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 soil-borne 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 22-30°C (environmental temperatures), not 37°C. All transcojugants were able to transfer pRErm46 back to R. equi. Conjugation could not be detected with Mycobacterium and Corynebacterium spp. or several members of the more distant phylum Firmicutes such as Enterococcus, Streptococcus or Staphylococcus. Thus, 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 R. 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 rhodocci as well as Nocardia and Arhtrobacter spp. The reported data support the notion that environmental microbiota may act as reservoirs for the endemic maintenance of antimicrobial resistance in an antibiotic pressurized farm habitat.

结合是细菌种群中抗生素耐药性扩散和维持的主要机制之一。我们最近表明，在土传马和人畜共患病病原体马红球菌中新兴的大环内酯类抗药性是由87kb结合质粒pRErm46上的erm（46）基因赋予的。在这里，我们调查了pRErm46的夫妻转移性可能遇到的14种代表细菌马红球菌的环境栖息地。体外交配实验证明与红球菌属的不同成员以及诺卡氏菌和节杆菌属的结合。在〜10的频率范围内^-2至10 ^-6。在土壤和马粪中的交配实验中也观察到了pRErm46的转移，尽管频率很低，并且在22-30°C（环境温度）而不是37°C下长时间孵育后也是如此。所有的转结合剂都可以将pRErm46转移回R. equi。分枝杆菌和棒状杆菌无法检测到结合。或距离较远的门扇菌的几个成员，例如肠球菌，链球菌或葡萄球菌。因此，pRErm46宿主范围似乎跨越了多个放线菌顺序，并且在棒状杆菌内有一定的宿主限制。所有获得pRErm46的细菌种类均表现出增加的大环内酯类抗药性，并且对体质没有明显的有害影响，除了罗丹红球菌以外。我们的结果表明，环境微生物群的放线菌成员既可以获取和传播马R. equi pRErm46质粒，也可能有助于马场中erm（46）介导的大环内酯类抗药性的维持和传播。

重要性

这项研究表明，在高效率水平传送马红球菌接合质粒pRErm46，最近被鉴定为这样的病原体分离物的马，以及从不同的环境之间的新兴大环内酯类耐药性的原因放线菌在内的各种rhodocci的以及诺卡氏菌和Arhtrobacter spp。报告的数据支持以下观点：环境微生物群可以充当抗生素加压农场生境中特有维持抗菌素耐药性的储存库。