A MRSA mystery: how PBP4 and cyclic-di-AMP join forces against β-lactam antibiotics

Abstract

The high-level resistance to next-generation β-lactams frequently found in Staphylococcus aureus isolates lacking mec, which encodes the transpeptidase PBP2a traditionally associated with methicillin-resistant Staphylococcus aureus (MRSA), has remained incompletely understood for decades. A new study by Lai et al. found that the co-occurrence of mutations in pbp4 and gdpP, which respectively cause increased PBP4-mediated cell wall crosslinking and elevated cyclic-di-AMP levels, produces synergistic β-lactam resistance rivaling that of PBP2a-producing MRSA (L.-Y. Lai, N. Satishkumar, S. Cardozo, V. Hemmadi, et al., mBio 15:e02889-23. 2024, https://doi.org/10.1128/mbio.02889-23). The combined mutations are sufficient to explain the high-level β-lactam resistance of some mec-lacking strains, but the mechanism of synergy remains elusive and an avenue for further research. Importantly, the authors establish that co-occurrence of these mutations leads to antibiotic therapy failure in a Caenorhabditis elegans infection model. These results underscore the need to consider this unique and novel β-lactam resistance mechanism during the clinical diagnosis of MRSA, rather than relying on mec as a diagnostic.

Keywords: MRSA; PBP4; cyclic-di-AMP; gdpP; methicillin-resistant lacking mec (MRLM); β-lactam resistance.

FIG 1

Schematic of S. aureus β-lactam resistance mechanisms. S. aureus resistance to early generations of β-lactams is achieved by production of β-lactamases, encoded by blaZ. Resistance to methicillin and other NGB is most commonly attributed to production of the alternate transpeptidase PBP2a, which has low β-lactam affinity and is encoded by either mecA or mecC. In a new study by Lai et al. published in mBio, high-level resistance to NGBs in strains lacking mec and blaZ is explained by synergistic mutations co-occurring in pbp4 and gdpP (22). The pbp4 mutations include regulatory promoter mutation(s) and missense mutation(s) in the coding region, ultimately resulting in increased production and/or stability of PBP4, whereas the gdpP mutation(s) generally causes loss of function. The increase in PBP4 activity leads to increased secondary peptidoglycan crosslinking, which enhances β-lactam resistance. The loss of GdpP causes elevated intracellular cyclic-di-AMP concentrations, which raises both resistance and tolerance to β-lactams. This may be in part because high cyclic-di-AMP levels can activate the CWSS and increase cell wall thickness (23–25). Additionally, high c-di-AMP levels increase resistance to fosfomycin, which targets an early step in peptidoglycan monomer synthesis; this may indicate an impact of c-di-AMP early on in peptidoglycan synthesis (24). The mechanism for how the individual effects of pbp4 and gdpP mutations synergize to produce high-level β-lactam resistance remains a mystery.