152 BACTERIOLOGY OF WATER, AIR, MILK, ETC. 



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

 any particular value. 



The American Public Health Association recognizes the importance of the informa- 

 tion obtained from a comparison of the number of organisms developing at 38C. 

 and those developing at 20C. 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 

 number of organisms developing at 38C. at all approximates the number developing 

 at 2oC., there is a strong suspicion that sewage organisms may be present. Normal 

 waters give proportions of i to 25 or i to 50, while in sewage contaminated waters the 

 proportion may be as i to 4 or less. 



In addition, the appearance of pink colonies on the lactose litmus 

 agar is of great assistance in judging of a water. Both sewage strepto- 

 cocci and the colon bacillus give pink colonies those of the streptococci 

 are smaller and more vermilion in color. Microscopic examination 

 will differentiate the cocci from the bacilli. It is well to bear in mind 

 that the pink colonies after twenty-four hours may turn blue in forty- 

 eight hours from the development of ammonia and amines. Conse- 

 quently the lactose litmus agar plates should be studied after twenty- 

 four hours. 



A good water supply will rarely show a pink colony, while in a sewage-contami- 

 nated one the pink colonies will probably predominate. 



A commission composed of eminent American bacteriologists and 

 sanitarians have recommended the following as maximum limits of 

 bacteriological impurity : 



1. The total number of bacteria developing on standard agar plates, 

 incubated twenty-four hours at 37C.; shall not exceed 100 per c.c. 

 Provided that the estimate shall be made from not less than two plates, 

 showing such numbers and distribution of colonies as to indicate that 

 the estimate is reliable and accurate. 



2. Not more than one out of five 10 c.c. portions of any sample 

 examined shall show the presence of organisms of the Bacillus coli 

 group when tested as follows: 



(a) Five 10 c.c. portions of each sample tested shall be planted, each in a fermenta- 

 tion tube containing not less than 30 c.c. of lactose peptone broth. These shall be 

 incubated forty-eight hours at 37C. and observed to note gas formation. 



(6) From each tube showing gas, more than 5% of the closed arm of fermenta- 

 tion tube, plates shall be made, after forty-eight hours' incubation, upon lactose 

 litmus agar or Endo's medium. 



(c) When plate colonies resembling B. coli develop upon either of these plate 



