How NADH Respiration Regulates Staphylococcus aureus Virulence via Fatty Acid and SaeRS Pathways Original paper

What was studied?

This original research explored how NADH-dependent aerobic respiration regulates virulence in Staphylococcus aureus, a major human pathogen. Specifically, the study examined the physiological and pathogenic consequences of disabling the two type-2 respiratory NADH dehydrogenases (NDH-2s: NdhC and NdhF) that S. aureus possesses, focusing on their role in energy metabolism, redox balance, biofilm formation, and production of virulence factors such as alpha-toxin. The work also investigated the regulatory connections between these metabolic pathways and key two-component regulatory systems—most notably, the SaeRS system, which responds to fatty acid metabolism and controls virulence gene expression. Through a combination of genetic knockouts, biochemical assays, and mouse infection models, the study aimed to reveal how respiratory metabolism influences S. aureus pathogenicity and to suggest potential antimicrobial strategies targeting these processes.

Who was studied?

The research used laboratory strains and clinical isolates of Staphylococcus aureus, including the Newman, USA300 LAC JE2, and UAMS-1 strains. Both single and double gene knockouts for ndhC and ndhF were created to assess their role in vitro (growth, respiration, redox status, biofilm, and toxin production) and in vivo (using murine models). Mouse infection experiments utilized outbred CD-1 and C57BL/6 female mice, which were systemically infected to evaluate organ colonization and bacterial burden. The study also employed genetic complementation and regulatory mutants (e.g., saeRS, srrAB) to dissect the pathways connecting respiration, fatty acid metabolism, and virulence regulation.

Most important findings

The study revealed that while neither NdhC nor NdhF is individually essential for S. aureus respiration or growth under standard conditions, their absence profoundly affects virulence. Inactivation of either NDH-2 gene drastically reduced the bacterium’s ability to form biofilms, produce alpha-toxin, and colonize specific organs (liver, heart, spleen) in mice. These phenotypes were not associated with general growth defects or increased oxidative stress susceptibility but rather with disruptions in fatty acid metabolism and redox balance, specifically, increased NADH/NAD+ ratios and altered menaquinol/menaquinone pools. Notably, the double knockout of both ndhC and ndhF paradoxically restored virulence levels near those of wild-type S. aureus, likely by triggering a compensatory metabolic switch involving alternative electron donors (e.g., lactate, pyruvate). The SaeRS two-component system was found to be the key regulatory link: NDH-2 deficiency led to fatty acid accumulation, which repressed SaeRS activity and thus downregulated alpha-toxin production and biofilm formation. Manipulating fatty acid levels (e.g., by addition of BSA or overexpression of acyl-ACP synthetase) could rescue these virulence phenotypes. This work establishes a direct mechanistic connection between respiratory NADH oxidation, redox, and lipid homeostasis, and the regulation of major S. aureus virulence determinants.

Key implications

These findings significantly enhance our understanding of how S. aureus integrates metabolic signals with virulence regulation, emphasizing the centrality of NADH-dependent respiration in controlling redox and fatty acid balance, and thereby pathogenicity. Identifying NDH-2s and the SaeRS system as pivotal nodes in this regulatory network suggests new antimicrobial targets: drugs that disrupt NADH oxidation or manipulate fatty acid homeostasis could impair S. aureus biofilm formation and toxin production, attenuating infection. For clinicians, this research underscores the importance of metabolic adaptability in S. aureus pathogenesis. It supports the rationale for therapies aimed at metabolic vulnerabilities, particularly in biofilm-associated or persistent infections.