Bactericidal Mechanisms of Ofloxacin

Ciprofloxacin and ofloxacin are known to exert a second bactericidal mechanism (termed B) against Escherichia coli which functions even when protein synthesis is inhibited by chloramphenicol or when RNA synthesis is inhibited by rifampicin. However, the bactericidal activity of ciprofloxacin against a coagulase-negative staphylococcus (Staphylococcus warneri) was found to be abolished by chloramphenicol so the 4-quinolone does not exert mechanism B against this species. On the other hand, ofloxacin did exhibit mechanism B against S. warneri because the drug remained bactericidal in the presence of chloramphenicol. When S. aureus was investigated results similar to those observed in S. warneri were obtained throughout the range of clinically achievable concentrations of ofloxacin and ciprofloxacin. Ofloxacin seems to exhibit mechanism B against the staphylococci while ciprofloxacin does not. This may explain why ciprofloxacin is more potent than ofloxacin against Gram-negative bacteria but against staphylococci both drugs are equipotent.