20 
BULLETIN 303, U. S. DEPABTMENT OF AGBICULTUBE. 
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 En do'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 1 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 
from colonies which would be called colon colonies is shown in Table 14. 
