Bacterial biofilms are difficult to eradicate and can complicate many infections by forming on tissues and medical devices. Phage+antibiotic combinations (PAC) may be more active on biofilms than either type of agent alone, but it is difficult to predict which PAC regimens will be reliably effective. To establish a method for screening PAC combinations against Staphylococcus aureus biofilms, we conducted biofilm time-kill analyses (TKA) using various combinations of phage Sb-1 with clinically relevant antibiotics. We determined the activity of PAC against biofilm versus planktonic bacteria and investigated the emergence of resistance during (24 h) exposure to PAC. As expected, fewer treatment regimens were effective against biofilm than planktonic bacteria. In experiments with isogenic strain pairs, we also saw less activity of PACs against DNS-VISA mutants versus their respective parentals. The most effective treatment against both biofilm and planktonic bacteria was the phage+daptomycin+ceftaroline regimen, which met our stringent definition of bactericidal activity (>3 log10 CFU/mL reduction). With the VISA-DNS strain 8015 and DNS strain 684, we detected anti-biofilm synergy between Sb-1 and DAP in the phage+daptomycin regimen (>2 log10 CFU/mL reduction versus best single agent). We did not observe any bacterial resensitization to antibiotics following treatment, but phage resistance was avoided after exposure to PAC regimens for all tested strains. The release of bacterial membrane vesicles tended to be either unaffected or reduced by the various treatment regimens. Interestingly, phage yields from certain biofilm experiments were greater than from similar planktonic experiments, suggesting that Sb-1 might be more efficiently propagated on biofilm. IMPORTANCE Biofilm-associated multidrug-resistant infections pose significant challenges for antibiotic therapy. The extracellular polymeric matrix of biofilms presents an impediment for antibiotic diffusion, facilitating the emergence of multidrug-resistant populations. Some bacteriophages (phages) can move across the biofilm matrix, degrade it, and support antibiotic penetration. However, little is known about how phages and their hosts interact in the biofilm environment or how different phage+antibiotic combinations (PACs) impact biofilms in comparison to the planktonic state of bacteria, though scattered data suggest that phage+antibiotic synergy occurs more readily under biofilm-like conditions. Our results demonstrated that phage Sb-1 can infect MRSA strains both in biofilm and planktonic states and suggested PAC regimens worthy of further investigation as adjuncts to antibiotics.
Keywords: MRSA; bacteriophages; biofilms.