FURTHER NOTE ON BILE SALT 
LACTOSE AGAR 
By ALFRED MacCONKEY 
In a previous article of this Volume (p. 40) of the Thompson Yates Laboratories 
Reports, 1 gave an account of some rough experiments made with this medium. These 
experiments have been continued, and Grubler's pure salts have been used instead of 
the impure commercial salts. Cholalic acid, glycocholic acid, taurocholic acid, sodium 
glycocholate and sodium taurocholate have been experimented with, and it has been 
found that it is only the sodium taurocholate which gives the characteristic reaction, 
namely, a cloudiness or haze in the medium in the vicinity of the colonies. This salt 
also seems to exert a more pronounced inhibitory effect than the glycocholate. The 
medium was made in the following way : — 
1,000 c.c. of tap water were put into a flask, and 2 per cent, peptone, and 
0.5 per cent, of sodium taurocholate, and 1.5 per cent, of agar added. The flask 
was then put into the autoclave and kept at 105° to i io° C. for about one-and-a-half 
hours. The mixture was cleared with white of egg, using as little as possible, and 
filtered. Then 1 per cent, of lactose was added to the filtrate, the medium 
was distributed into test tubes, ten c.c. in each, and sterilisation effected by fifteen 
to twenty minutes in Koch's steriliser on each of three successive days.* The plates 
made were incubated at 42° C. for forty-eight hours. Organisms which produce acid 
with lactose in forty-eight hours give a characteristic reaction in this medium. The 
acid causes a precipitation of the taurocholate with the formation of a cloudiness 
round the colony. Thus there is a marked difference between the colonies of such 
organisms as the B. typhi abdominalis and those of the B. coli communis group. 
B. Typhi Abdominalis. 
Surface colonies — Small, round, raised, and semi-transparent. 
Deep colonies — Lens-shaped, white, and opaque. 
The medium remains quite clear. 
B. Coli Communis. 
Surface colonies -Roundish or irregular in shape with flattened top, 
opaque-white with a small spot ot yellow or orange in 
* If the medium be overheated, changes take place in the lactose and the B. typhus will give a haze. 
