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K-State researchers discover houseflies in fast-food restaurants carry antibiotic-resistant bacteria

By Keener Tippin


Those annoying houseflies buzzing around your meal at a fast-food restaurant may be more than a nuisance -- they also may pose a health threat.

According to researchers at Kansas State University, the flies can carry and have the capacity to transfer antibiotic-resistant and potentially virulent bacteria to your food.

Ludek Zurek, a K-State assistant professor of entomology, and Lilia Macovei, an entomology research associate, wrote about their findings in the June 2006 issue of the journal Applied and Environmental Microbiology.

Resistance to antibiotics is a serious concern because the number of antibiotics effective at treating human infections continues to decline. It's a concern, according to the researchers, that may extend to the food supply as well.

Zurek and Macovei cite the two main sources of antibiotic-resistant bacteria are places where antibiotics are used the most: in clinical settings and in the agricultural industry, which heavily uses antibiotics to promote growth of poultry, swine and cattle.

"That causes a great selective pressure on bacteria," Zurek said. "As a result, we very often find antibiotic-resistant bacteria in the feces of domestic animals. Since houseflies develop in decaying organic material, especially in animal manure, we looked at the gut of these flies to see what kind of microbes, in terms of antibiotic resistance, they carry."

Zurek said Enterococci bacteria are commonly found in animal and human digestive tracts, and are known for their multi-antibiotic resistance. Two of its 26 species, Enterococcus faecalis and Enterococcus faecium, are responsible for the majority of human infections, he said.

Zurek and Macovei screened houseflies from five different restaurants in mid-size communities in Kansas. They found that the majority of the flies carried antibiotic-resistant bacteria. This indicates the flies either developed in the manure of animals that were heavily exposed to antibiotics or that they were in contact with feces of some animals that were exposed to antibiotics, Zurek said.

Of the flies found to carry antibiotic-resistant bacteria, 97 percent tested positive for the bacteria with E. faecalis identified in the majority of the isolates. E. faecalis was found to carry virulence genes and have varying percentages of resistance to tetracycline, erythromycin, streptomycin, ciproflaxin and kanamycin. E. faecium showed up at a rate of nearly 7 percent.

"This study showed that houseflies in food-handling and serving facilities carry antibiotic-resistant and potentially virulent Enterococci that have the capacity for horizontal transfer of antibiotic-resistant genes to other bacteria," Zurek said. "It shows great potential for the food in those fast-food restaurants to be contaminated by bacteria from the houseflies."

Because the flies are attracted to sugar and protein, prepared foods in fast-food restaurants that are ready to be served are a prime target, he said.

"It's not before cooking, it's not before frying, etc. It's basically the food that is being served and then ingested by people," Zurek said. "While it is more of a nuisance issue, it is a public safety and hygiene issue as well."

Zurek and Macovei think there should be a higher standard of insect control, primarily flies, in restaurants.

"You don't see houseflies in hospitals and it's not just because it's a nuisance issue," Zurek said. "It's known that flies do carry and can transmit bacteria. That's why hospitals, at least in the United States, have very high and strict standards for that. Similar standards should be applied to restaurants because the flies that are in restaurants, at least based on our study, carry the same bacteria you find in manure."

Zurek and Macovei are currently working on two follow-up studies to their research. One examines exposing food from fast-food restaurants to different numbers of flies, collected from various animal farms, for different amounts of time. The level of contamination from a few flies in an hour will be studied, as well as the level from more flies for a longer period of time.

The second experiment samples foods from the fast-food restaurants during the winter when flies are not as prevalent.


Spring/Summer 2007