Amongst all the insects public health authorities have to deal with in urban environments, cockroaches occupy an important place. Nowadays cockroach populations appear to have more or less replaced populations of domestic flies as the main domestic urban pests, but their presence in homes and particular on food, elicits similar reactions of disgust and aversion. "Equally important as disease carriers are flies and roaches" -- this quotation from a 1952 U.S. Public Health Service publication and reported by Roth and Willis (1) is still true. |
Cockroaches are known to carry various microorganisms like bacteria, helminths, viruses and fungi. Many of the bacterial species identified from cockroaches are either opportunist or potential pathogens for man and vertebrates (1, 2, 3, 4, 5). However, direct evidence that indicates that cockroaches could be involved in the transmission of bacterial diseases is largely circumstantial (1). Cockroaches are merely passive vectors and only carry bacteria that are already present in the environment (6). Contamination could occur through other paths without the intervention of cockroaches.
Demonstrating that cockroaches can be implied in the transmission of pathogenic bacteria includes several steps. The first step was to prove that cockroaches could carry pathogenic bacteria by isolating these pathogenic species from wild-caught cockroaches in different types of urban environments. Such reports already exist (1, 4, 5). As the samples increase in number, the number of different bacterial species identified from cockroaches also appears to increase. This first point is particularly significant because it concerns microorganisms that cockroaches acquired naturally through their own activities.
The second step was to inoculate cockroaches experimentally with bacteria in food and to check that the bacteria were still viable in the guts and the feces (7, 8, 9). However, cockroaches can carry bacteria on their cuticle, collecting them simply by passive contact (5, 6). This fact diminishes the importance accorded to transition through the digestive tract in bacterial transmission. These first two steps indicate that cockroaches are potential vectors of infectious agents.
The third step to this investigation was to demonstrate that bacteria are actually deposited by cockroaches on non-contaminated areas and that cockroaches actually play a part in disseminating bacteria. The experiment described below aimed at testing the potential of cockroaches for contaminating a food source.
Material and methods
Cockroaches were trapped in kitchens of low-income flats following the method described in Rivault (10). The traps were placed on the floor behind the refrigerator, the cooker and other electric appliances, inside cupboards and sideboards, near stored food and plates and dishes.
As soon as the cockroaches were captured, they were placed in 125 ml sterile glass containers. The cockroaches were divided into groups of five adults. One group of cockroaches was placed in each container with a 5 mg piece of fresh French bread. The cockroaches were left 16 hours with the bread, in the container. This period included a night period when cockroaches are active. This experiment was replicated 11 times.
Four control pieces of bread were placed in similar sterile containers and manipulated like in the other samples of bread. All containers were placed under the same light and temperature conditions (27 ~degrees~ C).
The pieces of bread were collected the following morning and masserated in 10 ml sterile water. Serial dilutions of each bread sample were inoculated on various bacteriological nutritive media (AES Laboratory, France) and incubated for 48 hours at 37 ~degrees~ C.
The identification of gram-negative bacteria was made after incubation by use of standard methods (API System, France) after incubation on Drigalski medium. Incubation on Chapman medium and identification by slide agglutination and respiratory tests using Staphyslide tests (BioMerieux, France) were used to investigate the presence of Staphylococcus aureus.
No bacteria were found on the control pieces of bread.
However, the 11 samples of bread from the containers with cockroaches were all contaminated. One or two bacterial species were identified from each sample (mean = 1.27 ~+ or -~ 0.47) (Table 1). In the bread samples contaminated by cockroaches, the bacteria came from 11 different species. These species included five species previously listed as pathogens by Roth and Willis (1) and by Cochran (2). However, no Staphylococcus sp. were found.
Our data show that during one night, a small group of cockroaches left one or two bacterial species on the bread. In previous studies, whether a sample of five cockroaches was washed, dissected or ground, we identified, ...
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