The method of feeding of the house-fly has an important bearing on the question of its ability to transmit pathogenic organisms. Graham-Smith (1910) has shown that when feeding, flies frequently moisten soluble substances with "vomit" which is regurgitated from the crop. This is, of course, loaded with bacteria from previous food. When not sucked up again these drops of liquid dry, and produce round marks with an opaque center and rim and an intervening less opaque area. Fly-specks, then, consist of both vomit spots and feces. Graham-Smith shows a photograph of a cupboard window where, on an area six inches square, there were counted eleven hundred and two vomit marks and nine fecal deposits.
From a bacteriologist's viewpoint a discussion of the possibility of a fly's carrying bacteria would seem superfluous. Any exposed object, animate or inanimate, is contaminated by bacteria and will transfer them if brought into contact with suitable culture media, whether such substance be food, or drink, open wounds, or the sterile culture media of the laboratory. A needle point may convey enough germs to produce disease. Much more readily may the house-fly with its covering of hairs and its sponge-like pulvilli ([fig. 109]) pick up and transfer bits of filth and other contaminated material.
For popular instruction this inevitable transfer of germs by the house-fly is strikingly demonstrated by the oft copied illustration of the tracks of a fly on a sterile culture plate. Two plates of gelatine or, better, agar medium are prepared. Over one of these a fly (with wings clipped) is allowed to walk, the other is kept as a check. Both are put aside at room temperature, to be examined after twenty-four to forty-eight hours. At the end of that time, the check plate is as clear as ever, the one which the fly has walked is dotted with colonies of bacteria and fungi. The value in the experiment consists in emphasizing that by this method we merely render visible what is constantly occurring in nature.
A comparable experiment which we use in our elementary laboratory work is to take three samples of clean (preferably, sterile) fresh milk in sterile bottles. One of them is plugged with a pledget of cotton, into the second is dropped a fly from the laboratory and into the third is dropped a fly which has been caught feeding upon garbage or other filth. After a minute or two the flies are removed and the vials plugged as was number one. The three are then set aside at room temperature. When examined after twenty-four hours the milk in the first vial is either still sweet or has a "clean" sour odor; that of the remaining two is very different, for it has a putrid odor, which is usually more pronounced in the case of sample number three.
Several workers have carried out experiments to determine the number of bacteria carried by flies under natural conditions. One of the most extended and best known of these is the series by Esten and Mason (1908). These workers caught flies from various sources in a sterilized net, placed them in a sterile bottle and poured over them a known quantity of sterilized water, in which they were shaken so as to wash the bacteria from their bodies. They found the number of bacteria on a single fly to range from 550 to 6,600,000. Early in the fly season the numbers of bacteria on flies are comparatively small, while later the numbers are comparatively very large. The place where flies live also determines largely the numbers that they carry. The lowest number, 550, was from a fly caught in the bacteriological laboratory, the highest number, 6,600,000 was the average from eighteen swill-barrel flies. Torrey (1912) made examination of "wild" flies from a tenement house district of New York City. He found "that the surface contamination of these 'wild' flies may vary from 570 to 4,400,000 bacteria per insect, and the intestinal bacterial content from 16,000 to 28,000,000."
Less well known in this country is the work of Cox, Lewis, and Glynn (1912). They examined over four hundred and fifty naturally infected house-flies in Liverpool during September and early October. Instead of washing the flies they were allowed to swim on the surface of sterile water for five, fifteen, or thirty minutes, thus giving natural conditions, where infection occurs from vomit and dejecta of the flies, as well as from their bodies. They found, as might be expected, that flies from either insanitary or congested areas of the city contain far more bacteria than those from the more sanitary, less congested, or suburban areas. The number of aerobic bacteria from the former varied from 800,000 to 500,000,000 per fly and from the latter from 21,000 to 100,000. The number of intestinal forms conveyed by flies from insanitary or congested areas was from 10,000 to 333,000,000 as compared with from 100 to 10,000 carried by flies from the more sanitary areas.
Pathogenic bacteria and those allied to the food poisoning group were only obtained from the congested or moderately congested areas and not from the suburban areas, where the chances of infestation were less.
The interesting fact was brought out that flies caught in milk shops apparently carry and obtain more bacteria than those from other shops with exposed food in a similar neighborhood. The writers explained this as probably due to the fact that milk when accessible, especially during the summer months, is suitable culture medium for bacteria, and the flies first inoculate the milk and later reinoculate themselves, and then more of the milk, so establishing a vicious circle.
They conclude that in cities where food is plentiful flies rarely migrate from the locality in which they are bred, and consequently the number of bacteria which they carry depends upon the general standard of cleanliness in that locality. Flies caught in a street of modern, fairly high class, workmen's dwellings forming a sanitary oasis in the midst of a slum area, carried far less bacteria than those caught in the adjacent neighborhood.
Thus, as the amount of dirt carried by flies in any particular locality, measured in the terms of bacteria, bears a definite relation to the habits of the people and to the state of the streets, it demonstrates the necessity of efficient municipal and domestic cleanliness, if the food of the inhabitants is to escape pollution, not only with harmless but also with occasional pathogenic bacteria.