Active efflux of ofloxacin by a highly quinolone-resistant strain of Proteus vulgaris

Abstract

A clinical isolate of Proteus vulgaris, strain 881051, highly resistant to ofloxacin (MIC for ofloxacin; 800 mg/L) possessed three types of resistance mechanisms to ofloxacin. The supercoiling activity of DNA gyrase from this strain was barely inhibited by ofloxacin, the ID₅₀, value of ofloxacin against the enzyme was 2700 times higher than that against the enzyme from a susceptible strain, P. vulgaris 08602. In the outer membrane of this resistant strain, no porin protein of 37 kDa was detectable. In the presence of carbonylcyanide m-chloro-phenylhydrazone (CCCP), we found that the accumulation of ofloxacin in the cells of strain 881051, treated with the drug, was about 40-fold higher than that in the absence of CCCP. The CCCP-induced increase in the intracellular drug was 10 and 30 times higher than those in strain 880561 (a moderately resistant strain, MIC 50 mg/L) and 08602 (a fully sensitive strain), respectively. Simultaneous treatment of the strain 881051 with ofloxacin and CCCP resulted in bactericidal synergy. These results indicated that in addition to alterations of DNA gyrase and outer membrane proteins, the active efflux, as the third mechanism, could operate the resistance of strain 881051 to ofloxacin.