54 



both media concludes that if both lactose broth and lactose bile are positive 

 in 24 hours, the probability of the presence of the colon group is very 

 great (about 98% confirmed by subsequent tests) and that, in general, 

 if there is fermentation in both bile and broth media the presumptive test 

 is reliable in about 75 percent, but if either medium is negative, then the 

 proportion of positively confirmed tubes is very much less, even as low 

 as 25 to 45%. This high proportion of positively confirmed presumptive 

 tests, when both broth and bile are positive, may find a ready explanation 

 in the observations of Creel. Most of the spurious presumptive tests are 

 due to the presence of spore forming anaerobic lactose fermenters. Creel 

 found that some of these anaerobes are capable of growing in broth 

 but not in bile. He calls this "Group A" whereas another "Group B" 

 will grow readily in bile but not in broth. Thus a positive test in both 

 lactose broth and lactose bile automatically eliminates both of these anaer- 

 obic spore formers and thereby accounts for the high proportion of pos- 

 itively confirmed tests. 



Opposed to the views expressed above is that of Hale who recently 

 (1917) stated that from a careful detailed, and extensive comparative 

 study on the two media at the Mount Prospect Laboratory, he considers 

 the results were all in favor of bile, that gas formation was more rapid, 

 produced in larger amounts, and that the B. clostridium welchii types (an 

 aerobic spore formers) were less frequent. Probably his experiences 

 may be explained by the fact that, in these experiments, he employed a 5 per- 

 cent dried bile, whereas the observations of other investigators were con- 

 cerned with the original medium, containing either undiluted ox gall or 

 10 percent of the dried bile. 



The work of Salter and the author in the Laboratory of the Iowa En- 

 gineering Experiment Station indicated quite clearly that bile may inhibit 

 or stimulate multiplication of Bact. coli, depending upon the concentration 

 of bile salts. With less than 0.5 percent bile salts a stimulating action 

 on the rate of multiplication was observed, whereas higher concentrations 

 were markedly inhibitory. The effect was studied by observing changes in 

 the generation time (See Table XXIV.). 



Since bacteria divide by simple fission, the number of organisms pres- 

 ent at any time in an actively growing culture may be expressed thus: 

 b = B X 2 n 



Where "B" is the initial number of bacteria 

 "n" the number of generations 

 "b" the number of bacteria after "n" generations 



If the time elapsed is "t" then "g" the generation time equals 

 t t 



or "n" equals . 



n g 



Thus b = B X 2 n = B X 2% 



t Iog2 

 ~ logb logB 



