404 APPLIED BACTERIOLOGY 



solution. The suspected water is examined as follows : To 

 100 c.c. of the water are added 1 gramme each of pure 

 peptone and common salt ; the mixture is made faintly 

 alkaline with sodium carbonate, and then incubated at 

 37° C. At intervals of ten, fifteen, and twenty hours 

 respectively, cover-glass preparations are prepared from the 

 top of the liquid ; these are then microscopically examined 

 for spirilla. At the same time agar plates are prepared, 

 and incubated at blood-heat. Any colonies that appear 

 which resemble the cholera spirillum are examined micro- 

 scopically ; if the organisms are comma-shaped, they are at 

 once subcultured into broth and other media. The broth- 

 tubes after incubation are tested for the indol reaction, and 

 if possible by animal inoculation. 



It is well known that many impure, especially sewage- 

 contaminated waters, contain spirilla and comma-shaped 

 bacteria, many of which strongly resemble the cholera 

 organism in many ways ; care must therefore be taken that 

 none of these are mistaken for the true cholera organism. 

 None of these spirilla forms, however, give the indol 

 reaction, and Koch is of opinion that the presence of the 

 cholera bacillus in the water is proved if comma-shaped 

 organisms are found which exhibit the indol reaction, and 

 which give rise to the characteristic symptoms on inocula- 

 tion into the peritoneum of guinea-pigs. 



BACTERIOLOGICAL EXAMINATION OF FILTERS. 



When chemical analysis was the only means at com- 

 mand for examining water, it was found that in a majority 

 of cases those waters which had been statistically con- 

 victed of spreading disease contained an excess of organic 

 matter. Hence it was inferred that the organic matter 

 was the cause of the disease : and filters were constructed 



