9.B.414.  The Putative Transporter Auxiliary Factor AuxA (AuxA) Family 

A major determinant of beta-lactam resistance in methicillin-resistant Staphylococcus aureus (MRSA) is the drug insensitive transpeptidase, PBP2a, encoded by mecA. However, full expression of the resistance phenotype requires auxiliary factors. Mikkelsen et al. 2021 identified two such factors, auxiliary factor A (AuxA, SAUSA300_0980) and B (AuxB, SAUSA300_1003) in a screen against mutants with increased susceptibility to beta-lactams in the MRSA strain JE2. auxA and auxB encode transmembrane proteins, with AuxA predicted to be a transporter with 10 or 11 TMSs. Inactivation of auxA or auxB enhanced beta-lactam susceptibility in community-, hospital- and livestock associated MRSA strains without affecting PBP2a expression, peptidoglycan cross-linking or wall teichoic acid synthesis. Both mutants displayed increased susceptibility to inhibitors of lipoteichoic acid (LTA) synthesis and alanylation pathways, and released LTA even in the absence of beta-lactams. The beta-lactam susceptibility of the aux mutants was suppressed by mutations inactivating gdpP, which was previously found to allow growth of mutants lacking the lipoteichoic synthase enzyme, LtaS. Using the Galleria mellonella infection model, Mikkelsen et al. 2021 observed enhanced survival of larvae inoculated with either auxA or auxB mutants compared to the wild type strain following treatment with amoxicillin. Thus, AuxA and AuxB are central for LTA stability and potential inhibitors can be used to re-sensitize MRSA strains to beta-lactams and combat MRSA infections.

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