β-Lactam-Induced Cell Envelope Adaptations, Not Solely Enhanced Daptomycin Binding, Underlie Daptomycin-β-Lactam Synergy in Methicillin-Resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is a serious clinical threat due to innate virulence properties, high infection rates, and the ability to develop resistance to multiple antibiotics, including the lipopeptide daptomycin (DAP). The acquisition of DAP resistance (DAP-R) in MRSA has been linked with several characteristic alterations in the cell envelope. Clinical treatment of DAP-R MRSA infections has generally involved DAP-plus-beta-lactam combinations, although definable synergy of such combinations varies in a strain-dependent as well as a beta-lactam-dependent manner. We investigated distinct beta-lactam-induced cell envelope adaptations of nine clinically derived DAP-susceptible (DAP-S)/DAP-R strain pairs following in vitro exposure to a panel of six standard beta-lactams (nafcillin, meropenem, cloxacillin, ceftriaxone, cefaclor, or cefoxitin), which differ in their penicillin-binding protein (PBP)-targeting profiles. In general, in both DAP-S and DAP-R strains, exposure to these beta-lactams led to (i) a decreased positive surface charge; (ii) decreased cell membrane (CM) fluidity; (iii) increased content and delocalization of anionic phospholipids (i.e., cardiolipin), with delocalization being more pronounced in DAP-R strains; and (iv) increased DAP binding in DAP-S (but not DAP-R) strains. Collectively, these results suggest that beta-lactam-induced alterations in at least three major cell envelope phenotypes (surface charge, membrane fluidity, and cardiolipin content) could underlie improved DAP activity, not mediated solely by an increase in DAP binding. (Note that for ease of presentation, we utilize the terminology DAP-R instead of DAP nonsusceptibility.)

Automated summary

The clinical treatment of MRSA infections with beta-lactam combinations may induce changes in the cell envelope, potentially improving the activity of lipopeptide daptomycin against MRSA strains.