ELIMINATION OF SPURIOUS TESTS FOR B. COLI 337 



modification (1914) of the Durham fermentation tube is less 

 apt to provide anaerobic conditions than the usual type. 



In order to test the applicability of gentian violet in actual 

 water examinations, arrangement was made to secure samples 

 showing positive presumptive tests from the Bureau of Sanitary 

 Engineering of the State Board of Health of California. Our 

 sincere thanks are due Messrs. C. G. Gillespie and Frank Bach- 

 man for their courtesy in this matter. Inasmuch as the samples 

 were frequently several days old before we received them it is 

 not surprising that our findings do not coincide exactly. 



TECHNIC 



Upon receipt the bottles were thoroughly shaken and the 

 water divided into two parts; one portion was heated to 56°- 

 60°C for thirty minutes, the other being left unheated. Each 

 of these was again subdivided into two parts, one of which was 

 inoculated into a Durham fermentation tube containing 2 per 

 cent lactose broth with gentian violet, the other being inocu- 

 lated into a tube of similar medium without gentian violet. The 

 tubes contained 10 cc. of double strength media and the amount 

 of water inoculated was 10 cc. in each case. The final concen- 

 tration of lactose was therefore 1 per cent; the final concentra- 

 tion of dye was 1-100,000 in the first 21 samples, after that 

 1-20,000. Tests at the latter concentration were also made 

 with what remained of the first 21 samples. The change was 

 made because gas was encountered several times in the unheated 

 sample inoculated into dye broth from which no aerobic gas 

 former could be isolated; we interpreted this result as due to 

 imperfect inhibition of gas forming anaerobes, indeed we had 

 anticipated that B. coli should be isolated from every unheated 

 sample showing gas in the presence of 1-100,000 gentian violet. 

 As later shown even 1-20,000 is not sufficient to give this re- 

 sult, due, we believe, to the somewhat better conditions of an- 

 aerobiosis provided by the presence of aerobic organisms. 



Incubation was at 37° until gas formation occurred, if within 

 five days. Plates of litmus lactose agar were then streaked 



