Synergistic Antibiotic‒Antibiotic and Antibiotic‒Phage Combinations Against Biofilm-Associated Staphylococcus aureus in Dual-Species Biofilms

Treating polymicrobial biofilms often requires the combination of antibiotics and the participating species play a major role in the response of these bacteria to antimicrobial agents. S. aureus and P. aeruginosa are among the most common species found in polymicrobial biofilms. We used in-house dual-species biofilms of S. aureus and P. aeruginosa , to evaluate their response to antibiotics, combinations of antibiotics and selected antibiotics‒phage combinations. The interaction of antibiotic combinations on the inhibition and eradication of biofilms were assessed by calculating the fractional inhibitory concentration index (FICI). Coculture of P. aeruginosa and S. aureus altered their antibiotic sensitivity and S. aureus became highly tolerant to antibiotics. Most antibiotic combinations have synergistic effects on biofilm inhibition. The combinations of vancomycin‒ofloxacin and vancomycin‒ciprofloxacin had FICI values of 0.034 and 0.055 against S. aureus respectively. Combinations of glycopeptide and fluoroquinolone antibiotics also showed synergistic interactions in biofilm eradication. The addition of phage Stab 21 restored the efficacy of ciprofloxacin and vancomycin against S. aureus in dual-species biofilms, reducing their biofilm inhibitory concentration to 0.5 µg/mL from 33 µg/mL for ciprofloxacin and > 2048 µg/mL for vancomycin. In summary, we describe the susceptibility profile of S. aureus to combination treatments within a dual-species biofilm and report effective antibiotic–antibiotic and phage–antibiotic combinations against antibiotic-tolerant S. aureus .