Increase in nalidixic acid resistance among non-Typhi Salmonella enterica isolates in the United States from 1996 to 2003.

Fluoroquinolones commonly are used to treat adult Salmonella infections. Fluoroquinolone treatment has failed for persons infected with nalidixic acid-resistant Salmonella. From 1996 to 2003, state public health laboratories forwarded 12,252 non-Typhi Salmonella enterica isolates to the Centers for Disease Control and Prevention for antimicrobial susceptibility testing; 203 (1.6%) of the isolates were nalidixic acid resistant, and 14 (7%) of those were ciprofloxacin resistant. Resistance to nalidixic acid significantly increased from 0.4% in 1996 to 2.3% in 2003. All ciprofloxacin-resistant isolates had at least one point mutation in the quinolone resistance determining region (QRDR) of gyrA and did not harbor qnr or have point mutations in the QRDR of gyrB, parC, or parE. Continued surveillance of antimicrobial resistance among non-Typhi S. enterica isolates is needed to mitigate the increasing prevalence of nalidixic acid resistance.