Prolonged bacterial exposure to minocycline/rifampicin-impregnated vascular catheters does not affect antimicrobial activity of catheters.

OBJECTIVES: We assessed the in vitro effect of exposing various bacteria to minocycline/rifampicin-impregnated vascular catheters on the antimicrobial activity of the catheters and the antimicrobial susceptibility of tested organisms.
METHODS: Segments of minocycline/rifampicin-impregnated catheters were placed in agar plates inoculated with methicillin-susceptible Staphylococcus aureus (MSSA), methicillin-resistant S. aureus (MRSA), methicillin-resistant Staphylococcus epidermidis (MRSE) and vancomycin-resistant Enterococcus (VRE). Zones of inhibition were measured at 24 h, and colonies from the edge of this zone were retrieved after 72 h and inoculated onto new agar plates. A total of seven 72 h cycles were completed. We then measured the MICs of minocycline, rifampicin, vancomycin and linezolid for the collected strains.
RESULTS: The zones of inhibition of the four organisms remained stable after 21 days of sequential exposure to the impregnated catheters. The MICs of the antimicrobials remained constant, except for the MICs of rifampicin for MRSA and linezolid for MRSE, which increased slightly but remained within the susceptible range.
CONCLUSIONS: Minocycline/rifampicin-impregnated catheters remain effective against MSSA, MRSA, MRSE and VRE, as evidenced by stable zones of inhibition following 21 day sequential exposure to these catheters. The increase in MIC of rifampicin for MRSA may be clinically relevant if the catheter remains in place for >12 days though the strain remained susceptible to minocycline, there was no concurrent increase in the MIC of other tested drugs, and the zones of inhibition remained stable.