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.



Mikkelsen, K., W. Sirisarn, O. Alharbi, M. Alharbi, H. Liu, K. Nøhr-Meldgaard, K. Mayer, M. Vestergaard, L.A. Gallagher, J.P. Derrick, A.J. McBain, J. Biboy, W. Vollmer, J.P. O'Gara, T. Grunert, H. Ingmer, and G. Xia. (2021). The Novel Membrane-Associated Auxiliary Factors AuxA and AuxB Modulate β-lactam Resistance in MRSA by stabilizing Lipoteichoic Acids. Int J Antimicrob Agents 106283. [Epub: Ahead of Print]


TC#NameOrganismal TypeExample

Putative transporter/auxiliary subunit, AuxA or FmnA, that influences beta-lactam sensitivity; of 431 aas and 10 (or 11) TMSs  in a 1 + 5 + 4 TMS arrangement (Mikkelsen et al. 2021). This protein and its homologs may function as a subunit of an ABC transport system (see TC# 3.A.1 and 9.B.414.1.1.

AuxA of Staphylococcus aureus


Uncharacterized protein of 415 aas and 10 or 11 TMSs.

UP of Nosocomiicoccus sp.


Uncharacterized protein, an AuxA homolog, that may be an auxiliary subunit of a transporter that confirs resistance to beta-lactams in S. aureus (Mikkelsen et al. 2021).

AuxB of Salinicoccus qingdaonensis


Uncharacterized protein of 430 aas and 11 TMSs in a 7 + 4 TMS arrangement (1 +2 + 2 + 2 + (HYDROPHILIC GAP) + 1 + 1 + 2 TMS arrangement. 77% identical to TC# 9.B.414.1.1; it may function with 9.B.415.2.1.

UP of Staphylococcus haemolyticus