Quinolone resistance mechanisms in pneumococci.

Quinolones are widely used in the treatment of respiratory infections, in large part because of their activity against Streptococcus pneumoniae and other commonly encountered respiratory tract pathogens. Pneumococcal isolates that are resistant to these "respiratory quinolones" have now begun to emerge. Resistance is attributable to mutations affecting the intracellular targets of these drugs, topoisomerase IV and DNA gyrase; drug efflux contributes to quinolone resistance in some isolates. Most commonly, strains fully resistant to the newer quinolones have one or more mutations affecting DNA gyrase and topoisomerase IV. Although various agents of this class exhibit selectivity in primarily targeting one or the other of these enzymes, the passage of isolates in the presence of any agent can result in selection of mutations affecting both enzymes. Quinolone resistance in S. pneumoniae has arisen in heterogeneous genetic backgrounds but, ominously, has now appeared in strains that are well adapted for regional and global transmission.