Fitness cost conferred by the novel erm(51) and rpoB mutation on environmental multidrug resistant-Rhodococcus equi.

The research investigated the impact of the new erm(51) gene and associated rpoB mutation on the growth of multidrug-resistant Rhodococcus equi bacteria under different nutrient conditions, and found that its fitness is reduced in nutrient-rich conditions, but not under restricted conditions.

Objectives of the Study

• The main objective of this study was to understand how the recently discovered erm(51) gene and a mutation in the rpoB gene affect the growth of multidrug resistant Rhodococcus equi (MDR-R. equi) bacteria under varying nutrient conditions.
• This was significant because R. equi is a common cause of serious pneumonia in young horses (foals), and there has been a recent emergence of macrolide-resistant forms of the bacteria detected in foals and in their environments, particularly in Kentucky horse farms.

Methods Used in the Study

• Bacterial growth curves were created for three isolates of MDR-R. equi and three isolates of a wild-type non-resistant variety of R. equi that were recovered from environmental samples in central Kentucky horse farms.
• Growth of these isolates was measured over a period of 30.5 hours in three different mediums: a nutrient-rich brain-heart infusion broth (BHI), a minimal medium (MM), and a minimal medium with no iron (MM-I).

Key Findings of the Study

• All isolates of the bacteria demonstrated higher growth in the nutrient-rich BHI broth than in either of the nutrient-restricted media, MM or MM-I.
• Interestingly, the MDR-R. equi demonstrated significantly lower growth compared to the wild-type isolates in the BHI broth, which indicates a lower fitness. However, the growth of MDR-R. equi was not significantly different from the wild type bacteria in either of the nutrient-restricted conditions.
• This suggests that in nutrient-rich environments, MDR-R. equi could potentially be outcompeted by wild-type isolates, especially if antimicrobial usage is reduced, which lowers the pressure for resistance to develop and persist.
• However, the study also highlights the concern that MDR-R. equi may be able to survive and persist in more challenge environments, with potential selective pressures such as antimicrobials or nutrient restriction.