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Precise methods are described which made it possible to handle individual mounted flies fed controlled amounts of known culture suspensions. Collections of droppings from such flies were then studied for their aerobic bacterial content.
Normal stool flora studies were conducted on 36 Musca domestica with 61 isolations being made prior to feeding the test organism to the flies. Rechecks were carried out on these same flies immediately after their death to ascertain what changes, if any, had occurred in the aerobic bacterial flora during the period of confinement of the flies. While the presence of soil and water species was common, it is of interest to note that Proteus morganii was isolated from 28 of the 36 flies studied, and Aerobacter aerogenes was found in 20 of the 36 flies 24 hours after the insects were mounted and again at the death of the flies.
During the period of their confinement, mounted flies were fed daily with known quantities of sterile 0.5 molar sucrose solution and sterile skim milk from potometers. Droppings were collected in tubes containing 2.0 ml. of physiological saline or on the surface of non-nutrient agar plates from which isolations of the specific test organisms were attempted. Controlled studies eliminated the possibility that the stools, in themselves, might supply sufficient nutrients to allow active multiplication of the organisms excreted into the saline.
Of 132 Musca domestica investigated in controlled feeding experiments, 36 were fed known numbers of Escherichia coli, 48 were given measured amounts of Salmonella schottmülleri, and a third group of 48 flies received known numbers of Shigella dysenteriae in an effort to determine passage of the test species of bacteria through the digestive tract of the flies.
Flies in groups of twelve fed small numbers (less than 1000) of the specified organisms in a single feeding apparently do not pass these bacteria in their stools, but as the numbers of ingested bacteria increase, decided multiplication occurs within the insects as evidenced by the excretion of significantly greater numbers of the test species than were initially fed. This excretion remains at a high level for at least six days, but the death of many of the flies limited the length of the observation period.
Now with the Test Department of the Baltimore and Ohio Railroad Company, Baltimore, 23, Maryland.