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f we share our homes with mice, then it means we’re also cohabitating with the bacteria they carry. In a new study that looked at New York City mice, researchers discovered that those strains might not be such pleasant houseguests.

The mice harbored a number of bacteria that can cause human gastrointestinal infections, including C. difficile, E. coli, and particular Leptospira species, researchers reported Tuesday in the journal mBio. Some of the bacteria also had genes that confer resistance to certain antibiotics.

The study did not document any cases of the bacteria passing from the mice to people, and some outside experts are skeptical the supposed threats identified in this and similar studies actually pose a serious danger to humans. But Dr. Ian Lipkin, the study’s senior author, said the findings at least raised the possibility that a person could acquire a bacterial disease that is difficult to treat from our murine roommates.

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“What’s really compelling is that these are animals that live very close to us,” said Lipkin, the director of the Center for Infection and Immunity at Columbia University’s Mailman School of Public Health. “People focus a lot on rats, but they don’t think that much about mice, and I think that’s unfortunate. This is a potential risk.”

He added: “There are hundreds of millions of these animals in urban environments. It’s time for us to start thinking about the built environment and the risks for all these sorts of things and how to mitigate them.”

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Just because certain bacteria are present does not suggest they are going to be pathogenic — meaning they can first infiltrate our systems and then cause disease, said Timothy Walsh, an expert on antimicrobial resistance at Cardiff University, who was not involved with the study. Past research has found similar bacterial warning signs in mice in other parts of the world, but they haven’t led to outbreaks of human disease.

“I don’t think it’s anything to be overly concerned about,” Walsh said about the new study’s findings. “By and large, the strains that inhabit mice are different than the ones that inhabit humans.”

Some diseases — both viral and bacterial — are known to spread from mice and other rodents to humans, including hantavirus, Lassa fever, and leptospirosis, often when the animals’ feces or urine contaminates people’s food. The new paper also comes at a time when there are growing concerns about antimicrobial resistance — with evermore-villainous strains of “superbugs” being discovered around the world — and greater recognition that the issue needs to be addressed.

For the research, Lipkin’s team tested more than 400 mice mostly scooped up from residential buildings around New York. The group had previously conducted a similar study in rats, but wanted to investigate the bacterial communities in mice because people typically have more exposure to them than their larger relatives.

In addition to strains of E. coli and C. diff that can cause gastrointestinal infections, the researchers also found species of shigella and salmonella bacteria. Some of the bacteria had genes that confer resistance to quinolone and macrolide antibiotics and certain beta-lactams.

Overall, the researchers found that more than a third of the mice carried at least one potentially pathogenic bacterium, while about a quarter of them had bacteria with at least one resistance gene.

In a second study published Tuesday, the researchers examined the viruses the mice were carrying. In all, they found 36 viruses but none that could cause human disease.

Lipkin acknowledged that it was perhaps unsurprising that mice would harbor these bacteria given the germs’ omnipresence. Just last month, researchers reported that some rubber duckies contained “potentially pathogenic bacteria,” though outside researchers expressed skepticism that playing with the toys at bath time was driving many cases of human disease.

But Lipkin said that the study should encourage health officials and clinicians to consider investigating the possibility that mice are culprits in additional kinds of human infections.

“You never know until you look,” he said.

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