- 

 taining the colon bacillus. The percentage of error with this method 

 is reported to be only 11%, while with glucose fermentation tubes the 

 error is more than 50%. Gas formation is usually shown in 48 hours, 

 but it is advisable to continue the incubation for 72 hours. Even with 

 this method plates should be made. 



3 . As the colon and sewage streptococci ferment lactose with the pro- 

 duction of acid and hence produce pink colonies on lactose litmus agar, 

 much information can be obtained from the proportion existing 

 between the number of pink colonies and those not having such a color. 

 Waters of fair degree of purity rarely give any pink colonies. 



Qualitative Bacteriological Examination. 



General Considerations. In some countries the proportion of 

 liquefying to nonliquefying colonies on gelatin plates is considered of 

 importance. Certain sewage organisms belonging to the proteus and 

 cloaca group liquefy gelatin; consequently, if the proportion of liquefy- 

 ing to nonliquefying be greater than as i to 10, the water is considered 

 suspicious. The test is not considered by American authorities as of 

 any particular value. 



The American Public Health Association recognizes the importance 

 of the information obtained from a comparison of the number of 

 organisms developing at 38 C. and those developing at 20 C. Bacteria 

 whose normal habitat is the intestinal canal naturally develop well at 

 body temperature, while normal water bacteria prefer the average 

 temperature of the water in rivers and lakes. Consequently when the 



