Efficacy of combination oral antimicrobial agents against biofilm-embedded methicillin-resistant Staphylococcus aureus

Background: The combination of fusidic acid and rifampicin has a demonstrated synergistic effect against methicillin-resistant Staphylococcus aureus (MRSA), including planktonic and biofilm-related organisms. However, the in vitro efficacy of other combinations of oral anti-MRSA antibiotics in biofilm models has not been established. Methods: The antibacterial activity of fusidic acid, linezolid, rifampicin, and minocycline against 33 biofilm-embedded MRSA isolates in low susceptibility and high resistance breakpoint concentrations was investigated using the 3-[4,5-dimethyl-2-thiazolyl]-2,5-diphenyl-2H-tetrazolium-bromide staining method. The compounds were further examined to determine their antibacterial efficacies in combination. The optical density ratio (ODr) was used to evaluate the antibacterial effects of these antibiotics, and the results indicate higher survival rates of MRSA on biofilm. A biofilm-positive phenotype (determined using the crystal violet stain) was defined as an optical density >= 0.17 at 492 nm, and strong biofilm formation was defined as an optical density >= 1.0. Results: One-third of the MRSA isolates demonstrated weak biofilm formation, and two-thirds demonstrated strong biofilm formation. At low concentrations, linezolid alone lowered the ODr to 0.55 and was effective against biofilm-embedded MRSA (p < 0.001). The activity of minocycline was concentration-dependent and more effective against MRSA isolates that demonstrated weak biofilm formation. The effect of minocycline seems to be further enhanced when used in combination with either fusidic acid or linezolid at low concentrations, with the obtained results equal to those obtained with rifampicin-based regimens (p < 0.001). Rifampicin plus minocycline was also effective against MRSA in biofilm. Conclusion: In comparison with monotherapy, minocycline-based combinations exhibit highly effective bactericidal effects against biofilm-embedded MRSA. Copyright (C) 2012, Taiwan Society of Microbiology. Published by Elsevier Taiwan LLC. All rights reserved.