Potentially harmful germs dwell inside the guts of urban mice, according to a team of scientists who trapped more than 400 of the rodents around New York City. The researchers, conducting the largest survey of microorganisms living in city mice, also identified several genes that give germs resistance to antibiotic drugs.
“This doesn't tell us that these mice are directly responsible for infecting humans,” said W. Ian Lipkin, director of Columbia University's Center for Infection and Immunity. “But they have the potential to do so.”
Lipkin is an author of a new report, published Tuesday in the journal mBio, that describes the mouse bacteria. A related mBio study, also conducted by Lipkin and his colleagues, presents six never-before-seen viruses found in the city mice. None of the detected viruses, including the newly discovered ones, were known to cause human diseases.
But 37 percent of the mice contained at least one bacterial pathogen, based on the mice's fecal matter or anal swab samples. These germs included: Clostridium difficile, which causes severe diarrhea in humans; Salmonella enterica, responsible for bacterial food poisoning; Klebsiella pneumoniae, which can cause pneumonia or bloodstream infections; and the toxic and invasive type of Escherichia coli. Roughly 1 in 4 mice, or 23 percent, tested positive for at least one antimicrobial-resistant gene.
The researchers caught mice from the Bronx, Brooklyn, Manhattan and Queens. Most of the mice lived in trash compactor rooms in apartment building basements. Lipkin said these mice were likely representative of the city's mouse population — a handful of mice captured in a New York bakery, the scientists found, contained similar bacteria.
And they probably resembled their cousins in other urban megacenters, too, Lipkin said. “I would suspect that if you looked in Washington, D.C., or Boston or Toronto, you’d have similar findings.”
Drug-resistant germs have been reported in hospital settings for years, where antibiotics are routinely administered and bacteria that can survive them are more likely to thrive. “The drugs that we have been using successfully since the 1940s have been losing their potency,” Lipkin said.
More recently, the hunt for superbugs has expanded beyond the medical pipeline. The current work is “an interesting study but not one that changes a whole lot,” said J. Scott Weese, a microbiologist at the University of Guelph, in Ontario, who was not involved with this research. “We know animals, including pets, food animals and wildlife, can carry a wide range of resistant bacteria.”
Wherever epidemiologists and infectious disease experts look, they find resistance. A web of antimicrobial resistant germs connects the bowels of farm animals, pets and wildlife. Monkeys in Mexico have antibiotic-resistant germs in their feces. In 2015, a survey of wild rats in British Columbia revealed drug-resistant E. coli.
Mice are particularly important to study, Lipkin said, given their close contact with humans. Rats, even in the city, live primarily outside of the home, in sewers and subways. A life indoors makes mice more “worrisome,” Lipkin said.
This research cannot determine in which direction the germs had traveled — it was not known if the bacteria moved from humans to mice or mice to humans. “We would like to very clearly sort out the extent” that mice contribute to antibiotic resistance, Lipkin said.
“Does a resistant bug in a mouse pose a risk to a person in the house? It’s hard to say,” Weese said. The microbiologist said that exposure to other people, pets and contaminated food were more likely to be sources of drug-resistant infections.
Only rarely do health officials trace fatal cases of infectious disease to mice. In 2017, one person in the Bronx died and two others were sickened by leptospirosis, a disease that spreads through contact with rodent urine. The New York City Department of Health and Mental Hygiene determined rats, not mice, caused that disease cluster.
Yet it was clear from the new report that bacteria have traveled between species, Lipkin said. The study authors took samples of Clostridium difficile from 18 mice and grew the bacteria in a laboratory. The Centers for Disease Control and Prevention matched the strains of C. difficile from these mice to types known to cause infections in humans.
Lipkin said he hoped studies like these demonstrated the importance of rodent control in public health. “This is something that people need to take seriously,” Lipkin said.