On the Survival of Specific Microorganisms in Pupae and Imagines of Musca Domestica raised from experimentally Infected Larvae. Experiments with B. Typhosus

Abstract

Although typhoid bacilli were liberally supplied to larvae of Musca domestica, all attempts to demonstrate B. typhosus in the pupae or imagines were unsuccessful, until recourse was had to disinfection of the ova. After this preliminary disinfection both larvae and pupae gave pure growths of B. typhosus but hitherto it has not been possible to examine the imagines.In the experiments with unsterilised ova great difficulty was experienced in determining whether B. typhosus was present in MacConkey plates owing to the almost invariable occurrence of the colourless typhoid-like colonies of the Bacillus “A” which was evidently an organism throughly adapted to the conditions prevailing in the interior of the larvae, pupae and imagines.The fact also that this organism like B. typhosus fermented mannite without the production of gas, rendered the search for B. typhosus still more difficult.By the employment of sorbite—in place of lactose-media, a preliminary differentiation might have been possible but was not considered practicable in the first place owing to the expense of sorbite, and secondly owing to the fact that the fermentation of sorbite by B. typhosus though constant, is sometimes delayed. The most satisfactory plan was to pick off as many colonies as possible and inoculate them on litmus-milk. Bacillus “A” invariably rendered litmus-milk intensely alkaline.From the practical point of view the main conclusion to be drawn from the experiments detailed in this communication is that the typhoid bacillus can lead only a very precarious existence in the interior of larvae or pupae which possess, at least in so far as these investigations warrant, a well-defined bacterial flora of their own.Even under the highly artificial conditions of the final series of experiments, it was not possible to decide whether the B. typhosus though recoverable from the pupa was really actively multiplying in the pupal interior or gradually dying out. There was some indication that the latter was the case, as the typhoid colonies recovered from the pupa in the one successful instance, were extremely few in number, while the larvae which had been feeding on B. typhosus contained enormous numbers as evidenced both by cultural and microscopical examination.Since the above experiments were concluded, some further researches by Graham-Smith (1911) on the carriage of bacteria by flies have appeared in the form of a Local Government Board Report. This author has succeeded in recovering B. anthracis from blow-flies bred from larvae fed on meat infected with spores of this organism, but has failed to recover B. typhosus or B. enteritidis under similar experimental conditions.