Clonal expansion and genetic diversity of nalidixic acid-resistant Salmonella enterica serotype Paratyphi A

The aim of this study was to understand the clonal expansion and genetic diversity of nalidixic acid -resistant (NAR) Salmonella enterica serotype Paratyphi A (SPA) from Yuxi city, China. Antimicrobial susceptibility testing was performed with 3980 SPA isolates from Yuxi city of China between 1999 and 2007. The incidence of resistance to nalidixic acid (NA) of 1999 and 2000 were 25.0 and 82.2%, respectively. More than 95.0% of the isolates obtained from 2001 to 2007 were resistant to NA. Amplification by PCR and sequencing of the genes within the quinolone resistance-determining region with subsets of 15 NAR strains revealed that the mechanisms of NA resistance resulted from single point mutations in the gyrA gene leading to the Ser83Phe substitution. Subtyping of 120 NAR isolates from seven counties and one Nalidixic acid-susceptible (NAS) isolate were studied using pulsed-field gel electrophoresis (PFGE) analysis, followed by digestion of chromosomal DNA with restriction endonucleases SpeI and XbaI. PFGE patterns were analyzed by cluster analysis. SpeI and XbaI digestion of 121 isolates generated five and four different PFGE patterns with predominance of the SpeI01 and SpeI02 (or the XbaI01) epidemic patterns, respectively. The incidence of resistance to NA of the isolates increased during the study period. NAS isolates predominated in 1999 but was replaced by NAR isolates after 2000. PFGE patterns SpeI01 and SpeI02 (or XbaI01), the main clones of the epidemics, are highly prevalent in Yuxi. Key words: Paratyphoid fever, Salmonella enterica serotype Paratyphi, antimicrobial susceptibility, resistance to nalidixic acid, PFGE, clonal expansion.