49 



water at 46 C.) and neutral red broth, but these are not employed to any 

 extent in the United States and will not be considered further. 



The Glucose Broth Presumptive Test. The test consists of the 

 determination of the amount of gas and the ratio of hydrogen to carbon 

 dioxide in glucose broth after 24 hours incubation at the body temperature. 



Production of 25 to 70 percent gas, one-third of which is carbon 

 dioxide and the remainder hydrogen (that is H/C0 2 =2 to 1) is con- 

 sidered a "typical" test and is regarded as indicating the presence of 

 Bact. coli or closely related bacilli. Thus Irons stated that when the pro- 

 portion of carbon dioxide is approximately 33 percent Bact. coli commune 

 is almost invariably present. 



If the amount of gas is (a) between 10 and 25 percent, (b) more than 

 70 per cent or (c) if the proportion of carbon dioxide is more than 40 

 percent of the total volume of gas formed, the reaction is considered 

 "atypical" and the probable presence of the colon group is doubtful. 



If there is less than 10 percent gas, or if no gas is formed at all, the 

 probability of isolation of the colon group is very slight and the test is 

 considered negative. 



The early studies indicated that the colon group was successfully iso- 

 lated from about 70 percent of positive presumptive tests. More recent 

 and detailed studies have shown, however, that the proportion of presump- 

 tive tests confirmed by further observation is not constant but is effected by 

 the character, treatment, etc. of the water under examination and the season 

 of the year. 



Objections to the Glucose Broth Presumptive Test. The ob- 

 jections to the glucose broth presumptive test may be summarized as fol- 

 lows: 



1. The volume of gas produced is not a significant criterion of the 

 probable presence of the Bact. coli or its close allies. 



2. The gas ratio, as determined in the Smith tube, is unreliable. 



3. There are many glucose fermenting species which are incapable of 

 attacking lactose. 



4. The reliability of the glucose presumptive test varies with (a) 

 the season of the year, and (b) differs for raw and treated waters. 



5. The glucose broth medium is particularly subject to overgrowths. 

 That 25 to 75 per cent gas is not necessarily characteristic of the colon 



group and that even pure cultures produce as little as 10 percent gas has 

 been demonstrated by Fuller and Ferguson, Longley and Batton, Hale and 

 Melia, and others. They also have shown that the so-called 'typical' gas 

 ratio is not at all characteristic of the colon group. Thus of 818 tubes of 

 sterilized water, inoculated with Bact. coli but containing no other gas 

 former, only 474 or 58 percent gave the typical gas volume and gas ratio. 

 The errors inherent in the gas ratio as determined in the Smith tube have 

 already been discussed in detail in Chapter II. 



Fermentation of glucose with gas production is not a very reliable in- 

 dication that the organism will also ferment lactose. There are many 

 species of bacteria capable of attacking the monosaccharids, but which 



