What happens (and what danger are we really in) when a fly lands on our food?

Flies that carry disease-causing microorganisms can easily contaminate the food they land on — but several factors determine whether or not the person consuming them will get sick.

Most people feel disgust when a fly lands on the food you are about to eat.

But after all, how dangerous can it be a simple housefly that decides to land where it shouldn’t?

This issue has two important components, explains Kevin Verstrepengeneticist and microbiologist at the Flemish Institute of Biotechnology and the Catholic University of Leuven, at .

“First, how many microorganisms does it carryon average, a single fly? And second, how many microorganisms are needed to cause illness in a human being?”

Several researchers have tried to answer these questions.

In a study published in 1999, Japanese scientists exposed houseflies to pathogenic bacteria.a Escherichia coli and quantified the number of bacterial cells that the flies were able to disseminate up to four days after feeding.

On the second day, 80% of flies deposited around 100 colonies of E. coli on agar plates. The researchers observed colonies of different sizes, which they attributed, respectively, to the contact with mouth parts and the feet of flies, or their waste.

The conclusions indicated that the bacteria could survive in the digestive system of flies for days and that these could propagate microorganisms both by contact and through feces.

Years later, a team led by Antonio De Jesusa researcher at the US public health regulator FDA, studied the amount of E. coli what a single housefly could transfer from contaminated food to a clean surface.

In this, 50 flies were exposed to milk, steak and potato salad contaminated with E. coli. Then, the scientists measured how many bacteria the flies were able to transfer into a sterilized vial.

They found that about half of the flies carried a detectable amount of bacteria — more than 50 viable cellsor colony forming units (CFU). Each of these flies could transmit between approximately 30 and 40 thousand UFC every time you landa.

How many microorganisms are needed for a person to get sick?

Verstrepen explains that, in theory, “just a single bacterial cell is enough to cause disease”, but experimentally this issue is difficult to prove. “You cannot simply infect a human being with a microorganism and observe whether they become ill”, observes the researcher.

In another conducted in Japan, in 2011, outbreak data of food poisoning in Japan to estimate the infectious dose of various pathogenic bacteria, including two types of E. coli.

The study authors concluded that a person would need to ingest somewhere between 25 and 1,000 CFU of E. coli enterotoxigenicthe most common cause of so-called travelers’ diarrhea to get sick.

In the case of E. coli producer of Shiga toxin, the infectious dose is much lower: between two and nine UFC, a value close to Verstrepen’s theoretical estimate.

In addition to the type of bacteria, other factors influence the risk of infectiono, such as the time bacteria have to multiply before being ingested. “If a fly lands on food while you are eating, no big problembecause the food is hot and microorganisms will not have time to multiply,” says Verstrepen.

“But it’s like a fly lands on a food display, in a store or in a kitchenand it stays there for some time, then yes it can become dangerousbecause bacteria multiply and can produce toxins”, notes the

O where the fly was is also important. In a 2019 study, Verstrepen’s team analyzed the microbiomes of flies collected from homes, hospitals and farms in Belgium and Rwanda. The researchers found that the internal microbiomes of flies were relatively similar regardless of the environment.

Already the bacterial populations present on the surface of the flies’ bodies varied by location, “which probably means that wherever the fly lands, the microorganisms present there can hitch a ride on the outside of your body”, explained the scientist.

In short, according to Verstrepen, people with a healthy immune system have little cause for concern if a fly lands on their food — or even if, by chance, they swallow one.

“It may be that they pass a night, or a few hours, in the bathroom”, he admits with humor, “but they will hardly have to fear for their lives”.