Abstract
Aims: Quinone compounds are electron carriers and have antimicrobial and toxic properties due to their mode of actions as electrophiles and oxidants. However, the regulatory mechanism of quinone resistance is less well understood in the pathogen Staphylococcus aureus.Results: Methylhydroquinone (MHQ) caused a thiol-specific oxidative and electrophile stress response in the S. aureus transcriptome as revealed by the induction of the PerR, QsrR, CstR, CtsR, and HrcA regulons. The SACOL2531-29 operon was most strongly upregulated by MHQ and was renamed as mhqRED operon based on its homology to the Bacillus subtilis locus. Here, we characterized the MarR-type regulator MhqR (SACOL2531) as quinone-sensing repressor of the mhqRED operon, which confers quinone and antimicrobial resistance in S. aureus. The mhqRED operon responds specifically to MHQ and less pronounced to pyocyanin and ciprofloxacin, but not to reactive oxygen species (ROS), hypochlorous acid, or aldehydes. The MhqR repressor binds specifically to a 9–9 bp inverted repeat (MhqR operator) upstream of the mhqRED operon and is inactivated by MHQ in vitro, which does not involve a thiol-based mechanism. In phenotypic assays, the mhqR deletion mutant was resistant to MHQ and quinone-like antimicrobial compounds, including pyocyanin, ciprofloxacin, norfloxacin, and rifampicin. In addition, the mhqR mutant was sensitive to sublethal ROS and 24 h post-macrophage infections but acquired an improved survival under lethal ROS stress and after long-term infections.Innovation: Our results provide a link between quinone and antimicrobial resistance via the MhqR regulon of S. aureus.Conclusion: The MhqR regulon was identified as a novel resistance mechanism towards quinone-like antimicrobials and contributes to virulence of S. aureus under long-term infections.
Highlights
Staphylococcus aureus is a major human pathogen, which can cause several diseases including lifethreatening systemic and chronic infections, such as sepsis, necrotizing pneumonia, or endocarditis [3, 8, 47].The increasing prevalence of multiple antibiotic resistant strains, such as methicillin-resistant S. aureus, leads to treatment failure and high mortality rates [15, 54]
Using RNA-seq transcriptomics, we identified the mhqRED operon as most strongly induced by methylhydroquinone (MHQ) in S. aureus, which is controlled by SACOL2531 (MhqR), a close homolog to MhqR of B. subtilis [69]
About 70 genes displayed the highest fold-changes under MHQ stress ranging from 10 to 536 (M-values of 3.3–9), which could be mainly allocated to the TetR, QsrR, PerR, Fur, CtsR, CstR, CsoR, SigB, and GraRS regulons (Figs. 1 and 2 and Supplementary Fig. S2; Supplementary Tables S1 and S2)
Summary
Staphylococcus aureus is a major human pathogen, which can cause several diseases including lifethreatening systemic and chronic infections, such as sepsis, necrotizing pneumonia, or endocarditis [3, 8, 47].The increasing prevalence of multiple antibiotic resistant strains, such as methicillin-resistant S. aureus, leads to treatment failure and high mortality rates [15, 54]. Understanding the defense and resistance mechanisms of S. aureus to antibiotics and the host immune response, including reactive oxygen species (ROS) and reactive electrophilic species, will lead to the discovery of novel resistance mechanisms and potential new drug targets to combat multiple antimicrobial resistance. We characterized the novel MhqR repressor as important quinone-sensing and regulatory mechanism in S. aureus, which controls quinone detoxification genes and conferred resistance to quinones and quinone-like antimicrobial compounds, including fluoroquinolones (ciprofloxacin, norfloxacin), rifampicin, and pyocyanin. The constitutive expression of quinone detoxification genes in the mhqR mutant was suggested to decrease respiratory chain activity and to limit ROS production as mechanism of l-form growth [34]. In S. aureus, the YodB homologue QsrR has been ascribed to be implicated in quinone detoxification, which controls related quinone reductases and a nitroreductase, an flavin mononucleotide-linked monooxygenase, and thiol-dependent dioxygenases [33]. Due to the increasing prevalence of multiple antibiotic resistant S. aureus isolates, these results are important to understand the underlying mechanisms of antimicrobial resistance
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