Staphylococcus aureus Does Not Synthesize Arginine from Proline under Physiological Conditions

Subminimum inhibitory concentrations of rifampicin attenuate methicillin-resistant staphylococcus aureus virulence by suppressing SaeRS two-component system and arginine metabolism-related pathways

OBJECTIVES

Methicillin-resistant Staphylococcus aureus (MRSA) poses a significant threat to global public health, prompting the exploration of alternative strategies to mitigate its virulence. This study investigates the impact of subminimum inhibitory concentrations (sub-MICs) of rifampicin on MRSA virulence, aiming to provide insights for optimizing antibiotic treatment strategies.

METHODS

Enzyme-linked immunosorbent assay and western blot analysis were used to assess α-hemolysin expression. Transcriptomic sequencing and RT-qPCR analyzed gene expression changes in MRSA treated with sub-MICs of rifampicin. Mutant strains (ΔsaeR and ΔargGH) were constructed to validate the roles of the SaeRS system and arginine metabolism. Thermal shift assays evaluated the interaction between L-arginine and SaeR protein. In vivo murine models and Galleria mellonella infection models were used to assess the anti-virulence effects of rifampicin.

RESULTS

Our findings reveal that sub-MICs of rifampicin significantly reduce the expression of MRSA α-hemolysin. Transcriptomic sequencing and RT-qPCR analysis suggest a dual-pathway mechanism, wherein rifampicin suppresses virulence by indirectly inhibiting the SaeRS two-component system and disrupting arginine metabolism-related pathways. The construction of a saeR knockout mutant (ΔsaeR) and an arginine biosynthesis deficient mutant (ΔargGH) further supports this mechanism. Notably, exogenous l-arginine supplementation reverses rifampicin's inhibitory effect on α-hemolysin expression, underscoring the pivotal role of l-arginine metabolism in MRSA virulence regulation. Thermal shift assays demonstrate a direct interaction between l-arginine and SaeR protein, elucidating the intricate interplay between metabolic pathways and virulence regulation. In vivo studies confirm that sub-MICs of rifampicin attenuate the severity of skin abscesses in a murine model, improve survival rates in bloodstream infection models, and mitigate inflammation in both skin and lung tissues.

CONCLUSION

This study highlights the potential of rifampicin as an anti-virulence agent and pave the way for the development of innovative therapeutic strategies targeting MRSA infections.

Regulatory dynamics of arginine metabolism in Staphylococcus aureus

Staphylococcus aureus is a highly significant pathogen with several well studied and defined virulence factors. However, the metabolic pathways that are required to facilitate infection are not well described. Previous data have documented that S. aureus requires glucose catabolism during initial stages of infection. Therefore, certain nutrients whose biosynthetic pathway is under carbon catabolite repression and CcpA, including arginine, must be acquired from the host. However, even though S. aureus encodes pathways to synthesize arginine, biosynthesis of arginine is repressed even in the absence of glucose. Why is S. aureus a functional arginine auxotroph? This review discusses recently described regulatory mechanisms that are linked to repression of arginine biosynthesis using either proline or glutamate as substrates. In addition, recent studies are discussed that shed insight into the ultimate mechanisms linking arginine auxotrophy and infection persistence.

Combined Transcriptome and Metabolome Analysis Reveals That Carbon Catabolite Repression Governs Growth and Pathogenicity in Verticillium dahliae

Carbon catabolite repression (CCR) is a common transcriptional regulatory mechanism that microorganisms use to efficiently utilize carbon nutrients, which is critical for the fitness of microorganisms and for pathogenic species to cause infection. Here, we characterized two CCR genes, VdCreA and VdCreC, in Verticillium dahliae that cause cotton Verticillium wilt disease. The VdCreA and VdCreC knockout mutants displayed slow growth with decreased conidiation and microsclerotium production and reduced virulence to cotton, suggesting that VdCreA and VdCreC are involved in growth and pathogenicity in V. dahliae. We further generated 36 highly reliable and stable ΔVdCreA and ΔVdCreC libraries to comprehensively explore the dynamic expression of genes and metabolites when grown under different carbon sources and CCR conditions. Based on the weighted gene co-expression network analysis (WGCNA) and correlation networks, VdCreA is co-expressed with a multitude of downregulated genes. These gene networks span multiple functional pathways, among which seven genes, including PYCR (pyrroline-5-carboxylate reductase), are potential target genes of VdCreA. Different carbon source conditions triggered entirely distinct gene regulatory networks, yet they exhibited similar changes in metabolic pathways. Six genes, including 6-phosphogluconolactonase and 2-ODGH (2-oxoglutarate dehydrogenase E1), may serve as hub genes in this process. Both VdCreA and VdCreC could comprehensively influence the expression of plant cell wall-degrading enzyme (PCWDE) genes, suggesting that they have a role in pathogenicity in V. dahliae. The integrated expression profiles of the genes and metabolites involved in the glycolysis/gluconeogenesis and pentose phosphate pathways showed that the two major sugar metabolism-related pathways were completely changed, and GADP (glyceraldehyde-3-phosphate) may be a pivotal factor for CCR under different carbon sources. All these results provide a more comprehensive perspective for further analyzing the role of Cre in CCR.

