Antimicrobial Peptides Improve Antibiotic-Mediating Killing Of Methicillin-Resistant Staphylococcus Aureus (MRSA) Biofilms

Staphylococcus aureus can cause drug-resistant, biofilm-associated infections. These infections place methicillin-resistant S. aureus (MRSA) as one of the sixth leading bacterial pathogens associated with deaths due to antimicrobial resistance. Traditional antibiotics are no longer enough to treat these resistant infections and novel therapeutics are needed. Antimicrobial adjuvants are a potential strategy for improving efficacy of existing antibiotics, and antimicrobial peptides (AMPs) may have adjuvant activity. Recent reports showed that AMPs improve antibiotic activity against planktonic staphylococci, but whether AMPs might alter antibiotic efficacy against biofilm-resident staphylococcus is less clear. We tested the effect of nine AMPs on staphylococcal biofilm susceptibility to the antibiotics linezolid, vancomycin, and daptomycin. We used AMPs with reported activity against either planktonic staphylococci (LL37, GF17, GF17d3, 17BIPHE2 and, LL23v9) or other bacterial biofilms (IDR-1018, WR-12, and DIK-8). Using biofilm viability assays, we found decreased bacterial biofilm viability following treatment for 48 hours with seven of the nine AMPs tested when used in conjunction with vancomycin. In addition, the AMP LL23v9 also improved biofilm effects on viability by linezolid and daptomycin. We also tested the effect of these AMPs on the structural integrity of S. aureus USA300 biofilms in conjunction with all antibiotics. Using a crystal violet staining assay, we found no correlation between changes in bacterial viability and biofilm structural integrity in any treatment group. Collectively, our findings suggest that some of our AMPs have the utility to serve as potential antibiotic adjuvants against S. aureus USA300 biofilms. Further testing is needed to understand the mechanism by which these AMPs improve biofilm susceptibility to antibiotics.