The MsrAB reducing pathway of Streptococcus gordonii is needed for oxidative stress tolerance, biofilm formation, and oral colonization in mice

Emergent emm4 group A Streptococcus evidences a survival strategy during interaction with immune effector cells

The major gram-positive pathogen group A Streptococcus (GAS) is a model organism for studying microbial epidemics as it causes waves of infections. Since 1980, several GAS epidemics have been ascribed to the emergence of clones producing increased amounts of key virulence factors such as streptolysin O (SLO). Herein, we sought to identify mechanisms underlying our recently identified temporal clonal emergence among emm4 GAS, given that emergent strains did not produce augmented levels of virulence factors relative to historic isolates. By creating and analyzing isoallelic strains, we determined that a conserved mutation in a previously undescribed gene encoding a putative carbonic anhydrase was responsible for the defective in vitro growth observed in the emergent strains. We also identified that the emergent strains survived better inside macrophages and killed macrophages at lower rates than the historic strains. Via the creation of isogenic mutant strains, we linked the emergent strain "survival" phenotype to the downregulation of the SLO encoding gene and upregulation of the msrAB operon which encodes proteins involved in defense against extracellular oxidative stress. Our findings are in accord with recent surveillance studies which found a high ratio of mucosal (i.e., pharyngeal) relative to invasive infections among emm4 GAS. Since ever-increasing virulence is unlikely to be evolutionarily advantageous for a microbial pathogen, our data further understanding of the well-described oscillating patterns of virulent GAS infections by demonstrating mechanisms by which emergent strains adapt a "survival" strategy to outcompete previously circulating isolates.

From ubiquity to specificity: The diverse functions of bacterial thioredoxin systems

The thioredoxin (Trx) system, found universally, is responsible for the regeneration of reversibly oxidized protein thiols in living cells. This system is made up of a Trx and a Trx reductase, and it plays a central role in maintaining thiol-based redox homeostasis by reducing oxidized protein thiols, such as disulfide bonds in proteins. Some Trxs also possess a chaperone function that is independent of thiol-disulfide exchange, in addition to their thiol-disulfide reductase activity. These two activities of the Trx system are involved in numerous physiological processes in bacteria. This review describes the diverse physiological roles of the Trx system that have emerged throughout bacterial evolution. The Trx system is essential for responding to oxidative and nitrosative stress. Beyond this primary function, the Trx system also participates in redox regulation and signal transduction, and in controlling metabolism, motility, biofilm formation, and virulence. This range of functions has evolved alongside the diversity of bacterial lifestyles and their specific constraints. This evolution can be characterized by the multiplication of the systems and by the specialization of cofactors or targets to adapt to the constraints of atypical lifestyles, such as photosynthesis, insect endosymbiosis, or spore-forming bacteria.

Emergent emm4 group A Streptococcus evidences a survival strategy during interaction with immune effector cells

The major gram-positive pathogen group A Streptococcus (GAS) is a model organism for studying microbial epidemics as it causes waves of infections. Since 1980, several GAS epidemics have been ascribed to the emergence of clones producing increased amounts of key virulence factors such as streptolysin O (SLO). Herein, we sought to identify mechanisms underlying our recently identified temporal clonal emergence amongst emm4 GAS, given that emergent strains did not produce augmented levels of virulence factors relative to historic isolates. Through the creation and analysis of isoallelic strains, we determined that a conserved mutation in a previously undescribed gene encoding a putative carbonic anhydrase was responsible for the defective in vitro growth observed in the emergent strains. We also identified that the emergent strains survived better inside macrophages and killed macrophages at lower rates relative to the historic strains. Via creation of isogenic mutant strains, we linked the emergent strain "survival" phenotype to the downregulation of the SLO encoding gene and upregulation of the msrAB operon which encodes proteins involved in defense against extracellular oxidative stress. Our findings are in accord with recent surveillance studies which found high ratio of mucosal (i.e., pharyngeal) relative to invasive infections amongst emm4 GAS. Inasmuch as ever-increasing virulence is unlikely to be evolutionary advantageous for a microbial pathogen, our data furthers understanding of the well described oscillating patterns of virulent GAS infections by demonstrating mechanisms by which emergent strains adapt a "survival" strategy to outcompete previously circulating isolates.

