Influence of the cell wall on ciprofloxacin susceptibility in selected wild-type Gram-negative and Gram-positive bacteria

Abstract

The susceptibility of several wild-type bacteria to ciprofloxacin and accumulation of the drug in these bacteria were evaluated. Species studied included Escherichia coli , Serratia marcescens , Pseudomonas aeruginosa , Staphylococcus aureus , Bacillus subtilis and Bacillus cereus . Ciprofloxacin susceptibility was measured for each strain using two different methods: the minimal inhibitory concentration and the bactericidal index. Significant differences were observed between the results derived from these two methods. Whereas the minimal inhibitory concentration was low in all strains tested, ciprofloxacin’s bactericidal activity, as indicated by the bactericidal index, varied with the species studied. To determine whether this finding was due to variations in cell envelope permeability to ciprofloxacin (i.e. to combined cell uptake and efflux), we studied ciprofloxacin accumulation using spectrofluorometry. In Gram-negative bacteria, differences in permeability can lead to altered susceptibility to antibiotics. In fact, the combination of slow uptake and efficient efflux seems to be crucial to the characteristic poor susceptibility of P. aeruginosa to ciprofloxacin. However, the low level of activity of ciprofloxacin against S. aureus and two Bacillus species may have resulted from the drug’s interaction with its target enzymes (i.e. topoisomerase IV in S. aureus and DNA gyrase in Bacillus spp.) rather than diminished permeability.