48 THOMPSON YATES LABORATORIES REPORT 
Several samples (about 12) of Severn water, from the neighbourhood of 
Shrewsbury, were plated in this bile salt agar, and in carbolic agar 1 °/ 00 . The 
results showed that the bile salt agar inhibited some 70 p.c. of the B. c. c. The 
problem, then, was to find out the composition which would give results equal to 
carbolic agar. 
Numerous alterations in, and additions to, the medium were made, and in the 
course of these experiments it was found that — 
Bile Salts — 1 p.c. and over — have an inhibitory effect upon the great majority 
of organisms, but favour the growth of moulds. 
KNO3, AmCl, NaCl — 0.5 p.c. — exercise an unfavourable influence upon 
B. c. c. while encouraging the growth of other organisms. 
Na 2 HP0 4 — 0.5 p.c. — seems specially favourable to earth organisms. 
Glucose — 0.5 p.c. — tends to inhibit the ordinary earth and water bacteria, 
and, if in a larger percentage, B. c. c. also ; but assists the growth of 
moulds and yeasts. A small percentage of glucose benefits B. c. c, 
especially the colonies in the depth. 
One example will show the effect of glucose and of salts upon B. c. c. A pure 
culture of B. c. c. was taken and diluted with sterile water, plated, and incubated 
at 42 0 C. 
Nutrient 
Agar 
Bile salt i% 
Peptone 0.2% 
Bile salt 1% 
Peptone 0.2% 
AmCl. 0.5% 
KN0 3 0.5% 
Bile salt 1% 
Peptone 0.2% 
Glucose 0.5% 
Bile salt 1% 
Peptone 0.2% 
Glucose 1% 
Bile salt 1% 
Peptone 0.2% 
Glucose 1.5% 
870 
440 
260 
960 
496 
270 
JOO 
230 
200 
800 
600 
240 
960 
300 
168 
650 
484 
228' 
Average No. of Colonies per 
plate 
844 
323 
209 
803 
527 
246 
It B. c. c. be plated in a medium containing bile salts, peptone, and glucose 
only, the colonies at the end of 24 hours appear much larger than usual, and have a 
hazy outline, owing to the formation of a precipitate round each colony. If there are 
many colonies the plate looks ' milky,' and has a yellowish colour. This precipitate 
is caused by an acid produced by the organism. If a drop of NH 3 solution be placed 
upon the colony the haze soon disappears, and the colony is seen to be of the usual 
size. The addition of salts to the medium tends to prevent the formation of this 
precipitate, and thus to remove a means of distinguishing one kind of colony from 
