350 IVAN C. HALL AND LILLIAN J. ELLEFSON 



tive test a successful isolation was made. The highest total 

 number of positive tests in this series, from which B. coli could 

 not be isolated, consists of course of tests made of heated samples 

 in the standard lactose broth. We feel justified in interpreting 

 these as due mainly to sporulating anaerobes although the 

 figures contain those few cases in which lactolytic aerobic spores 

 were isolated, as do also the few cases showing a test in the 

 presence of the dye. The general tendency of tests not showing 

 B. coli is delayed gas production; taking all the tests in table 7 

 more than 32 per cent occurred after the second day. 



A general conclusion from tables 6 and 7 is that the time of 

 appearance of gas gives no certain clue to the nature of the gas 

 former. 



AMOUNT OF GAS 



Any arbitrary amount of gas required in the presumptive test 

 to be interpreted as a positive test is apt to be. misleading, be- 

 cause the presumptive test frequently deals with mixed cultures, 

 and we believe usually containing other gas formers aside from 

 B. coli. And by no means most of these produce less than 10 

 per cent gas in the closed arms ; on the other hand many cultures 

 which we can not consider other than members of the colon 

 group produce less than 10 per cent gas and indeed certain 

 coliform bacilli ferment lactose with the formation of acid only. 

 While none of the latter has been so designated in this study 

 because we are concerned with the question of gas formers in 

 the presumptive test, yet the writers are strongly inclined to- 

 ward Kligler's (1914) view that acid formation is a true criterion 

 of fermentation by B. coli. Levene (1916), on the contrary, 

 has held that gas formation is the true evidence of fermentation. 

 Frequently, as Levene suggests, no doubt usually, in the case 

 of B. coli, the two phenomena are correlated. In the case of 

 many anaerobes, however, and possibly of some aerobic organ- 

 isms, gas is produced abundantly even in the absence of fer- 

 mentable carbohydrates and it is just this which has made the 

 former attempts at classification of the anaerobic spore bearers 

 on the basis of gas formation so confusing. 



