New Attempts to Inhibit Methicillin-Resistant Staphylococcus aureus Biofilm? A Combination of Daptomycin and Azithromycin

OBJECTIVE: To investigate the antibacterial impact of daptomycin and azithromycin in vitro on methicillin-resistant Staphylococcus aureus (MRSA) biofilm. METHODS: (1) Measure the strain growth curve and the biofilm formation curve. (2) Determine the minimum inhibitory concentrations (MICs) of daptomycin and azithromycin. (3) Investigate the antibacterial impact of the combination of daptomycin and azithromycin. (4) Perform the evaluation of the intervention impact of antimicrobial agents on MRSA biofilm. (5) Observe the biofilm after intervention with the antibacterial agent. RESULTS: (1) MRSA exhibited three phases: lag phase (0–4 h), logarithmic growth (4–8 h) and stationary phase after 18 h; its biofilm began to form at 6 h, semi-matured at 24 h, and reached maturity after 48 h. (2) The MICs of daptomycin and azithromycin were 8 μg/mL and greater than 256 μg/mL, respectively. (3) The combination of daptomycin and azithromycin has an additive effect on MRSA (Fractional Inhibitory Concentration Index [FICI] 0.625) (FICI = MIC of drug A in combination/MIC of drug A alone + MIC of drug B in combination/MIC of drug B alone). Evaluation criteria: Synergistic effect is considered when FICI ≤ 0.5; additive effect is considered when 0.5 < FICI ≤ 1; irrelevant effect is considered when 1 < FICI ≤ 2; antagonistic effect is considered when FICI > 2). (4) Daptomycin or azithromycin at MICs inhibited not only the growth of planktonic bacteria but also the formation of biofilm. (5) The combination of both, in which group the ratio of live/dead bacteria is low and the biofilm morphology was incomplete, was more productive than monotherapy in against biofilm. CONCLUSION: Both daptomycin and azithromycin have anti-MRSA biofilm activity, and daptomycin is dominant. The fact that the combination of both can significantly inhibit the further maturation of MRSA biofilm and destroy already formed biofilm demonstrates the superiority of the combination over the monotherapy.