Studies of the Common Aerobic Spore-Forming Bacilli

Abstract

Agar slants with deep butts, inoculated into the butt as well as over the slant, were found superior to broth (Durham fermentation tubes) or soft agar stabs for fermentation tests with the common spp. of aerobic spore-forming bacilli. The butt-slants, made with 1.5% agar and containing brom cresol purple as indicator, proved more sensitive than the other types of media for the detection of slight acid production and often gave positive evidence of fermentation when the broth or stab cultures remained negative. When closely observed over a considerable period of time the butt-slants also revealed more clearly other properties which were characteristic of individual strains of these bacilli, such as the relative vigor of fermentation, the speed and extent of reversion to a neutral or alkaline reaction after the initial acid change, and the appearance of zones of acid reaction in particular locations in the medium. The fermentation reactions of the 10 principal vars. of aerobic spore-bearers, as detd. by cultures in brom cresol purple agar butt-slants containing familiar carbohydrates, were found to be of definite, though limited, differential value, especially when the capacity or lack of capacity of the organisms to store fat is also known. Among the fat-positive vars. Bacillus megatherium, B. circulans, and B. brevis are readily recognized by distinctive fermentations in lactose, maltose, and mannitol. Among the fat-negative spp., reactions in the same sugars are of special aid in distinguishing a newly recognized var. of B. subtilis (designated here B. subtilis Ford) from the Marburg type of B. subtilis.