Antibiotic-resistant gene discovered in soil bacterium that commonly infects foals


R. equi is one of the most important causes of disease in young horses, researchers say.

A research team based in the University of Georgia College of Veterinary Medicine has discovered a novel gene-erm(46)-that confers antibiotic resistance in Rhodococcus equi, a soil-dwelling bacterium that commonly infects foals and causes opportunistic infections in immunocompromised people, according to a release from the university.

The finding was made in collaboration with researchers at the University of Edinburgh, Texas A&M University and the University of Washington.

R. equi, a gram-positive intracellular pathogen, is one of the most important causes of disease in foals between 3 weeks and 5 months of age, says team lead Dr. Steeve Giguère, DVM, PhD, DACVIM, chair of equine studies at UGA.

The researchers sequenced the genomes of antibiotic-resistant and antibiotic-susceptible R. equi isolates collected from foals in four states. They searched each isolates' genome for genes with similar sequences to known genes that cause bacterial resistance to the macrolide class of antibiotics in other bacterial species. Through their search, they discovered the new gene, named erm(46) by the Nomenclature Center for MLS Genes at the University of Washington.

When the team cloned erm(46) into susceptible R. equi isolates normally inhibited by antibiotics, they found that erm(46) induced a high level of resistance to macrolide, lincosamide and streptogramin B antibiotics. They also found that the gene can be transferred from resistant to susceptible isolates of R. equi during bacterial mating.

“This process likely contributes to the spread of resistance,” Giguère says.

Their finding, recently published in the Journal of Antimicrobial Chemotherapy, is the first molecular characterization of resistance to these three classes of antibiotics in R. equi.

“Before, we knew we had resistant isolates, but we did not know how resistance occurred, and we had no molecular markers to identify and track the resistant bacteria,” Giguère says.

So far, Giguère and his team have identified antibiotic-resistant R. equi isolates carrying erm(46) in New York, Florida, Texas and Kentucky-where, on one farm producing 100 to 170 foals a year, as many of 40 percent of infected foals were found to carry resistant isolates. 

The bacterium is present in soil year-round, but because it typically causes disease only in foals up to 5 or 6 months of age, illnesses typically manifest in spring and summer.

“It is believed that most foals become infected through inhalation of R. equi within the first few weeks of life, and they start showing clinical signs of pneumonia between 3 weeks and 5 or 6 months of age,” says Giguère.

People may come into contact with R. equi on farms, while gardening or during other activities that disturb dirt; however, it typically only causes infection in immunocompromised individuals. 

Not all R. equi causes disease in foals. A piece of DNA located outside the chromosome, called a plasmid, is responsible for making R. equi virulent in foals. Variations on this plasmid can be found in pigs and also cattle.

The spread of drug-resistant R. equi may be a growing problem, Giguère says. “It's something that was unheard of 15 years ago, and now we've found it in multiple states. We need to conduct a well-designed epidemiological study to really know the prevalence of resistant isolates across the country.”

The work was funded by the Morris Animal Foundation.

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