Abstract
Fluoroquinolone-resistant variants of pandemic clones Spain(23F)-1, Spain(6B)-2, Spain(9V)-3, and Spain(14)-5 have been seen in various regions of the United States and the world, leading to the speculation that fluoroquinolone resistance among US Streptococcus pneumoniae may increase because of clonal spread. Using levofloxacin as a representative of the fluoroquinolone class, all 196 levofloxacin-resistant pneumococci from a total of 22 794 isolates in the Tracking Resistance in the United States Today (TRUST) 5-8 studies (2001-2004) were subjected to pulsed-field gel electrophoresis (PFGE), serotyping, and sequencing of parC/E and gyrA/B quinolone resistance-determining regions (QRDR) to measure the extent of clonality. In addition, susceptibility testing of these isolates to ciprofloxacin, gatifloxacin, levofloxacin, and moxifloxacin was performed. ATCC type strains of Spain(23F)-1, Spain(6B)-2, Spain(9V)-3, and Spain(14)-5 clones were included as comparators. Levofloxacin-resistant isolates with Spain(23F)-1-related PFGE patterns decreased from 28% of the resistant isolates in 2001 to 6% in 2004. These isolates, with serotypes 23F (n = 17), 19F (n = 17), or 19A (n = 1), had 15 different QRDR profiles and were all ciprofloxacin- and gatifloxacin-resistant. Levofloxacin-resistant isolates with Spain(9V)-3-related PFGE patterns decreased from 13% of the resistant isolates in 2001 to 2% in 2004. The Spain(9V)-3-related isolates were serotype 9V (n = 9), 9A (n = 2), and 9N (n = 1) and exhibited 6 different QRDR profiles. All were resistant to all fluoroquinolones tested. None of the levofloxacin-resistant isolates had PFGE patterns related to Spain(6B)-2 or Spain(14)-5. Resistance to respiratory fluoroquinolones among pneumococci has remained constant at about 1% (0.8%-1.1%) since 2001, and there has been a decrease in the prevalence of levofloxacin-resistant isolates similar to Spain(23F)-1 or Spain(9V)-3. Considerable QRDR variability among these strains appears to be the result of sporadic independent mutational events as opposed to clonal expansion.
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