BACTERIA AND RURAL WATER SUPPLIES 15 



than were obtained with S. M. lactose broth, and both of these media gave much 

 better results than did B. C. P. "Standard Methods" (9) includes B. G. B. as a 

 tentative method, and it has been proposed as a confirmatory medium in the 

 place of Endo's or eosin methylene blue agars. In this study, however, no par- 

 ticular advantage in the use of B. G. B. was discovered that would give it prefer- 

 ence over S. M. lactose broth for presumptive tests. Use was made also of B. G. B. 

 as a confirmatory medium. The reader is referred to the published article (6) 

 for details. 



Table 2. — Comparison of S. M. Lactose Broth, B. G. B. Broth, 

 AND B. C. P. Broth as Presumptive Media. 



Lactose broth B. G. B. broth B. C. P. broth 



Number of tubes inoculated 

 Percent positive, 24 hours 

 Percent confirmed, 24 hours 

 Percent positive, 48 hours* 

 Percent confirmed, 48 hours* 



*The 48-hour figures are totals for the whole period and include the 24-hour figures. 



D. False Presumptive Tests 



This study (10) was made to test the value of several media in detecting or 

 eliminating false presumptive reactions. Three fluid media and four solid media 

 were employed. The fluid media were Standard Methods lactose broth, brilliant 

 green bile (B. G. B.), and brom cresol purple (B. C. P.), all of which had been 

 employed in the preceding study. The solid media were Endo's agar, eosin 

 methylene blue agar (E. M. B.), MacConkey's agar, and the special agar of Ton- 

 ney and Noble (11). The latter medium was obtained in dehydrated form (Difco) 

 and the others were prepared according to directions in the Standard Methods 

 of Water Analysis (9). 



Several combinations of bacterial species previously reported to cause false 

 presumptive gas production in lactose broth were investigated. Pure laboratory 

 stock cultures were employed .for making the combinations, which were as follows: 



A. Staphylococcus and Pseudomonas aeruginosa. 



B. Staphylococcus and Proteus vulgaris. 



C. Staphylococcus and Salmonella schottmuelleri. 



D. Streptococcus faecalis and Proteus vulgaris. 



E. Streptococcus faecalis and Salmonella schottmuelleri. 



Three staphylococcus strains, S. aureus, S. citreus, and S. albus, were employed 

 separately in the combinations. 



The media were inoculated with these combinations of organisms and examined 

 after 24 hours' incubation. All negative results were incubated an additional 24 

 hours and e.xamined again. Results are shown in table 3. 



Lactose broth failed to give a false presumptive with combination A, even 

 though such a combination has. been reported to form gas from lactose. B. C. P. 

 gave false presumptive tests with all of the combinations, although fermentation 

 was slow. B. G. B. gave the best results of the fluid media and should be the 

 best of the three for eliminating false presumptives. It has been claimed that 



