Hospitals spend a lot of time and money keeping things clean and sterile. Despite those efforts, a new study in Great Britain found that nine in ten insects caught in seven hospitals harbored dangerous bacteria, much of which was found to be resistant to antibiotics.

Federica Boiocchi, a PhD student at Aston University and lead author of the study in the Journal of Medical Entomology, writes for The Conversation that scientists have previously studied insects in hospitals, but mainly focused on species that breed inside the medical facilities, like ants and cockroaches, overlooking flying insects.

To see whether they could pose a hazard to patients, Boicchi and her colleagues set up ultraviolet light fly traps and sticky traps in seven National Health Service (NHS) hospitals in England, covering various areas including food preparation sites, neonatal units and general hospital wards. Over 18 months, they collected 20,000 insects, 76 percent of which were in the Diptera, or true fly genus. The other 14 percent were mostly “true bugs,” including leafhoppers, froghoppers and aphids believed to have come into the buildings from outside. A small number of bees, ants and moths were also collected.

But it’s not the bugs themselves that are of concern. It’s the bugs on the bugs. The researchers found 86 bacterial strains on the exoskeletons and inside the insects, including many that can infect humans. Enterobacteriaceae, a group that includes E. coli, made up 41 percent of the strains while Bacillus bacteria, including some that cause food poisoning, made up 24 percent. Staphylococci, including the nasty bug S. aureus which causes skin, bone infections and pneumonia, made up 19 percent.

More concerning, 53 percent of the bacteria strains found were resistant to at least one antibiotic and 19 percent were resistant to two or more antibiotics, including penicillin, vancomycin and levofloxacin.

“The results from this large-scale microbiological analysis show that a variety of flying insects collected from UK hospitals do indeed harbor pathogenic bacteria of different species,” Boiocchi says in a press release. “What’s quite interesting, though, is the high proportion of drug-resistant bacteria found in these samples. It’s a vivid reminder of how our overuse of antibiotics in healthcare settings is making infections more difficult to treat.”

The prevalence of germy insects doesn’t necessarily mean they are spreading these bacteria around. “Mostly it depends by the bacterium carried by the fly and where the fly lands,” Boiocchi tells George Dvorsky at Gizmodo. “Our study showed that some flies carry pathogenic bacteria but the amount of bacteria recovered was not enough to cause infection. The risk is mainly related to the fact that flies represent a reservoir of bacteria.”

In a rare case, she says, a fly could land on something like an apple slice left out overnight, infecting it with a few bacterial cells from its legs. Over the course of hours, these cells might be able to proliferate to levels high enough to make someone sick. But with proper sanitation and food handling, that type of situation shouldn’t be a problem.

“NHS hospitals are extremely clean environments and the risk of insects carrying bacteria and transferring these to patients is very low,” Anthony Hilton, the study's senior author, also from Aston University says in the press release. “What we are saying in this paper is that even in the cleanest of environments, it’s important to take steps to prevent bacteria being brought into hospitals by insects. NHS hospitals will already be implementing many of these measures, but there are simple steps that can be taken to improve this further.”

The team suggests installing UV light traps for flying insects to help control the insects.

Flies aren’t the only things that can transmit bugs around hospitals. Studies have found that neckties worn by doctors can be a source of infection. But the biggest bug transmitter is something most people have been taught since toddlerhood to keep clean: their hands. A study released last April found that 14 percent of 400 hospital patients tested had antibiotic-resistant bacteria on their hands in their nostrils early on in their hospital stay, meaning everyone in the facilities, doctors and patients, should scrub down frequently.