Restriction of arginine induces antibiotic tolerance in Staphylococcus aureus

Staphylococcus aureus is responsible for a substantial number of invasive infections globally each year. These infections are problematic because they are frequently recalcitrant to antibiotic treatment. Antibiotic tolerance, the ability of bacteria to persist despite normally lethal doses of antibiotics, contributes to antibiotic treatment failure in S. aureus infections. To understand how antibiotic tolerance is induced, S. aureus biofilms exposed to multiple anti-staphylococcal antibiotics are examined using both quantitative proteomics and transposon sequencing. These screens indicate that arginine metabolism is involved in antibiotic tolerance within a biofilm and support the hypothesis that depletion of arginine within S. aureus communities can induce antibiotic tolerance. Consistent with this hypothesis, inactivation of argH, the final gene in the arginine synthesis pathway, induces antibiotic tolerance. Arginine restriction induces antibiotic tolerance via inhibition of protein synthesis. In murine skin and bone infection models, an argH mutant has enhanced ability to survive antibiotic treatment with vancomycin, highlighting the relationship between arginine metabolism and antibiotic tolerance during S. aureus infection. Uncovering this link between arginine metabolism and antibiotic tolerance has the potential to open new therapeutic avenues targeting previously recalcitrant S. aureus infections.

Glutamate-dependent arginine biosynthesis requires the inactivation of spoVG, sarA, and ahrC in Staphylococcus aureus

Genome sequencing has demonstrated that Staphylococcus aureus encodes arginine biosynthetic genes argDCJBFGH synthesizing proteins that mediate arginine biosynthesis using glutamate as a substrate. Paradoxically, however, S. aureus does not grow in a defined, glutamate-replete medium lacking arginine and glucose (CDM-R). Studies from our laboratory have found that specific mutations are selected by S. aureus that facilitate growth in CDM-R. However, these selected mutants synthesize arginine utilizing proline as a substrate rather than glutamate. In this study, we demonstrate that the ectopic expression of the argDCJB operon supports the growth of S. aureus in CDM-R, thus documenting the functionality of this pathway. Furthermore, suppressor mutants of S. aureus JE2 putA::Tn, which is defective in synthesizing arginine from proline, were selected on CDM-R agar. Genome sequencing revealed that these mutants had compensatory mutations within both spoVG, encoding an ortholog of the Bacillus subtilis stage V sporulation protein, and sarA, encoding the staphylococcal accessory regulator. Transcriptional studies document that argD expression is significantly increased when JE2 spoVG sarA was grown in CDM-R. Lastly, we found that a mutation in ahrC was required to induce argD expression in JE2 spoVG sarA when grown in an arginine-replete medium (CDM), suggesting that AhrC also functions to repress argDCJB in an arginine-dependent manner. In conclusion, these data indicate that the argDCJB operon is functional when transcribed in vitro and that SNPs within potential putative regulatory proteins are required to alleviate the repression.IMPORTANCEAlthough Staphylococcus aureus has the capability to synthesize all 20 amino acids, it is phenotypically auxotrophic for several amino acids including arginine. This work identifies putative regulatory proteins, including SpoVG, SarA, and AhrC, that function to inhibit the arginine biosynthetic pathways using glutamate as a substrate. Understanding the ultimate mechanisms of why S. aureus is selected to repress arginine biosynthetic pathways even in the absence of arginine will add to the growing body of work assessing the interactions between metabolism and S. aureus pathogenesis.

Restriction of Arginine Induces Antibiotic Tolerance in Staphylococcus aureus

Staphylococcus aureus is responsible for a substantial number of invasive infections globally each year. These infections are problematic because they are frequently recalcitrant to antibiotic treatment, particularly when they are caused by Methicillin-Resistant Staphylococcus aureus (MRSA). Antibiotic tolerance, the ability for bacteria to persist despite normally lethal doses of antibiotics, is responsible for most antibiotic treatment failure in MRSA infections. To understand how antibiotic tolerance is induced, S. aureus biofilms exposed to multiple anti-MRSA antibiotics (vancomycin, ceftaroline, delafloxacin, and linezolid) were examined using both quantitative proteomics and transposon sequencing. These screens indicated that arginine metabolism is involved in antibiotic tolerance within a biofilm and led to the hypothesis that depletion of arginine within S. aureus communities can induce antibiotic tolerance. Consistent with this hypothesis, inactivation of argH, the final gene in the arginine synthesis pathway, induces antibiotic tolerance under conditions in which the parental strain is susceptible to antibiotics. Arginine restriction was found to induce antibiotic tolerance via inhibition of protein synthesis. Finally, although S. aureus fitness in a mouse skin infection model is decreased in an argH mutant, its ability to survive in vivo during antibiotic treatment with vancomycin is enhanced, highlighting the relationship between arginine metabolism and antibiotic tolerance during S. aureus infection. Uncovering this link between arginine metabolism and antibiotic tolerance has the potential to open new therapeutic avenues targeting previously recalcitrant S. aureus infections.