Lack of interaction of fluoroquinolones with lipopolysaccharides

Fluoroquinolones are known to chelate with di- and trivalent cations, and it has accordingly been claimed that they perturb the integrity of the outer membrane (OM) of gram-negative bacteria. So far, chelation has not been assessed in biologically relevant test systems. Therefore, we investigated the interaction of ciprofloxacin and moxifloxacin in the absence and presence of Mg2+ with whole bacteria and isolated lipopolysaccharide (LPS) from various rough mutant strains of Salmonella enterica chemotypes by applying different biophysical techniques. We found that the fluoroquinolones did not disturb the integrity of the OM and neither were incorporated into LPS monolayers nor displaced Ca2+ from LPS monolayers, suggesting that chelation of fluoroquinolones with divalent cations does not contribute to the antibacterial effect of fluoroquinolones.