Investigation of the function and role of FmtA in the Staphylococcus aureus response to antibiotic stress

Staphylococcus aureus is a human pathogen of great medical concern due to its ability to evolve into antibiotic resistant strains. Several factors are involved in determining high level of resistance in S. aureus. Among them, fmtA has been identified as a gene whose transcription increases in presence of cell wall active antibiotics hence it is considered as a part of cell wall stress-stimulon. Inactivation of fmtA gene has been reported to be associated with reduced cell wall cross-linking, methicillin resistance and biofilm formation. This study provides for the first time insights into the function of fmtA and its role in antibiotic stress response of S. aureus . We have demonstrated that FmtA is the fifth penicillin binding protein (PBP) and is intrinsically resistant to inactivation by beta-lactams. Investigation of the function of FmtA revealed that the protein has transpeptidase activity. We constructed a S. aureus mutant whereby fmtA is placed under the control of the inducible promoter P spac, referred to as the S. aureus RN4220 spacfmtA conditional mutant, to investigate the role of FmtA in S. aureus response to cell wall damage. Analysis of antibiotic susceptibility of S. aureus conditional mutant revealed that expression of high levels of FmtA increase antibiotic resistance. In addition, analysis of S. aureus conditional mutant revealed that expression of FmtA enhances synthesis of peptidoglycan. Moreover, we provide for the first time evidence that FmtADelta27 binds specifically to teichoic acid (TA). We have proposed that this property of FmtA is linked with the process of biofilm formation in S. aureus. By using GFP-FmtADelta27, we have determined that FmtADelta27 localizes at the cell septum in S. aureus. However, removal of the teichoic acid results in delocalization of FmtADelta27 . This evidence is indicative of the role of teichoic acids in targeting FmtA to cell wall synthesis site. Further, our study shows that septal localization of GFP-FmtADelta27 is not lost in the presence of antibiotics which is an indication that FmtA is able to compensate for the function of other PBPs under antibiotic stress conditions.