20 



BULLETIN 303, U. S. DEPARTMENT OF AGRICULTUKE. 



bacteria were present in 106, or 88.33 per cent, of the samples, and 

 absent in one-tenth of a cubic centimeter in 14, or 11.67 per cent, of 

 the samples. Of the 14 negative samples, 13 were of the winter 

 series and the other one was examined during October. The number 

 of gas formers ranged from 10 to 600,000 per cubic centimeter. 

 Among the entire number of samples the average number of gas- 

 forming bacteria was 16,298 per cubic centimeter. Of the 57 summer 

 samples, the average was 29,544. The winter samples showed an 

 average of 889 gas formers per cubic centimeter. Apparently ice 

 cream contains a far greater number of colon bacilli during the 

 summer months than in the winter season. 



The question naturally arises, Does this asparagin agar bring out all 

 the gas-forming or colon bacilli ? To gain some information on this 

 point a comparison of this medium was made with Endo's medium, 

 which is highly recommended by some authorities for the determina- 

 tion of the colon bacillus. The Endo medium was prepared according 

 to the directions given by Kinyoun and Dieter. 



TableI 3. — Comparison of colon counts on litmus-lactose-asparagin agar and on Endo's 



medium. 



Sam- 



Number of B. coli. 



Sam- 



Number of B. coli. 











ple 



Litmus- 





ple 



Litmus- 





No. 



lactose- 



Endo's 



No. 



lactose- 



Endo's 





asparagin 



medium. 





asparagin 



medium. 





agar. 







agar. 





1 



90 



125,000 



10 



2,700 



11,000 



2 



800 



75,000 



11 



300 



10,000 



3 



1,200 



98, 000 



12 



700 



12,000 



4 



100 



25,000 



13 



30 



5,700 



5 



300 



92,000 



14 



300 



11,400 



6 



800 



3,000 



15 



140 



600 



7 



600 



92,000 



16 



700 



2,200 



8 



4,000 



39,000 



17 



50 



1,200 



As shown in Table 13, the number of colon bacilli determined in 

 litmus-lactose-asparagin agar was decidedly lower than when Endo's 

 medium was used. It was then a question of what percentage of the 

 colonies on Endo plates called colon bacilli were generally gas formers. 

 To determine this percentage typical colonies were picked off from 

 six Endo plates. Inoculations were made in lactose-broth fermenta- 

 tion tubes and the presence of gas recorded after 48 hours' incubation 

 ! at 37° C. (97.5° F.). The percentage of the gas-forming cultures 

 I from colonies which would be called colon colonies is shown in Table 14. 



