Department of Large Animal Clinical Sciences, College of Veterinary Medicine & Biomedical Sciences, Texas A&M University, College Station, TX 77843, USA.
Pathobiology Department, College of Veterinary Medicine, Auburn University, 1130 wire Rd, Auburn, AL 36832, USA.
Vet Microbiol. 2022 Oct;273:109531. doi: 10.1016/j.vetmic.2022.109531. Epub 2022 Aug 5.
Rhodococcus equi is a common cause of severe pneumonia in foals. Emergence of macrolide-resistant R. equi isolated from foals and their environment has been reported in the United States. A novel erm(51) gene was recently identified in R. equi in soil from horse farms in Kentucky. Our objective was to determine the effect of the erm(51) gene and associated rpoB mutation on the fitness of multidrug resistant-R. equi (MDR-R. equi) under different nutrient conditions. Bacterial growth curves were generated for 3 MDR-R. equi isolates and 3 wild-type (WTN) R. equi isolates recovered from environmental samples of farms in central Kentucky. Growth was measured over 30.5 h in brain-heart infusion broth (BHI), minimal medium (MM), and minimal medium without iron (MM-I). All isolates had significantly (P < 0.05) higher growth in BHI compared to either MM or MM-I. MDR-R. equi exhibited significantly lower growth compared to WTN isolates in BHI (nutrient-rich condition), but not in either MM or MM-I (nutrient-restricted conditions). This study indicates that under nutrient-rich conditions fitness of MDR-R. equi is reduced relative to susceptible isolates; however, under nutrient-restricted conditions MDR-R. equi isolates grow similarly to susceptible isolates. These findings indicate that MDR-R. equi might be outcompeted by susceptible isolates in nature when practices to reduce antimicrobial pressure, such as reducing antimicrobial use in foals, are implemented. But it also raises the concern that these resistant genotypes might persist in the environment of horse-breeding farms in the face of selective pressures such as antimicrobials or nutrient restriction.
马红球菌是驹肺炎的常见病因。在美国，已报道从驹及其环境中分离出的大环内酯类耐药马红球菌的出现。最近在肯塔基州马场的土壤中发现了一种新型 erm(51) 基因。我们的目的是确定 erm(51) 基因和相关 rpoB 突变对不同营养条件下多药耐药马红球菌（MDR-R. equi）的适应性的影响。为 3 种 MDR-R. equi 分离株和 3 种从肯塔基州中部农场环境样本中回收的野生型（WTN）马红球菌分离株生成细菌生长曲线。在脑心浸液肉汤（BHI）、最小培养基（MM）和最小培养基中没有铁（MM-I）中测量了 30.5 小时的生长。与 MM 或 MM-I 相比，所有分离株在 BHI 中的生长均显著（P < 0.05）更高。与 WTN 分离株相比，MDR-R. equi 分离株在 BHI 中的生长速度显著降低（营养丰富条件），但在 MM 或 MM-I 中没有降低（营养限制条件）。本研究表明，在营养丰富的条件下，MDR-R. equi 的适应性相对于敏感分离株降低；然而，在营养限制的条件下，MDR-R. equi 分离株的生长与敏感分离株相似。这些发现表明，在减少抗菌压力的实践（例如减少驹的抗菌药物使用）实施时，MDR-R. equi 可能会在自然界中被敏感分离株所淘汰。但这也引起了人们的关注，即在面临选择性压力（如抗生素或营养限制）时，这些耐药基因型可能会在马养殖场的环境中持续存在。
