A New Culture Medium for the Isolation of Bacillus Typhosus from Stools

Abstract

isolation of typhoid bacilli from feces the one at present most widely used is that devised by Endo in 1903. It allows the development of a high percentage of the typhoid organisms inoculated on it, but does not inhibit to any practical extent the growth of those fecal bacteria which develop on ordinary nutrient agar. The great value of the method lies in its sharp differentiation between the lactose-fermenting organisms and those organisms which do not ferment lactose. The colonies of the lactose-fermenting organisms are colored red after 24 hours' incubation, while the other colonies remain colorless. We have found that better results are obtained if both lactose and saccharose are added to the Endo medium, since certain members of the colon-bacillus group ferment saccharose more rapidly than lactose and hence their colonies take on the red color sooner and can no longer be regarded as slow colonies; the typhoid colony is without color in 24 hours, as on the Endo medium containing lactose alone. The chief disadvantage of the Endo medium lies in the fact that the red color is not confined to colonies of B. coli themselves, but spreads out through the medium adjacent to them. If the colonies of B. coli are close together the whole plate soon becomes red and then colorless colonies on it can no longer be distinguished from the red ones. We have tested a great number of stains separately and in combinations of varying strengths with the view of overcoming this difficulty while at the same time preserving the effectiveness of the medium for the sure growth of typhoid. In every instance where the acid or the basic fuchsin was tried, the color diffused into the medium around the red colonies, so that it was difficult, if not impossible, to recognize the typhoid colonies which may have lain in these areas. We have finally devised a medium that gives even better dif-