Antibacterial antagonism between fusidic acid and ciprofloxacin.

A routine laboratory disk susceptibility testing of a resistant Staphylococcus aureus strain showed that around the ciprofloxacin disk, placed by chance in proximity to a fusidic acid disk, the inhibition zone was truncated. Follow-up of this observation by a planned disk approximation method showed that there is a real antagonism between these two antibacterial agents. The antagonism was observed while testing S. aureus isolates including the standard ATCC 25923 strain, with Bacillus subtilis ATCC 6633 spores and also with a mutant Escherichia coli made fusidic acid susceptible. The antagonistic property was found structure-specific, only associated with those fluoroquinolones containing the cyclopropyl substituent at the N1-position: ciprofloxacin, enrofloxacin, sparfloxacin and WIN 57273. Fluoroquinolones without this substituent such as enoxacin, norfloxacin, pefloxacin and ofloxacin were not antagonized by fusidic acid, the steroidal Gram-positive active antibiotic.