New York City’s Mice Carry Human Pathogens with Antibiotic Resistance Genes
Washington, DC—April 17, 2018— Researchers have found that mice living in New York City’s residential buildings harbor multiple bacteria species that can cause human fever and gastrointestinal disease, including Salmonella, Clostridium difficile, Escherichia coli and Leptospira species. The two studies, available here and here, published this week in mBio®, an open-access journal of the American Society for Microbiology. The team also shows that the house mice’s microbiomes contained several antimicrobial resistance genes to three common classes of antibiotics.
“Mice are more than just a nuisance—they are a potential source of infections,” says Ian Lipkin, director of the Center for Infection and Immunity at Columbia University’s Mailman School of Public Health in New York City and senior researcher on the two studies.
After completing similar microbiome studies on the city’s very visible rats in 2014, Lipkin’s team wanted to tackle what made up the microbiome of the less visible, but just as ubiquitous, house mice, Mus musculus.
“They are much more intimate in that they live with us, inside our walls and we see their droppings,” says Lipkin. “We wanted to see whether there were human pathogens that might be lurking in mice.”
The project, headed up by Lipkin’s graduate student Simon Williams, collected 416 mice and their feces at 7 sites from predominantly residential buildings across four boroughs of New York City (Manhattan, Queens, Brooklyn and the Bronx) over the course of one year.
In the first study, the team used unbiased high throughput sequencing of the mouse fecal samples to look for viruses. The team identified 36 viruses, including 6 novel viruses. However, none of the viruses were known human viral pathogens—including one specifically transmitted between mice and humans, lymphocytic choriomeningitis virus (LCMV). To take a closer look, the team tested the blood of 395 mice for LCMV antibodies and found none.
“This was the only thing that surprised me,” says Lipkin. “Although documentation of movement of this virus to humans is not very common, it was thought to explain many non-bacterial cases of meningitis.”
In the second study, the team characterized the bacteria present in the mice’s fecal microbiome using 16S ribosomal RNA sequencing. Next, the researchers used targeted sequencing to measure the prevalence of several species of pathogenic bacteria found to be present, including C. difficile, Salmonella, E. coli, Shigella, Klebsiella pneumoniae, Clostridium perfringens and Leptospira.
The bacterial pathogen C. difficile can cause severe and difficult-to-treat diarrhea and inflammation of the colon in humans. Patients most often acquire the infection from healthcare facilities or after antibiotic treatment, but another form of the infection called community-acquired C. difficile infection, is on the rise among healthy people who have no risk factors.
Lipkin’s team cultured C. difficile from fecal pellets of the mice carrying it. Sequencing revealed three unique genetic profiles of C. difficile, which matched those isolated from human C. difficile infections by the US Centers for Disease Control and Prevention. One of the mice-derived profiles matched a human profile associated with a community-acquired infection. Although this study holds no direct evidence that C. difficile is passing from mice to humans, Lipkin says, the findings merit closer examination of that question.
Equally concerning, Lipkin’s team found that the NYC house mice harbored bacteria with 22 different genes that could confer resistance to a number of common classes of antibiotic drugs, including the quinolones, macrolides, and ß-lactams. Again, although the researchers found no direct evidence that these genes were being actively expressed by the bacteria to create antimicrobial resistance, Lipkin sees room for further investigation.
“These antibiotic resistance genes are out there in the environment and mice are carrying them everywhere,” he says. “My concern is that they are a reservoir of antibiotic resistance.” As such, if humans were infected by a bacterial strain carried by these mice, then the infection could be resistant to treatment, says Lipkin.
On the whole, the team documented that more than a third (37%) of the mice carried at least one potentially pathogenic bacterium and almost one quarter (23%) of the mice harbored at least one antimicrobial resistance gene in their fecal bacteria.
“We used to think of mosquitoes as the source of just an itchy bite, but now we know they carry Zika virus and West Nile virus,” says Lipkin. “We should be thinking of mice in the same way, as potential sources of infection. And that means we should control them as vectors of disease.”
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