Impermeability to quinolones in gram-positive and gram-negative bacteria.

The initial step in the accumulation of fluoroquinolone antimicrobial agents is binding to cell surface components reduced by lowered pH and divalent cations. Uptake into gram-negative and gram-positive bacteria is by simple diffusion. Entry through the outer membrane occurs preferentially for most agents by the porin route but a second process using the self-promoted uptake pathway is active especially for more hydrophobic agents. Fluoroquinolones bind to vesicles of phospholipid which may be the initiating step in cross-cytoplasmic membrane diffusion. An active efflux system has been described in Escherichia coli with evidence supporting its presence in several other bacteria. Total upset is not altered by a resistant gyrase. Resistant isolates associated with reduced total quinolone accumulation due to lowered uptake have been described for laboratory mutants and clinical isolates. Most but not all of these have had alterations in outer membrane proteins. A functionally dominant resistance gene has been cloned from resistant Staphylococcus aureus and codes for a highly hydrophobic protein most likely membrane associated. This gene is expressed in Escherichia coli and specifies resistance especially to hydrophilic quinolones, possibly by altered accumulation.