

46 



Glucose broth consists of ordinary broth, neutral to phenolphthalein, 

 with 1 percent glucose. This medium was the one most commonly em- 

 ployed in the United States prior to 1906. The probable presence of the 

 colon group is indicated by the production of gas after 24 to 48 hours 

 incubation at body temperature. 



Phenol broth consists of ordinary broth containing 0.1 percent car- 

 bolic acid which exerts a selective antiseptic action on the normal water 

 bacteria thereby supposedly favoring the growth of the colon types. This 

 medium is employed by the French, but it has been observed that the phenol 

 exerts an antiseptic action on some members of the colon group and that 

 its employment is particularly objectionable with water of fairly good 

 quality. 



Liver broth has been found particularly efficacious for isolation of 

 colon types and in 1912 the Standard Methods for Water Analysis of the 

 A. P. H. A. recommended that, "If a further study of all gas forming 

 bacteria, including attentuated forms, is desirable, then liver broth should 

 be employed in preference to the usual dextrose broth, as it gives a larger 

 number of attentuated forms, has better rejuvenating powers, and gives 

 fewer anomalies, and greater and more rapid gas production." For routine 

 water work, however, liver broth has not found favor. 



Since 1906, there has been a tendency to substitute lactose for dextrose 

 in the preliminary enrichment mediums. It is quite surprising that this 

 was not done sooner considering that the essential characteristic of the 

 colon group is lactose fermentation. 



Lactose broth, which is nutrient broth containing 0.5 to 1.0 percent 

 lactose, is most commonly employed in the United States at the present 

 time and is the medium recommended in the Standard Methods of Water 

 Analysis of the A. P. H. A. of 1917 and 1920. It is supplanting the lac- 

 tose peptone bile medium of Jackson, which was extensively used between 

 1912 and 1917. In both of these mediums the probable presence of the 

 colon group is indicated by gas production from the lactose. 



Lactose bile salt broth of MacConkey consists of 0.5 percent lactose, 

 2% peptone, 0.5 percent sodium taurocholate with neutral red as an in- 

 dicator. The colon group will produce gas and acid indicated by red 

 coloration of the liquid after 24 to 48 hours at 37 C. 



Technique for Isolation. The steps in the isolation of the colon 

 group from water may be summarized as follows: 



(1) Place various quantities of the water under examination (10 c. c., 

 1 c. c., 0.1 c. c., etc.) into Smith or Durham fermentation tubes containing 

 the preliminary enrichment medium. 



(2) Incubate at the body temperature (37 C.) for 24 hours. If 

 gas is formed, it is taken as presumptive evidence that Bact. coli or its close 

 allies are present. 



(3) The next step is then to isolate these organisms on some one of 

 the solid media enumerated above. This may be done in one of the fol- 

 lowing ways: 



