Antibiotic susceptibility of ica-positive and ica-negative MRSA in different phases of biofilm growth.

Methicillin-resistant Staphylococcus aureus (MRSA) has emerged as a clinically relevant pathogen because of its resistance to antibiotics and its ability to form potent biofilm. Both ica-negative as well as ica-positive MRSA strains are known to produce biofilm. In the present study, these strains were grown in biofilm mode and susceptibility of these to antibiotics was assessed. Our study suggests that antibiotic susceptibility of MRSA biofilm depends on the biochemical composition of its matrix. The biofilm matrix of ica-positive MRSA was mainly composed of poly-intercellular adhesion (PIA), whereas eDNA was a major constituent of ica-negative MRSA. The results showed that MRSA in planktonic growth was susceptible to clindamycin, vancomycin and minocycline. However, the MIC and MBC of vancomycin for the mature biofilm of ica-negative MRSA was 16 and 32 μg ml(-1), respectively. On the contrary, the MIC and MBC of vancomycin for ica-positive MRSA was >1024 μg ml(-1). The effect of vancomycin and minocycline on young and old biofilms was also determined. Vancomycin was quite effective in eradicating the young biofilm formed by ica-negative MRSA; however, it was completely ineffective on the biofilm of ica-positive MRSA. Minocycline at its highest clinical achievable concentration was found to be quite effective in eradicating the young biofilm formed by both the strains. The enzyme-linked immunosorbent assay (ELISA) results and dot blot assay suggest that the presence of ica locus influenced PIA production, which probably contributed towards the failure of vancomycin in eradicating the biofilm formed by ica-positive strain. However, none of the antibiotics used in this study was effective in eradicating the mature biofilms.