In the arms race between diseases and modern medicine, antibiotic-resistant bacteria are the nukes. Scientists are constantly developing new antibiotics, but just as quickly, bacteria are developing resistance to these drugs. And while many cases of antibiotic-resistant bacteria occur in hospitals, a new study published Tuesday shows just how close to home these superbugs can be.

In a paper published in the journal mBio, a team of public health and infectious disease researchers outline the findings of their investigation into the diseases carried by mice that live in residential buildings in New York City. In the 416 house mice (Mus musculus) that they sampled from seven different sites in New York City over 13 months, they found a number of bacteria that make humans sick, including Shigella, Salmonella, Clostridium difficile, and Escherichia coli. Even more concerning is that the researchers identified antibiotic resistance genes in those pathogens.

Methicillin-Resistant Staphylococcus aureus (MRSA)
Methicillin-resistant Staphylococcus aureus (MRSA) is one of the best-known antibiotic-resistant bacteria, but there are many more out there.

Antibiotic-resistant bacteria are a serious health concern because, well, they’re resistant to the only drugs we’ve got to fight them. The U.S. Centers for Disease Control and Prevention reports that antibiotic-resistant bacteria sicken more than 2 million Americans and kill 23,000 Americans a year, and public health experts predict that the problem is only going to get worse in the near future because of irresponsible antibiotic use. In light of these growing concerns, the authors of the new study turned their attention to house mice, which are uniquely positioned to harbor and transmit antibiotic-resistant pathogenic bacteria — and come into close contact with humans.

“Recent studies of rats in North American cities have reported the presence of pathogenic bacteria,” the authors write. “House mice occupy a niche position within many urban structures (e.g., homes, restaurants, and schools) wherein they have even more intimate contact with humans.” To find out which specific pathogens the mice in New Yorkers’ homes were carrying, the researchers sampled the mice’s fecal microbiomes and conducted rRNA sequencing on the bacteria to identify antibiotic resistance genes.

The fact that the researchers found at least four disease-causing bacteria in the mice — as well as genes for antibiotic resistance in those bacteria — is a significant public health concern, since the bacteria that are resistant can pass those genes on to ones that aren’t. It’s well documented that animal droppings can be a source of Salmonella outbreaks, but further research will be necessary to identify the role of mice in spreading antibiotic-resistant infections.

The authors point out that most people consider rats a major health concern but don’t often think about mice. With this study, they demonstrate that mice could play a significant role in spreading disease, especially treatment-resistant disease.