Mycobacterium smegmatis secreting methionine sulfoxide reductase A (MsrA) modulates cellular processes in mouse macrophages

Methionine sulfoxide reductase A (MsrA) is an antioxidant repair enzyme that reduces the oxidized methionine (Met-O) in proteins to methionine (Met). Its pivotal role in the cellular processes has been well established by overexpressing, silencing, and knocking down MsrA or deleting the gene encoding MsrA in several species. We are specifically interested in understanding the role of secreted MsrA in bacterial pathogens. To elucidate this, we infected mouse bone marrow-derived macrophages (BMDMs) with recombinant Mycobacterium smegmatis strain (MSM), secreting a bacterial MsrA or M. smegmatis strain (MSC) carrying only the control vector. BMDMs infected with MSM induced higher levels of ROS and TNF-α than BMDMs infected with MSC. The increased ROS and TNF-α levels in MSM-infected BMDMs correlated with elevated necrotic cell death in this group. Further, RNA-seq transcriptome analysis of BMDMs infected with MSC and MSM revealed differential expression of protein and RNA coding genes, suggesting that bacterial-delivered MsrA could modulate the host cellular processes. Finally, KEGG pathway enrichment analysis identified the down-regulation of cancer-related signaling genes in MSM-infected cells, indicating that MsrA can potentially regulate the development and progression of cancer.

Methionine oxidation in bacteria: A reversible post-translational modification

Methionine is a sulfur-containing residue found in most proteins which are particularly susceptible to oxidation. Although methionine oxidation causes protein damage, it can in some cases activate protein function. Enzymatic systems reducing oxidized methionine have evolved in most bacterial species and methionine oxidation proves to be a reversible post-translational modification regulating protein activity. In this review, we inspect recent examples of methionine oxidation provoking protein loss and gain of function. We further speculate on the role of methionine oxidation as a multilayer endogenous antioxidant system and consider its potential consequences for bacterial virulence.

Carvacrol inhibits Streptococcus pyogenes biofilms by suppressing the expression of genes associated with quorum-sensing and reducing cell surface hydrophobicity

Streptococcus pyogenes is a leading cause of chronic and acute infections, including streptococcus pharyngitis. Biofilm formation by S. pyogenes can cause tolerance against antibiotics. Although penicillin remains the first choice of antibiotic for S. pyogenes, alternative approaches have gained interest due to treatment failures and hypersensitive individuals. Carvacrol is a monoterpenoid from herbal plants with selective biocidal activity on S. pyogenes. Therefore, the present study reveals the efficacy of carvacrol in inhibiting and eradicating S. pyogenes biofilm. The antibiofilm activities were investigated using colorimetric assays, microscopy, cell surface hydrophobicity, gene expression analysis, and in-silico analysis. Carvacrol also showed a minimum biofilm inhibitory concentration (MBIC) against S. pyogenes of 125 μg/mL. The electron microscopic and confocal microscopic analyses revealed a dose-dependent suppression of biofilm formation and a reduction in the biofilm thickness by carvacrol. Carvacrol also inhibited the biofilm-associated virulence factors such as cell surface hydrophobicity. Quantitative real-time polymerase chain reaction analysis showed the downregulation of speB, srtB, luxS, covS, dltA, ciaH, and hasA genes involved in biofilm formation. The results suggested the therapeutic potential of carvacrol against biofilm-associated streptococcal infections.

The Fused Methionine Sulfoxide Reductase MsrAB Promotes Oxidative Stress Defense and Bacterial Virulence in Fusobacterium nucleatum

