Results of a new study show that poultry raised on farms that have shifted to organic practices have significantly lower levels of antibiotic- and multi-drug resistant enterococci bacteria.

Use of antibiotics in conventional animal food production has been linked to the rise in strains of disease-causing bacteria that are resistant to one or multiple antibiotics. The results suggest that removing antibiotic use from large-scale poultry farms in the US "can result in immediate and significant reductions in antibiotic resistance for some bacteria," according to "Lower Prevalance of Antibiotic-resistant Enterococci on U.S. Conventional Poultry Farms that Transitioned to Organic Practices," published in the August 10 online edition of Environmental Health Perspectives.

We initially hypothesized that we would see some differences in on-farm levels of antibiotic-resistant enterococci when poultry farms transitioned to organic practices. But we were surprised to see that the differences were so significant across several different classes of antibiotics even in the very first flock that was produced after the transition to organic standards, explained Dr. Amy R. Sapkota, the study's lead researcher, an assistant professor with the Maryland Institute for Applied Environmental Health. "It is very encouraging.

The scientists found that 67% of Enterococcus faecalis obtained from poultry from conventional poultry farms were resistant to the antibiotic erythromycin, while only 18% of the bacteria from newly organic poultry farms were resistant to it. Significant differences were also found in levels of multi-drug resistant bacteria - organisms resistant to three or more antimicrobial classes, according to the report.

Forty-two percent of Enterococcus faecalis samples from conventional farms were multi-drug resistant compared to only 10% from newly organic farms, and 84% of Enterococcus faecium from conventional farms were multi-drug resistant compared to 17% of those from newly organic farms.

The research team conducted its investigation by studying poultry-raising practices at ten conventional and ten organic farms in the mid-Atlantic US in order to find out the effects of removing antibiotics would have.

Enterococci bacteria were chosen because "these microorganisms are found in all poultry, including poultry on both organic and conventional farms. The enterococci are also notable opportunistic pathogens in human patients staying in hospitals," Sapkota said.

In addition, many of the antibiotics included in farm animal feed are active against Gram-positive bacteria, such as enterococci. These and enterococci's ability to easily exchange resistance genes with other bacteria make them a good choice for studies that aim to understand the effects of antibiotic use on animal farms and the effects of eliminating the use of antibiotics.

"While we know that the dynamics of antibiotic resistance differ by bacterium and antibiotic, these findings show that, at least in the case of enterococci, we begin to reverse resistance on farms even among the first group of animals that are grown without antibiotics. Now we need to look forward and see what happens over 5 years, 10 years in time," says Sapkota.