100 BACTERIOLOGICAL EXAMINATION OF WATER. 



(4) In the investigations of the State Board of Health, 

 Massachusetts, the method employed in testing waters for B. 

 coli varied slightly with the water under examination. For 

 waters in which B. coli was very rarely found, a qualitative test 

 was used ; while, for waters generally containing that organism 

 in some numbers, a quantitative method was used. 



The qualitative test consists in inoculating a Smithes fer- 

 mentation tube (a bent tube with one end closed), containing 

 glucose-bouillon, with one cubic centimetre of the water. This 

 tube is incubated at 38 C., and if after twelve hours growth 

 gas has collected in the closed arm the tube is taken out and 

 species tests made. The absence of gas is considered final, 

 there being no B. coli in the water. In order to make the 

 species tests, some of the bouillon from the fermented tube is 

 taken out and diluted with sterile water; a portion is then 

 plated on lactose-litmus-agar. This plate is grown for twelve 

 hours at 38 C. ; if at the end of this time any red colonies are 

 found on the plate a few of these are fished, planted out on 

 agar-slopes, and grown for twelve hours as before. If the culture 

 resembles B. coli it is then sub-cultured in milk, nitrate solu- 

 tion, Dunham's solution (1 per cent, salt and 1 per cent. Witters 

 peptone), and Smith's solution (meat bouillon containing 2 per 

 cent, glucose and 1 per cent, peptone) ; a gelatine-stab is also 

 made. The Smith tubes, nitrate solution and milk, are grown 

 for twelve hours at 38 C. The Dunham's solution is grown at 

 the same temperature for three days, when it is tested for indol 

 in the usual manner. The gelatine must be grown at 20 C. for 

 at least seven days, as some species of bacteria resembling B. 

 coli liquefy gelatine very slowly. 



In quantitative work, when dealing with waters in w r hich B. 

 coli is known to be present, the preliminary test in Smith's 

 solution is omitted. One cubic centimetre of the water is 

 plated direct on lactose-litmus-agar, and the plates grown for 

 twelve hours at 38 C. At the end of this time the plates are 

 taken out and the number of red colonies counted. The red 

 colonies are fished and sub-cultured as in the qualitative 

 method. The qualitative method is considered the more 

 delicate, as duplicate tests often showed B. coli present by the 

 gas test but absent by the plate method. This is probably due 



