Selection of resistant variants of respiratory pathogens by quinolones.

Quinolones are widely used in the treatment of respiratory tract infections. However, some disquiet has been expressed over using quinolones for community-acquired pneumonia since their activity is generally rather poor against Streptococcus pneumoniae. In addition, it is known that resistant variants emerge at a fairly high frequency during exposure of Enterobacteriaceae to quinolones; if this also occurred during quinolone treatment of community-acquired pneumonia it could lead to an increased risk of clinical failure. We therefore determined the selection rate of quinolone-resistant variants for six strains of S. pneumoniae, Haemophilus influenzae and Moraxella catarrhalis with nalidixic acid (except for S. pneumoniae), ciprofloxacin, ofloxacin and levofloxacin. We were only able to select resistant variants at low frequency from two of the six strains of S. pneumoniae with ciprofloxacin: no resistant variants were selected by either ofloxacin or levofloxacin. Variants of H. influenzae and M. catarrhalis with decreased susceptibility to quinolones were produced both with more strains and with a greater frequency; however, these variants still remained susceptible according to the NCCLS guidelines. Our study suggests that resistant variants of S. pneumoniae are relatively unlikely to occur in individuals treated with fluoroquinolones especially if they are given quinolones with enhanced anti-gram-positive activity compared to ciprofloxacin.