EFFICACY OF FUSIDIC ACID ALONE AND IN COMBINATION WITH OTHER ORAL ANTIMICROBIAL AGENTS AGAINST CLINICAL METHICILLIN-RESISTANT STAPHYLOCOCCUS AUREUS ISOLATES IN VITRO

Abstract

Fusidic acid (FA), an anti-methicillin-resistant Staphylococcus aureus (MRSA) antibiotic is recommended for use in combination with other antibiotics to prevent antimicrobial resistance development. However, optimal choice of the partner agent has been poorly studied. The study assessed in vitro activity of FA alone or combined with doxycycline (DOX), ciprofloxacin (CIP), clindamycin (CLI), rifampicin (RIF), and trimethoprim/sulfamethoxazole (TMP/SMX), against clinical MRSA isolates at standard and high inoculums. Minimum inhibitory concentration (MIC) of the six antimicrobial agents against MRSA was determined by a broth microdilution method. The summation of fractional inhibitory concentration (ΣFIC) was used to determine the effects of drug combinations at standard and high inoculums. Sixty-five of 71 (92 %) MRSA isolates were sensitive to FA, RIF and TMP/SMX. Using standard and high inoculums, antagonism was observed in two MRSA isolates for FA+TMP/SMX at standard inoculum and synergism at low frequencies, except for FA+DOX combination at high inoculum. These findings should assist in further studies to determine the appropriate antibiotic combinations for treatment of MRSA infection in Thailand.