BACTERIOLOGICAL STUDY OF EETAIL ICE CREAM. 21 



Table 14. — Cultures picked from Endo's plates shotving the number of gas formers . 



Endo plate. 



Colonies 

 picked. 



Cultures showing; gas in lac- 

 tose broth. 



Gas+ 



Gas- 



Gas+ 



1 



Number. 



160 



32 



73 



9 



10 

 21 



Nurriber. 

 91 



7 

 69 

 1 

 

 16 



Nurnber. 



69 



25 



4 



8 



10 



5 



Per cent. 

 56 S7 

 21.87 

 94.52 

 11.11 

 .00 

 76.19 



9 



3 



4 



5 



6 





Froni one plate, of the 10 colonies picked off, none produced gas 

 in lactose broth. Of 160 cultures from suspected colon colonies from 

 sample 1 only 56.87 per cent were gas formers. In sample 3 a high 

 percentage — that is, 94.52 per cent of cultures — were gas formers. 

 It is evident that in many cases it is impossible to count the typical 

 colonies on Endo plates and say that they represent gas formers or 

 colon bacilli. Typical colon bacilli are supposed to form on Endo's 

 agar medium-sized colonies which are distinctly red and have a 

 metallic sheen, but doubtless the appearance of colon colonies is 

 quite variable. In a study of the colonies of 10 different strains of 

 Bacillus coli on Endo's plates it was found that many of the colonies 

 showed no metallic sheen. Some were pinkish instead of red; there 

 was a red coloration of the medium around some colonies and none 

 around others, and many of the colonies were very small. We also 

 found that a number of nongas-forming bacteria of the acid group 

 and some peptonizers would grow readily on Endo's medium, and 

 the colonies had all the appearance of those of some of the 10 strains 

 of B. coli used during these experiments. It must be conceded, 

 however, that a much larger number of gas-forming bacteria can be 

 found in ice cream when using Endo's medium than when plating 

 on litmus-lactose-asparagin agar, and the asparagin plates showed a 

 much higher number than the lactose-bile tubes. The use of Endo's 

 medium for the determination of colon bacilli in ice cream is com- 

 plicated by the fact that bacteria other than gas formers may give 

 the typical reaction of the colon bacilli; also because when a dilution 

 low enough to determine the number of B. coli is used the plate is 

 sometimes overgrown by other bacteria which are present in large 

 numbers. 



It is not the purpose of this paper to discuss the best method for 

 the determination of B. coli in ice cream, for the reason, which is 

 obvious, that an entirely satisfactory method has not yet been per- 

 fected, ftjic.li process has ils merits and objectionable features, but 

 in view of our present knowledge of the colon group of bacteria it is 

 impossible to say llial any one method is the best. 



