The fused methionine sulfoxide reductase MsrAB promotes oxidative stress defense and bacterial virulence in Fusobacterium nucleatum

Recognizably associated with oral infections, adverse pregnancy outcomes, and colorectal cancer, the Gram-negative oral bacterium Fusobacterium nucleatum is known to colonize many extraoral sites including placenta and colon. How this anaerobe copes with oxidative stress imposed by the host immune response in various tissues is unknown. Here, we revealed that F. nucleatum harbors a gene locus coding for a single, fused methionine sulfoxide reductase (MsrAB), a two-component transduction system (ModRS), thioredoxin (Trx)- and cytochrome c (CcdA)-like proteins. Considering their role in oxidative stress responses, we demonstrated that these genes were increasingly expressed when fusobacterial cells were exposed to hydrogen peroxide. Comparative transcriptome analysis of the parental and modR mutant strains revealed that the response regulator ModR regulates a large regulon including trx, ccdA, and many metabolic genes. Significantly, mutants lacking individual genes within the msrAB locus, including msrAB, were sensitive to reactive oxygen species and defective in adherence/invasion of colorectal epithelial cells, as compared to the parental strain. The msrAB mutant was also defective in survival in macrophage, unlike the rescued and parental strains. Strikingly, in an experimental model of preterm birth this mutant was severely attenuated in virulence, for it failed to colonize the placenta and spread to the amniotic fluid. The results support that the MsrAB system represents a major pathway that protects fusobacteria against oxidative damage and confers virulence and that MsrAB-mediated oxidative stress defense is subjected to regulation by the two-component transduction system ModRS. Overall design: Examination of modR deficient mutant compared to parental strain under normal growth or H2O2 stress in biological triplicates