Truncation of GdpP mediates β-lactam resistance in clinical isolates of Staphylococcus aureus.

Abstract

Objectives: High-level β-lactam resistance in methicillin-resistant Staphylococcus aureus is mediated in the majority of strains by a mecA or mecC gene. In this study, we identified ten mec gene negative MRSA human isolates from Austria and eleven bovine isolates from the UK showing high levels of β-lactam resistance and sought to understand the molecular basis of the resistance observed. Methods: Different antimicrobial resistance testing methods (disk diffusion, Etest and Vitek®2) were used to establish the β-lactam resistance profiles for the isolates and the isolates were further investigated by whole-genome sequencing. Results: A number of mutations (including novel ones) in penicillin-binding proteins, AcrB, YjbH and pbp4-promoter were identified in the resistant isolates but not in closely related susceptible isolates. Importantly, a truncation in the c-di-AMP phosphodiesterase enzyme, GdpP, was identified in seven of the ten Austrian isolates and ten of the eleven UK isolates. Complementation of four representative isolates with an intact copy of gdpP gene restored susceptibility to penicillins and abolished the growth defects caused by the truncation. Conclusion: This study reports naturally occurring inactivation of GdpP protein in S. aureus of both human and animal origins, and demonstrated clinical relevance to a previously reported association between this truncation and increased β-lactam resistance and impaired bacterial growth in laboratory generated mutants. It also highlights possible limitations of genomic determination of antibiotic susceptibility based on single gene presence or absence when choosing the appropriate antimicrobial treatment for patients.