Nalidixic acid resistance and clonal expansion of Salmonella enterica serotype Paratyphi A in Yuxi city,China

Objective To understand the nalidixic acid resistance and clonal expansion of Salmonella enterica serotype Paratyphi A(SPA) recovered from patients with paratyphoid fever A in Yuxi City from 1999 to 2008.Method Antimicrobial susceptibility testing was performed with 4060 SPA isolates by the controlled Kirby-Bauer disc diffusion technique.A group of 166 NAR and 20 NAS isolates of SPA were randomly selected.186 isolates were typed by pulsed-field gel electrophoresis(PFGE) with SpeI PFGE patterns were analyzed by cluster analysis.The MICs for nalidixic acid,ciprofloxacin,levofloxacin,ofloxacin,norfloxacin,and gatifloxacin were determined.Result The incidence of resistance to nalidixic acid of the isolates increased from 12.5% in 1999,82.2% in 2000,93.0% in 2001,to 100% in 2008.Nalidixic acid-susceptible(NAS) isolates predominated in 1999 but was replaced by nalidixic acid-resistant(NAR) isolates after 2000.SpeI digestion of 186 isolates gave nine different PFGE patterns with predominance of the SpeI01 and SpeI02 epidemic patterns.All NAR isolates studied had a reduced susceptibility to flu-oroquinolone,with the MICs being higher than that for 20 NAS isolates.Conclusion We validate the use of the nalidixic acid screening test in the detection of decreased fluoroquinolone susceptibility in SPA.PFGE patterns SpeI01 and SpeI02,the main clones of the epidemics,are highly prevalent in Yuxi.Recommendation for the emergency treatments of outbreaks and epidemics caused by a fully fluoroquinolone-resistant strain can be made.