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Hygiene risk from flies greater than previously thought

Flies are a well-known risk for spreading diseases and spoilage organisms in the food, healthcare and pharmaceutical sectors. In these sectors, standard hygiene practices, physical barriers and electric fly killers are required components of a good hygiene regime. This ensures the prevention of contamination from flies that are attracted to food.

A team of researchers from the US, Singapore, Brazil and Germany have shown that houseflies and blowflies can carry a far wider range of bacteria. With one key discovery being that the major route of contamination is from walking on a surface.

The researchers collected houseflies and blowflies from three continents in a range of environments, including tropical rainforest in Brazil and urban areas. The researchers used new genetic analysis techniques to identify all the microorganisms carried by the flies and the parts of their bodies they were present on.

The legs and wings show the highest microbial diversity of parts of the fly body,  which shows that just by landing on food or surfaces, flies could spread many types of bacteria.

Disease-carrying flies

Most studies of flies have concentrated on microorganisms in the gastrointestinal tract of the insects. Blowflies and houseflies feed on and breed in faeces and rotting matter. Through their normal habits, flies will encounter many microorganisms that can cause disease or degrade plant and animal matter. It is, therefore, likely that mechanical means of dispersal are important factors.

Dr Ana Junqueira, a molecular entomologist at the Federal University of Rio de Janeiro who co-led the research said,

“Carrion flies have microscopic hairs on every part of the body excluding the eye and these bristles make them the perfect carrier for pollen and also bacteria. It is an evolutionarily optimised vehicle for the dispersal of microorganisms in the environment.”

Several techniques were used to analyse the genetic information to determine which microorganisms were present. The majority of DNA “reads” belonged to bacteria, while only one fly collected had a significant quantity of viral DNA.

The study found that individual flies can carry hundreds of species of bacteria picked up from the environment in which they live. One analysis technique identified 1396 species, another 316, and a third identified 49. There were 33 species of bacteria identified by all three techniques. The types of bacteria were similar in all environments and varied more between individual flies at each location.

There was a large overlap between houseflies and blowflies, with about 50% of the microorganisms found on both types of fly. This likely reflects the common environment that the flies inhabit.

The abdomen had more than twice the number of DNA reads as other body sections, which the researchers say is likely due to it containing the guts of the flies. The legs and wings, however, had the highest diversity of bacteria species.

Flies on a watermelon

Disease-causing bacteria

By far the most common microorganism in blowflies was Wolbachiaa genus of parasitic bacteria that has been found in more than 60% of insect species worldwide. Wolbachia has a range of complex relations with its host insects, including prevention of reproduction, killing of males, parthenogenesis, feminization of males and protection from viruses such as Norovirus, West Nile virus and chikungunya. It is not known to have any effect on humans, however.

In flies the most common bacteria was Psychrobacter sp. PRwf-1, accounting for 25% of the DNA reads detected in housefly samples. This is a strain of bacteria normally found in cold environments and permafrost soils but this strain is adapted to warmer environments and is associated with food spoilage.

Out of the hundreds of species of bacteria found on the flies, a number were identified by all the analysis techniques used. Some of the bacteria found on the blowflies and houseflies are well known for causing human diseases, especially nosocomial infections (caught in hospitals), or food spoilage. Some of the important bacteria found on the flies were:

  • Enterobacter cloacae: widespread in the environment, gut of humans and animals; nosocomial  infections of lower respiratory tract and urinary tract, endocarditis, skin and soft tissue infections; contaminates medical, intravenous and other devices
  • Salmonella enterica: common cause of food-borne illness
  • Staphylococcus: common in the environment; cause of gastroenteritis and wound infections in hospitals
  • Bacillus cereus: causes foodborne illness; spores can survive undercooking; creates toxins that cause food poisoning
  • Acinetobacter baumannii: widely distributed in nature, but well known for hospital-acquired infections such as bacteremia, urinary tract infections (UTIs), secondary meningitis, infective endocarditis, and wound and burn infections
  • Vibrio parahaemolyticus: common cause of gastroenteritis from undercooked seafood, but can also cause eye, ear and skin infections from being in infected water
  • Enterococcus faecalis: normal component of the gut flora, but can cause life threatening infections in hospital environments. High levels of antibiotic resistance
  • Morganella morgana: normal flora in the intestinal tracts of humans and many  animals;  causes nosocomial infections, usually in surgical wounds or urinary tract
  • E. coli: normal component of gut flora, but six types cause gastroenteritis through contaminated food, water or surfaces
  • Helicobacter pylori: cause of stomach ulcers and some cancers
  • Clostridium butyricum: a normal component of the gut flora; used as a probiotic in some Asian countries and is common in soured milk and cheese

The researchers compared the bacteria they discovered in the houseflies and blowflies (carrion flies) with those found in the human gut microbiomes project and those found in the New York subway in another project. They found that 19 species of bacteria were common to all three studies, 49 found in the fly samples have been recorded in the human gut and 79 species of bacteria found in the flies were also found in the New York subway.

One of the surprising findings of the research was the high incidence of Helicobacter pylori, which can cause peptic ulcers and stomach cancer. The bacterium has never previously been found in insects in the wild. It was found in 15 out of 116 blowflies sampled and in all body parts, but mainly on the legs and wings. All these flies were collected in Brazil.

The researchers considered that the blowflies had most likely acquired the bacterium from sewage sites or outside toilets. They concluded that further research is needed into alternative routes of transmission of H. pylori.

Close up picture of a fly

Contamination — every step of the way

Flies can transmit microorganisms to food and surfaces through three mechanisms:

  • Regurgitation during feeding
  • Excretion of faeces
  • Mechanical contamination through contact with body parts

The researchers investigated these routes of contamination by allowing some blowflies to land on a petri dish that had been inoculated with a special strain of E. coli and then let them walk on sterile agar plates.

These agar plates were incubated so that any E. coli spread onto the agar by the flies would grow into a colony and become visible.

The pattern of the bacterial colonies matched the footprints of the flies and persisted over many footprints. The first and third pair of legs on each side produced a line, while the middle pair of legs produced spots outside the lines.

There were only a few signs of other parts of the body contaminating the agar — a few spots from the mouth parts — which shows that the legs or feet of the flies are the main means of contamination.

The experiment also showed that after even many steps sufficient bacteria remain on the feet of flies to continue contaminating the surface they were walking on.

Professor Donald Bryant of Penn State University said,

“We believe that this may show a mechanism for pathogen transmission that has been overlooked by public health officials, and flies may contribute to the rapid transmission of pathogens in outbreak situations.”

Fly control and public health surveillance

The bacteria detected in the blowfly and housefly samples in this study overlap with those found in the human gut and decaying material the urban environment. The study shows that the outer surfaces of flies, especially the legs, are efficient routes for transferring bacteria from surface to surface. This is not just true for human pathogens but also bacteria causing plant and animal diseases.

Houseflies, blowflies and other carrion flies can, therefore, act as indicators of the dangerous bacteria present in the local environment.

The researchers conclude that the genetic analysis techniques developed in this study can be used to predict and prevent routes of transmission of potential disease-causing bacteria. They could become an effective tool in vector control and public health surveillance programs.

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