55 



TABLE XXIV. EFFECT OF CONCENTRATION OF COMMERCIAL BILE SALTS 



ON GROWTH OF BACT. COLl IN 0.5 PERCENT PEPTONE WATER. 



(Temp, of incubation 37 C.) 



The experience of Hale, that Cl. Welchii forms were less frequently 

 encountered in bile than in broth, may merely mean that in the waters he 

 worked with Creel's anaerobe of "Group A" were more frequent than 

 "Group B". In other laboratories, the reverse might be true. 



The relative merits of lactose broth and lactose bile may be summed 

 up as follows: 



1. Lactose bile is a more reliable presumptive test but a greater 

 proportion of the colon group may be detected by preliminary enrichment 

 in lactose broth. 



2. If the proper concentration of bile salts could be determined the 

 bile medium would probably be preferable. For the present, considering 

 the difficulty of obtaining bile of constant composition or the chemically 

 pure salts, and in view of our insufficient knowledge as to the optimum 

 concentration of bile salts, it seems best to employ lactose broth as a more 

 uniform medium may thus be obtained in different laboratories. It is 

 very probable that if a standardized evaporated bile were available a con- 

 concentration of 1 to 2 percent in lactose peptone water would be superior 

 to lactose broth. 



THE LACTOSE BROTH PRESUMPTIVE TEST. 



The use of lactose broth for preliminary enrichment and as a pre- 

 sumptive test has received considerable impetus through the investigations 

 of the Public Health Service and has been accepted by the Committee 

 on Standard Methods of Water Analysis of the A. P. H. A. (1917). 



Factors Affecting the Preparation of Lactose Broth. In 1917, 

 the Standard lactose broth consisted of 0.3 percent beef extract and 0.5 

 percent peptone with 1.0 percent lactose, the reaction being neutral to 

 phenolphthalein. The medium was tubed and sterilized in the autoclave 

 at 15 pounds (120 C.) for 15 minutes. 



Hasseltine pointed out that sterilization in the autoclave was objection- 

 able as it brought about a breaking down of the lactose resulting in a con- 

 siderable increase in the number of unconfirmable presumptive tests as com- 



