Many bacteria can protect themselves from antibiotics by modifying the compounds to make them harmless or by pumping the compounds out of their cells entirely. And other bacteria in soil have been found capable of living off of pesticides and herbicides. But study author and Agri-Food Canada microbiology researcher Ed Topp thinks that his team's discovery is the first instance of a microbe that can both resist and consume antibiotics.
“I think it’s kind of a game changer in terms of how we think about our environment and antibiotic resistance,” he said in a statement Thurday.
To look at how long-term antibiotic exposure affects soil bacteria populations, Topp and his team treated plots of soil at an experimental farm in Canada with a mixture of three veterinary antibiotics: sulfamethazine, tylosin and chlortetracycline, which are commonly fed to pigs and other kinds of livestock. The treatment was applied once a year between 1999 and 2009.
As detailed in their new paper in the Journal of Environmental Quality, the researchers were surprised to see that antibiotics were degrading much faster in plots that had been treated for long periods of time than in fresh plots that had not been previously dosed with drugs. They also saw that sulfamethazine, in particular, was disappearing five times faster than the other antibiotics.
Topp and his team looked through the treated plots with a very fine tooth comb, so to speak, and found a new strain of a bacteria belonging to the microbacterium family, a kind of microbe that's very common in soil. The new bug was breaking down the sulfamethazine and feasting on the nitrogen and carbon bits.
As Darwin might have predicted, long-term exposure to the antibiotics meant that the bugs that could tolerate the compounds were more likely to survive than their weaker brethren. Over years and many generations, antibiotic-digesting bacteria would be able to dominate in that chemical-soaked environment.
In one respect, the evolution of antibiotic-eating bacteria could be seen as a good thing, as it means that the drugs in agricultural runoff will be broken down naturally and not linger in the environment for very long. That could mean that farmers could argue that agricultural antibiotics are having less of an impact on the environment in the long term. Enterprising biotech companies could also possibly engineer bacteria to clean up the runoff from treated livestock.
But don't start feeling better about your factory-farmed meat just yet.
Bacteria are notorious for swapping genes with each other through a process called horizontal gene transfer. Genes can migrate from one bacteria to another through several avenues -- they can be passed along by viruses, absorbed directly through a process called transformation or exchanged through connections in a copulation-like technique called bacterial conjugation.
So just because the antibiotic-eating trait is limited to soil bacteria right now, there's no reason that it couldn't migrate to a more threatening microbe.
“A reservoir of antibiotic resistance genes in the environment that is made larger through contamination with agricultural wastes may represent an enhanced threat to human health,” the authors warn.
SOURCE: Topp et al. “Accelerated Biodegredation of Veterinary Antibiotics in Agricultural Soil following Long-Term Exposure, and Isolation of a Sulfamethazine-degrading Microbacterium sp.” Journal of Environmental Quality published online 6 December 2012.