Fusobacterium nucleatum, an anaerobic Gram-negative bacterium frequently found in the human oral cavity and some extra-oral sites, is implicated in several important diseases: periodontitis, adverse pregnancy outcomes, and colorectal cancer. To date, how this obligate anaerobe copes with oxidative stress and host immunity within multiple human tissues remains unknown. Here, we uncovered a critical role in this process of a multigene locus encoding a single, fused methionine sulfoxide reductase (MsrAB), a two-component signal transduction system (ModRS), and thioredoxin (Trx)- and cytochrome c (CcdA)-like proteins, which are induced when fusobacterial cells are exposed to hydrogen peroxide. Comparative transcriptome analysis revealed that the response regulator ModR regulates a large regulon that includes trx, ccdA, and many metabolic genes. Significantly, specific mutants of the msrAB locus, including msrAB, are sensitive to reactive oxygen species and defective in adherence/invasion of colorectal epithelial cells. Strikingly, the msrAB mutant is also defective in survival in macrophages, and it is severely attenuated in virulence in a mouse model of preterm birth, consistent with its failure to spread to the amniotic fluid and colonize the placenta. Clearly, the MsrAB system regulated by the two-component system ModRS represents a major oxidative stress defense pathway that protects fusobacteria against oxidative damage in immune cells and confers virulence by enabling attachment and invasion of multiple target tissues. IMPORTANCE F. nucleatum colonizes various human tissues, including oral cavity, placenta, and colon. How this obligate anaerobe withstands oxidative stress in host immune cells has not been described. We report here that F. nucleatum possesses a five-gene locus encoding a fused methionine sulfoxide reductase (MsrAB), a two-component signal transduction system (ModRS), and thioredoxin- and cytochrome c-like proteins. Regulated by ModRS, MsrAB is essential for resistance to reactive oxygen species, adherence/invasion of colorectal epithelial cells, and survival in macrophage. Unable to colonize placenta and spread to amniotic fluid, the msrAB mutant failed to induce preterm birth in a murine model.

Streptococcus gordonii DL1 evades polymorphonuclear leukocyte-mediated killing via resistance to lysozyme

Streptococcus gordonii is an etiological bacterial agent of infective endocarditis. Although the pathogenesis mechanisms are not well understood, the interaction between streptococci and phagocytes is considered important for the development of infective endocarditis. Previous studies show that some S. gordonii strains, including DL1, survive in polymorphonuclear leukocytes (PMNs), whereas other strains such as SK12 are sensitive to PMN-dependent killing. In this study, we assessed the differences between the sensitivity of S. gordonii DL1 and S. gordonii SK12 to PMN-dependent killing. S. gordonii DL1 showed a higher survival when treated with PMNs than SK12. Both S. gordonii DL1 and S. gordonii SK12 showed high resistance to low pH condition. Compared to S. gordonii SK12, S. gordonii DL1 was sensitive to hydrogen peroxide. However, the resistance of S. gordonii DL1 to the tested bactericidal agents, especially lysozyme, was higher than that of SK12. Furthermore, we performed a bactericidal assay by treating a mixture of S. gordonii DL1 and SK12 with PMNs. S. gordonii DL1 did not enhance the survival of S. gordonii SK12 exposed to PMNs. These results indicated that S. gordonii DL1 is resistant to bactericidal agents that degrade bacteria in phagolysosomes. In addition, there was no secretory factor involved in the resistance to bactericidal agents. The findings of this study may help develop treatments for infective endocarditis caused by S. gordonii.

Effects of the Quinone Oxidoreductase WrbA on Escherichia coli Biofilm Formation and Oxidative Stress

The effects of natural compounds on biofilm formation have been extensively studied, with the goal of identifying biofilm formation antagonists at sub-lethal concentrations. Salicylic and cinnamic acids are some examples of these compounds that interact with the quinone oxidoreductase WrbA, a potential biofilm modulator and an antibiofilm compound biomarker. However, WrbA’s role in biofilm development is still poorly understood. To investigate the key roles of WrbA in biofilm maturation and oxidative stress, Escherichia coli wild-type and ∆wrbA mutant strains were used. Furthermore, we reported the functional validation of WrbA as a molecular target of salicylic and cinnamic acids. The lack of WrbA did not impair planktonic growth, but rather affected the biofilm formation through a mechanism that depends on reactive oxygen species (ROS). The loss of WrbA function resulted in an ROS-sensitive phenotype that showed reductions in biofilm-dwelling cells, biofilm thickness, matrix polysaccharide content, and H₂O₂ tolerance. Endogenous oxidative events in the mutant strain generated a stressful condition to which the bacterium responded by increasing the catalase activity to compensate for the lack of WrbA. Cinnamic and salicylic acids inhibited the quinone oxidoreductase activity of purified recombinant WrbA. The effects of these antibiofilm molecules on WrbA function was proven for the first time.