Abstract
Rhodococcus equi is a facultative intracellular pathogen that can survive within macrophages of a wide variety of hosts, including immunosuppressed humans. Current antibiotherapy is often ineffective, and novel therapeutic strategies are urgently needed to tackle infections caused by this pathogen. In this study, we identified three mycoredoxin-encoding genes (mrx) in the genome of R. equi, and we investigated their role in virulence. Importantly, the intracellular survival of a triple mrx-null mutant (Δmrx1Δmrx2Δmrx3) in murine macrophages was fully impaired. However, each mycoredoxin alone could restore the intracellular proliferation rate of R. equi Δmrx1Δmrx2Δmrx3 to wild type levels, suggesting that these proteins could have overlapping functions during host cell infection. Experiments with the reduction-oxidation sensitive green fluorescent protein 2 (roGFP2) biosensor confirmed that R. equi was exposed to redox stress during phagocytosis, and mycoredoxins were involved in preserving the redox homeostasis of the pathogen. Thus, we studied the importance of each mycoredoxin for the resistance of R. equi to different oxidative stressors. Interestingly, all mrx genes did have overlapping roles in the resistance to sodium hypochlorite. In contrast, only mrx1 was essential for the survival against high concentrations of nitric oxide, while mrx3 was not required for the resistance to hydrogen peroxide. Our results suggest that all mycoredoxins have important roles in redox homeostasis, contributing to the pathogenesis of R. equi and, therefore, these proteins may be considered interesting targets for the development of new anti-infectives.
Highlights
Rhodococcus equi is a widely distributed actinobacterial pathogen, the main causative agent of pneumonia in young foals [1,2] and, an emerging opportunistic pathogen in immunocompromised patients, where it is often misdiagnosed [3,4,5]
The mrx1 clusters of R. equi and M. tuberculosis were clearly differentiated from the clusters found in corynebacterial, which might be indicative of specific adaptations to the different environments colonized by each of these bacteria (Figure S3)
The main preventative mechanisms described in actinobacteria to resist high concentrations of reactive oxygen species (ROS) and reactive nitrogen species (RNS) are based on the action of catalases [46,47,48,49,50], superoxide dismutases [51], mycothiol peroxidases [52], and their transcriptional regulators, OxyR being the most studied [46,48,53]
Summary
Rhodococcus equi is a widely distributed actinobacterial pathogen, the main causative agent of pneumonia in young foals [1,2] and, an emerging opportunistic pathogen in immunocompromised patients, where it is often misdiagnosed [3,4,5]. R. equi is highly prevalent in farms due to its adaptation to the horse intestine. This facultative intracellular pathogen is transmitted by inhaling contaminated soil dust or respiratory particles produced by infected animals [6,7]. R. equi can survive within macrophages, and such intracellular survival is mediated by different members of a family of megaplasmids, known as pVAP [12]. The pathogenicity islands of pVAP plasmids comprise several genes that encode virulence-associated proteins (Vaps), VapA being the Antioxidants 2019, 8, 558; doi:10.3390/antiox8110558 www.mdpi.com/journal/antioxidants
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