P594 Intracellular killing of drug-resistant Staphylococcus aureus by different antibiotics

Objectives:  The emergence of epidemic multiple resistant Staphylococcus aureus (EMRSA) and vancomycin intermediate susceptible (VISA) strains has heightened concerns about the treatment of associated infections. It has also been suggested that intracellular survival of S. aureus in phagocytic cells play an important role in the pathogenesis of associated infections, moreover it may contribute to failure of antimicrobial chemotherapy. Therefore intracellularly active antibiotics may be of value in treatment. This study was aimed to investigate the effect of linezolid, moxifloxacin and vancomycin on the intracellular survival of S. aureus. Methods:  Two clinical isolates of methicillin resistant S. aureus (EMRSA 16 also resistant to macrolids and VISA 3759.v with intermediate susceptibility to vancomycin) were tested in J774 macrophage cell line. Susceptibility of the strains was determined by NCCLS broth microdilution method. Cells were infected with bacteria opsonised with 10% normal pooled human serum in Hank s balanced salt solution supplemented with 1% gelatine. After 2 h of incubation the supernatant was discarded and cells were washed three times in the buffer. Thereafter, antibiotics were added to the cells at the following concentration: 0 × MIC, 1/2 × MIC, 1 × MIC, 2 × MIC in triplicate. Samples were taken 1, 2, 3, 4 h after antibiotic addition. Cells were washed three times with phosphate buffered saline, then lysed with distilled water. Bacterial count of the cell lysate was determined by the microdilution followed by blood agar plating. Each test was performed three times. Results:  In the presence of vancomycin, intracellular killing was not enhanced, as linezolid and moxifloxacin facilitated the clearance of live intracellular bacteria even at sub-MIC concentrations. Discussion:  Whether this enhancement is due to inhibition of intracellular bacterial multiplication, or due to an effect on the host cells’ killing mechanisms, or both remains to be seen. According to our observation linezolid and moxifloxacin have been shown bioactive intracellularly against MRSA. Acknowledgement:  The project was supported by the FEMS.