52 



This lack of constant relation between the dextrose broth fermentation 

 and the actual presence of the colon group has resulted in a gradual abandon- 

 ment of the glucose broth presumptive test in favor of media contain- 

 ing lactose. 



The Lactose Peptone Bile Presumptive Test. The use of bile 

 containing lactose was first suggested in this country by Jackson in 1906. 

 He recommended fresh ox bile containing 1 percent lactose but the in- 

 clusion of 1 percent peptone was soon found to be desirable. The medium 

 is prepared by adding 1 percent peptone and 1 percent lactose to fresh 

 undiluted ox gall (or a 10% solution of dried ox gall) which is placed 

 in fermentation tubes and sterilized in the usual manner. He found, how- 

 ever, that a longer period of incubation than is employed in dextrose 

 broth presumptive test is necessary and recommended 72 hours. For or- 

 dinary work a period of 48 hours is ample. In this medium, the pro- 

 duction of 10 percent or more gas is considered a positive presumptive 

 test. The proportion of positive presumptive tests found on further 

 study to contain the colon group is very high, Stokes and Stoner reporting 

 95 and Gumming 87 percent. Its superiority to dextrose broth is quite 

 marked. Thus Prescott and Winslow, from a study of 176 surface waters 

 in eastern Massachusetts, obtained 120 positive fermentations with dextrose 

 broth and 78 with lactose bile; but Bact. coli was isolated from only 70 

 (58%) of the dextrose as compared with 64 (82%) of the positive bile 

 tubes. Reliance on the dextrose broth presumptive test would have intro- 

 duced an error of 70 percent as compared to an error of 11 percent for the 

 lactose bile medium. 



The superiority of lactose bile, -as a presumptive test, is evident. This 

 is due partly to the selective antiseptic action of the bile salts but also 

 to a considerable extent to the use of lactose which automatically elimin- 

 ates the dextrose positive-lactose negative organisms. It should also be 

 noted that the presence of lactose may indirectly inhibit non-lactose fer- 

 menters for by stimulating the growth of the colon group, the resultant 

 acid production through fermentation of the disaccharid may in turn in- 

 hibit markedly the growth of water forms. In lactose media the probabiK 

 ity of the colon group being overgrown is considerably reduced as com- 

 pared with glucose media where acid may be produced by many other 

 bacilli resulting in the retardation of the colon group itself. 



Consideration of the example cited above indicates, however, that 

 although lactose bile is superior as a presumptive test, the actual number 

 of positive isolations with this medium (64) was about 8.5 percent less 

 than was obtained with dextrose broth (70). Apparently therefore the 

 inhibitory action of the bile is not restricted to water forms, but is exerted 

 to some extent, on the colon group as well. 



The Committee on Standard Methods (1912) suggested that this in- 

 hibitory action is exerted on the weaker members of the colon group, 

 strains which have become attenuated, so to speak, and which are indicative 

 only of remote pollution and therefore of apparently little significance. 

 Thus they say, "In the interpretation of the sanitary quality of a water, it 



