Roughly 80 percent of all antibiotics dispersed are given to livestock, according to the FDA. The World Health Organization (WHO) has also cautioned that we are moving toward a post-antibiotic era, given the high levels of antibiotic resistance. For these reasons, it is simultaneously disturbing and puzzling that a new report has found that fertilizing with cow manure can contribute to antibiotic resistance, even if the animal had not been given antibiotics. The research was led by Nikolina Udikovic-Kolic of Yale University and the paper was published in the Proceedings of the National Academy of Sciences.
Manure is commonly used as a fertilizer because it adds nitrogen and organic material into the soil, helping plants to grow. However, a new report released by the FDA last week indicated that distribution of antibiotics to livestock is rising. This is adding to concerns about how those animals contribute to antibiotic resistance when their manure is used as a fertilizer for crops.
To explore this question, the team fertilized a patch of land with manure from cows that had never been given antibiotics and compared it against a patch that had been inorganically fertilized with nitrogen. Soil samples taken prior to application and two weeks after were compared.
Oddly, the patch treated with manure had higher levels of resistant bacteria, even though the cattle had never been treated with antibiotics. The resistant bacterial strains produce β-lactamases, an enzyme that contributes to resistance against penicillins and other antibiotics.
It will require further research to be sure, but there could be other factors within the manure that is causing these resistant bacterial strains to proliferate. Many microbes have antimicrobial properties against other microbes as a means of self defense, so there could be something within the manure that is causing a proliferation of resistant bacteria in the soil.
Strains that produce β-lactamases are also typically resistant to heavy metals found in manure, which could have allowed the bacteria to out-compete other species in the soil. While the mechanism is not fully understood, treating the soil with antibiotics to get rid of the resistant strains would probably make the situation much worse. However, the study does not indicate what happens to soil samples with manure from cows who have been given antibiotics.
There has been research into whether or not fields of transgenic plants have passed along drug-resistant bacteria to the soil, but it appears organic agriculture might not be completely off the hook either. As organic farmers prefer the use of manure over inorganic nitrogen fertilizers, they could also be contributing to higher levels of resistant bacteria in the soil, which could potentially pose a risk to human health.