ABSTRACT
Beta-lactam antibiotics sensitize Enterococcus faecium to killing by endogenous antimicrobial peptides (AMPs) of the innate immune system and daptomycin through mechanisms yet to be elucidated. It has been speculated that beta-lactam inactivation of select E. faecium penicillin binding proteins (PBPs) may play a pivotal role in this sensitization process. To characterize the specific PBP inactivation that may be responsible for these phenotypes, we utilized a previously characterized set of E. faecium PBP knockout mutants to determine the effects of such mutations on the activity of daptomycin and the AMP human cathelicidin (LL-37). Enhanced susceptibility to daptomycin was dependent more on a cumulative effect of multiple PBP deletions than on inactivation of any single specific PBP. Selective knockout of PBPZ rendered E. faecium more vulnerable to killing by both recombinant LL-37 and human neutrophils, which produce the antimicrobial peptide in high quantities. Pharmacotherapy targeting multiple PBPs may be used as adjunctive therapy with daptomycin to treat difficult E. faecium infections.