Researchers from Yale and the University of Connecticut report detecting genes relating to bacterial resistance to antibiotics in the manure of dairy cows.

The findings create some concern that introduction of these genes into the environment as manure is used as fertilizer could lead to the emergence of antibiotic-resistant pathogens in humans or animals. The research results are published in the April 22 issue of mBio the online open-access journal of the American Society for Microbiology (ASM).

The researchers used a new combination of methods to examine the resistome of dairy cow manure, and constructed metagenomic libraries with DNA extracted from manure screened for resistance to beta-lactams, phenicols, aminoglycosides, and tetracyclines. Functional screening identified 80 different antibiotic resistance genes whose protein sequences were on average 50 to 60 percent identical to sequences deposited in GenBank. The resistance genes were frequently found in clusters and originated from a taxonomically diverse set of species, suggesting that some microorganisms in manure harbor multiple resistance genes.

“The diversity of genes we found is remarkable in itself considering the small set of five manure samples,” says Jo Handelsman, PhD, senior study author and microbiologist at Yale. “But also, these are evolutionarily distant from the genes we already have in the genetic databases, which largely represent (antibiotic resistant) genes we see in the clinic.”

According to a release from ASM, that might signal good news that antibiotic-resistant genes from cow gut bacteria are not currently causing problems for human patients. But, Wichmann points out, another possibility is that “cow manure harbors an unprecedented reservoir of AR genes” that could be next to move into humans.

Potentially, manure bacteria that contain antibiotic-resistant genes could colonize humans, or the genes from benign bacteria could move to human pathogens through a process called horizontal gene transfer. Gene transfer enables genes to jump between microorganisms that are not related, and it occurs in most environments that host bacteria, according to the ASM release.

The researchers hope the study will open up a larger field of surveillance to find and identify new types of resistance before they affect humans.

The mBio article is available online.