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

Abstract

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 Rhodococcus equi is conferred by the erm(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 R. equi in the environmental habitat. In vitro mating experiments demonstrated conjugation to different members of the genus Rhodococcus as well as to Nocardia and Arthrobacter spp. at frequencies ranging from ∼10−2 to 10−6. 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 R. equi. Conjugation could not be detected with Mycobacterium or Corynebacterium spp. or several members of the more distant phylum Firmicutes such as Enterococcus, Streptococcus, or Staphylococcus. Thus, the pRErm46 host range appears to span several actinobacterial orders with certain host restriction within the Corynebacteriales. All bacterial species that acquired pRErm46 expressed increased macrolide resistance with no significant deleterious impact on fitness, except in the case of Rhodococcus rhodnii. Our results indicate that actinobacterial members of the environmental microbiota can both acquire and transmit the R. equi pRErm46 plasmid and thus potentially contribute to the maintenance and spread of erm(46)-mediated macrolide resistance in equine farms. IMPORTANCE This study demonstrates the efficient horizontal transfer of the Rhodococcus equi conjugative plasmid pRErm46, recently identified as the cause of the emerging macrolide resistance among equine isolates of this pathogen, to and from different environmental Actinobacteria, including a variety of rhodococci as well as Nocardia and Arthrobacter 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.

Spread of Multidrug-Resistant Rhodococcus equi, United States

“…All of these antibiotic drugs are listed as critically or highly important for human medicine by the World Health Organization (31). Around 9% of human R. equi infections are caused by equine-derived (pVAPApositive) strains, and about half of human cases are caused by porcine-derived (pVAPB-positive) isolates (5), which recent in vitro data demonstrate can also acquire pRErm46 (32). Therefore, in addition to compromising the therapeutic management of equine R. equi infection, these isolates represent a potential hazard to human health because of the risk of zoonotic transmission (or horizontal spread of the pRErm46 resistance plasmid to other pathogens, either directly or through environmental microbiota [32]).…”

Manure as a Potential Hotspot for Antibiotic Resistance Dissemination by Horizontal Gene Transfer Events

“…The molecular analysis of ermB genes from human, swine and poultry Enterococcus faecium isolates suggests that the HGT of antibiotic resistance genes plays a greater role in the dissemination of resistant determinants than the direct transmission of resistant strains [118]. The soil-borne equine and zoonotic pathogen Rhodococcus equi has been shown to be able to conjugate with other environmental Actinobacteria that share the habitat, leading to macrolide resistance due to the dissemination of erm(46) embedded in plasmid pRErm46; conjugation between two Rhodococcus equi strains also occurred in equine manure at environmental temperatures (22 and 30 • C) [119].…”

Responses to Ecopollutants and Pathogenization Risks of Saprotrophic Rhodococcus Species

“…Moreover, the spread of antibiotic resistance in the environment poses a particular risk. “ R. equi ” is a reservoir for the antibiotic resistance genes and is involved in the dissemination of resistance genes in nature through highly transferrable conjugative plasmids, which leads to the emergence of multidrug resistance among non-pathogenic rhodococci [ 89 , 97 ].…”