BACILLUS COLI AND SALTS OF BILE 49 
another. Further experiments showed that a percentage of glucose not greater than 
0.5 was most favourable, and that an increase of the peptone to 1 p.c. did not cause an 
increase in the number of organisms other than B. c. c. So a medium composed 
of bile salts 1 p.c, peptone 1 p.c, glucose 0.1 p.c, agar 2 p.c, and tap water, was 
compared with carbolic agar 1 p.c. ; 16 samples of Severn water being plated. The 
results were, however, disappointing as there was no definite correspondence between 
the numbers in the two media. In a few of the samples the numbers were practically 
the same, but in the others the bile salt agar results were much too low. 
An increase in the amount of bile salt caused a decrease in numbers, and the 
addition of 0.5 p.c. of carbolic acid decidedly inhibited. Thus a pure culture of B. c. c. 
gave — 
Nutrient Agar 
Bile salt 1% 
Peptone i% 
Glucose 0.3% 
Carb. Acid 0.5% 
Bile salt 1% 
Peptone 1% 
Glucose 0.3% 
Bile salt 1.5% 
Peptone 1% 
Glucose 0.3% 
Bile salt 2% 
Peptone 1% 
Glucose 0.3% 
109 
20 
67 
30 
33 
I02 
15 
IOI 
36 
78 
I 2 
47 
2 I 
31 
Average No. of Colonies per 
plate ... 
96 
«5 
7i 
24 
33 
On some occasions the addition of KNO s o. i p.c. seemed to increase the 
numbers of B. c. c, and on other occasions the numbers were decreased. So a series 
of trials was made with percentages of KNO s , varying from o. i to 0.5, and the 
conclusion come to was that this salt had an unfavourable effect upon the growth of 
B. c. c. Just prior to coming to this conclusion 20 samples of Severn water were 
plated in bile salt 0.5 p.c, peptone 2 p.c, glucose 0.5 p.c, KNO s o. 1 p.c, agar 
1.5-2 p.c, tap water q. s. The results corresponded more closely than before with 
those given by carbol agar ; but the inhibition was still too great. 
At this stage Dr. Herbert E. Durham suggested that I should once more try 
the effect of different kinds of sugars. The hypothesis he enunciated (as far as I 
understood it) was that if an organism ferments a sugar it presumably gives rise to 
products which are inimical to its own growth, and that, consequently, if of two 
organisms A and B, A ferments both glucose and lactose, while B only ferments 
glucose, then a medium containing lactose should be more favourable to B than to A. 
Glucose, lactose, and mannite were the sugars tried, and it seemed that when employed 
G 
