Even, Nature explains, from cows that have never been around antibiotics. Something about cow manure runoff helps resistant bacteria grow in the soil:
Because manure itself is known to change the composition of bacterial communities in soil, a team led by microbiologist Jo Handelsman, then at Yale University in New Haven, Connecticut, decided to examine whether it also affects drug resistance. The team treated soil samples with either a nitrogen-based fertilizer or with manure from cows that had never been fed antibiotics.
The researchers examined soil bacteria sampled before and after the treatment, searching for genes that encode enzymes called ?-lactamases, which break down a class of antibiotic that includes penicillin.
Two weeks after treatment, the soil spread with manure contained significantly higher numbers of bacteria producing ?-lactamases than did soil treated with only the nitrogen-based fertilizer. By tracing genetic markers in the resistant bacteria, the researchers found that these bacteria came from the soil rather than from the manure, suggesting that the manure treatment had helped these natural bacteria to grow by feeding them or eliminating their competitors. The manure was particularly beneficial for Pseudomonas species, which are common in human infections.
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But it is unclear how manure creates a better environment for antibiotic-resistant bacteria. Handelsman — now associate director for science at the White House Office of Science and Technology Policy — and her colleagues suggest that certain nutrients or heavy metals in the manure could be responsible, because bacteria with ?-lactamases are also more likely to be resistant to metals. The authors say that they plan to test this in the future.