Nalidixic acid resistance and clonal expansion of SALMONELLA ENTERICA serotype Paratyphi A Isolates in Yuxi city, China

Abstract

Antimicrobial susceptibility tests were performed with 4060 strains of SALMONELLA ENTERICA serotype 
Paratyphi A (SPA) isolated from patients at Yuxi City from 1999 to 2008. The incidence of resistance to 
nalidixic acid of the isolates increased from 12.5% in 1999 to 82.2% in 2000, 93.0% in 2001, and 100% in 
2008. The frequencies of intermediary and susceptibility (Kirby-Bauer disc diffusion technique) to 
ciprofloxacin were 17.0% and 83.0% before 2005, 65.3% and 34.7% in 2008, respectively. A group of 166 
NAR and 20 NAS isolates of SPA were typed by pulsed-field gel electrophoresis (PFGE) using SpeI into 
nine different PFGE patterns with predominance of the SpeI01 and SpeI02. All NAR isolates had higher 
MICs for fluoroquinolone. We validate the use of the nalidixic acid screening test for detection of 
decreased fluoroquinolone susceptibility in SPA.