With respect to the role of the Msr proteins, it is well documented that these enzymes contribute to the ability of a pathogen to adhere to host tissue, evade immune system, form biofilms, survive inside macrophages, and resist oxidative killing.MsrA protein contributes to cell wall integrity and maintenance of adhesion properties in Streptococcus gordonii. Msr proteins have also been shown to affect adherence properties of pathogenic Neisseria. In S. gordonii, the MsrA enzyme was shown to maintain the integrity of bacterial adhesins during oxidative stress. The current study confirms the role of Msr proteins, particularly the MsrAs in the adherence of S. aureus to human cells. The MsrA1-deficient S. aureus, the triple msrA and the quadruple msrAB null-mutants, all showed reduced adherence to lung MK-4827 PARP inhibitor epithelial cells. The role of Msr proteins in virulence of the bacterial pathogens is also well documented. Both MsrA and MsrB contributed to the enzymatic defenses of Mycobacterium tuberculosis from reactive oxygen species. In Pseudomonas aeruginosa, inactivation of either msrA or msrB or both reduced virulence and increased its killing by oxidants. In Campylobacter jejuni, the single msrA or msrB mutants showed no growth defect, but the msrA-msrB double mutant showed increased sensitivity to oxidative stress conditions. Mutation in the msrA or msrB gene in Enterococcus faecalis resulted in increased sensitivity to H2O2. In addition, an msrA msrB double mutant showed further increase in sensitivity suggesting that the effect of mutations were additive. In a later study, however, the msrA and msrB mutants were shown to behave differently; the msrA mutant was more sensitive to oxidative stress LEE011 conditions whereas the msrB mutant showed stimulated growth under similar conditions. In Salmonella Typhimurium, deletion of msrA increased bacterial susceptibility to H2O2 and reduced its virulence, but a mutation in msrB had no apparent phenotype. In Mycobacterium smegmatis also, MsrB was shown to have a limited role in protection from oxidative stress conditions. Thus, the role of MsrB protein in defense from oxidative stress is questionable in many bacterial species. It is possible that under oxidative stress the majority of the oxidized methionine is S-MetO and the MsrB protein has no activity against this epimer. This may be the reason why the MsrA-deficient bacteria showed a high sensitivity to conditions that impose oxidative stress. MsrB of S. aureus, seems to some extent, counterbalance the effect of MsrA1. For example, lack of MsrA1 reduces pigmentation and this may be due to previously shown higher level of MsrB in MsrA1-deficient S. aureus.