Think livestock poop loaded with antibiotics and antibiotic-resistant microbes is already terrifying? According to a new study, doped-up droppings aren’t just biohazards festering on farms across the country. They also contribute to climate change.

Soil microbial communities stressed by farm-borne superbugs and drugs can burn through up to 5.8 times the amount of soil carbon stores as their undisturbed counterparts. This is according to a new report in Proceedings of the Royal Society B. Though the study didn’t plunge into the exact cause of the revved-up carbon cycling, the researchers speculate that the heavy use of antibiotics on farms leads to soil microbes getting locked into molecular arms-races and wars—both of which are metabolically costly endeavors.

The findings are troubling given that global livestock production is on the rise. In lockstep, use of antibiotics is expected to leap 67 percent over the next two decades. Already, almost 80 percent of antibiotics sold in the US go to farms , and farm animals excrete anywhere from 40 to 95 percent of the drugs they ingest—alongside a steady stream of drug-resistant germs.

These menacing microbes can travel through waterways and wind, as well as on meat. They threaten public health and ecosystem functioning everywhere they go.

The new study “suggests that the expected increase in manure inputs and/or agriculturally derived antibiotics due to intensifying livestock production not only has human health implications but may also have substantial environmental impacts.”

The authors of the Proceedings study are a group of environmental and agricultural researchers led by Michael Strickland at Virginia Tech, and they came to their conclusion after studying soil samples from 11 dairy farms across the US. The cows on those farms had all taken cephapirin, a drug in the cephalosporin family of antibiotics, which are similar to penicillin. (This drug protects cows from bacterial infections on their udders.)

From each farm, the researchers collected soil from where cows congregated, which was sure to be packed with manure. To find soil devoid of manure, the researchers also collected samples from where cows didn’t hang out. Next, the researchers used genetic sequencing to identify the microbes in each soil sample, as well as genes that make microbes resistant to cephalosporins and other antibiotics. The researchers tested how active the microbes were, based on the amount of carbon they released from respiration.

Microbial muck

As expected, the turd-filled turf had drastically different microbial communities than the non-poopy samples. The former had drug-resistant and gut-dwelling bugs, plus more genetic blueprints for resistance. A gene responsible for making microbes resistant to cephalosporins was, on average, 421 percent more abundant in the manure samples than the control soil. The researchers also found resistance genes for drugs the cows weren’t even taking. This suggests that these resistance genes may have arrived in packages, making microbes resistant to multiple types of antibiotics in one fell swoop.

When the researchers dumped more antibiotics on soil samples in the lab—mimicking the direct dose of drugs left over in animal excrement—the bacteria in the soil sprang into metabolic action. This suggests that the genes they detected by sequencing were “live,” so to speak: The bacteria could power them to defend against antibiotics and their foes as needed.

But keeping that genetic weaponry around is costly. The more abundant the gene, the researchers found, the more the bacteria burned through carbon stores and released carbon dioxide. On average, the manure-filled soil respired 2.1 times more carbon per microbial biomass unit than the control samples. The most active samples showed a 5.8-fold increase.

And this activity may not fade quickly over time. Researchers had previously expected that microbes would simply ditch such costly resistance genes as soon as the genes weren't needed anymore. But the genes aren’t discarded, studies have shown. For instance, after a pig farm banned antibiotic use, researchers still found high levels of antibiotic resistance genes two-and-a-half years later.

This is bad news for the environment, as well as our health. The new findings, the authors write, “highlight that manure from cattle treated with antibiotics have the potential to markedly alter microbial community composition and the ecosystem processes that these communities regulate.”

Proceedings of the Royal Society B, 2017. DOI: 10.1098/rspb.2016.2233 (About DOIs).