Horizontal Spread of Rhodococcus equi Macrolide Resistance Plasmid pRErm46 across Environmental .

Conjugation is one of the main mechanisms involved in the spread and maintenance of antibiotic resistance in bacterial populations. We recently showed that the emerging macrolide resistance in the soilborne equine and zoonotic pathogen is conferred by the (46) gene carried on the 87-kb conjugative plasmid pRErm46. Here, we investigated the conjugal transferability of pRErm46 to 14 representative bacteria likely encountered by in the environmental habitat. mating experiments demonstrated conjugation to different members of the genus as well as to and spp. at frequencies ranging from ∼10 to 10 pRErm46 transfer was also observed in mating experiments in soil and horse manure, albeit at a low frequency and after prolonged incubation at 22 to 30°C (environmental temperatures), not 37°C. All transconjugants were able to transfer pRErm46 back to Conjugation could not be detected with or spp. or several members of the more distant phylum such as , , or Thus, the pRErm46 host range appears to span several actinobacterial orders with certain host restriction within the All bacterial species that acquired pRErm46 expressed increased macrolide resistance with no significant deleterious impact on fitness, except in the case of Our results indicate that actinobacterial members of the environmental microbiota can both acquire and transmit the pRErm46 plasmid and thus potentially contribute to the maintenance and spread of (46)-mediated macrolide resistance in equine farms. This study demonstrates the efficient horizontal transfer of the conjugative plasmid pRErm46, recently identified as the cause of the emerging macrolide resistance among equine isolates of this pathogen, to and from different environmental , including a variety of rhodococci as well as and spp. The reported data support the notion that environmental microbiotas may act as reservoirs for the endemic maintenance of antimicrobial resistance in an antibiotic pressurized farm habitat.