Antibiotic-eating microbe unearthed

Antibiotic-eating microbe unearthed

- Soil microbe degrades antibiotic for nutrition.

- Capability to break down sulfonamides may be widespread.

- Negative effects still occurring in soil.

IT is well known that bacteria, when exposed to antibiotics for long periods, will find ways to resist the drugs by quickly pumping them out of their cells, for instance, or modifying the compounds so they're no longer toxic, according to the American Society of Agronomy.

New research has uncovered another possible mechanism of antibiotic "resistance" in soil. In a paper to be published this week in the Journal of Environmental Quality, a group of researchers from Canada and France reported on a soil bacterium that breaks down the common veterinary antibiotic sulfamethazine and uses it for growth.

Certain soil bacteria are already known to live off, or "eat," agricultural pesticides and herbicides, according to the study's leader, Ed Topp, a soil microbiologist with Agriculture & Agri-Food Canada in London, Ont. In fact, the microbes' presence in farm fields can cause these agricultural chemicals to fail.

However, to Topp's knowledge, this is the first report of a soil microorganism that degrades an antibiotic both to protect itself and get nutrition.

"I think it's kind of a game-changer in terms of how we think about our environment and antibiotic resistance," he said.

While commonly fed to pigs and other livestock, antibiotics are thought to keep animals healthier, but they're also excreted in manure, which is then used as fertilizer in North American farm fields.

Concerns about widespread antibiotic resistance are what led Topp and his collaborators to set up an experiment 14 years ago in which they dosed soils annually with environmentally relevant concentrations of three veterinary antibiotics: sulfamethazine, tylosin and chlortetracycline.

The researchers first wanted to know whether these yearly applications were promoting higher levels of antibiotic resistance in soil bacteria. However, a few years ago, they also decided to compare the persistence of the drugs in soil plots that had been repeatedly dosed versus fresh soils where antibiotics were never applied.

They did this experiment because of previous work indicating that pesticides often break down more quickly in soils with a long history of exposure, indicating that pesticide-degrading microbes have been selected for over time, Topp explained.

Still, it came as a surprise when they also saw antibiotics degrading much faster in long-treated plots than in fresh, control soils, he said. In particular, sulfamethazine -- a member of the antibiotic class called sulfonamides -- disappeared up to five times faster (Table).

The researchers subsequently cultured from the treated plots a new strain of microbacterium, an actinomycete that uses sulfamethazine as a nitrogen and carbon source. Extremely common in soil, actinomycete bacteria are known to degrade a wide range of organic compounds.

Now, at least two other sulfonamide-degrading microbacterium strains have been reported, Topp said: one from soil and another from a sewage treatment plant.

Taken together, the findings suggest that the capability to break down sulfonamides could be widespread. If it is true that "the microbiology in the environment is learning to break down these drugs more rapidly when exposed to them, this would effectively reduce the amount of time that the environment is exposed to these drugs and, therefore, possibly attenuate the impacts," Topp said.

However, he cautioned that negative effects are still occurring. In particular, long-term exposure to antibiotics puts significant pressure on soil bacteria to evolve resistance, which they typically do by giving and receiving genes that let them detoxify drugs or keep the compounds out of their cells, Topp explained.

What the new research suggests, though, is that soil bacteria could be swapping genes for breaking down antibiotics at the same time.

"My guess is that's probably what's happening, but it remains to be determined," Topp said. "It's actually extremely fascinating."

The work was funded by Agriculture & Agri-Food Canada.

 

Days required to dissipate 50% (DT50) of the indicated antibiotic in field soil that had received five annual applications each of 10 mg/kg and a 1 mg/kg mixture of the three drugs, plus a control soil that had not been previously exposed to the drugs

 

-DT50 (days)-

 

With history

With no history

Antibiotic

of exposure

of exposure

Sulfamethazine

1.3 + 0.3*

5.3 + 2.0**

Tylosin

2.0 + 1.0

10.2 + 5.4**

Chlortetracycline

3.3 + 0.5

2.8 + 0.4

*Data presented as mean + standard deviation.

**Indicates a statistically significant treatment effect (P < 0.05).

Source: Journal of Environmental Quality.

 

Volume:84 Issue:52

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