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Background  Typhoid and paratyphoid fever are endemic in China. The objective of this investigation was to determine the molecular features of nalidixic acid-resistant  Salmonella enteric  serovar Typhi ( S. typhi ) and Paratyphi ( S. paratyphi ) from blood isolates in Shenzhen, China.    Results  Twenty-five  S. typhi  and 66  S. paratyphi  were isolated from 91 bacteriemic patients between 2002 and 2007 at a hospital in Shenzhen, Southern China. Fifty-two percent (13/25) of  S. typhi  and 95.3% (61/64) of  S. paratyphi  A were resistant to nalidixic acid. Sixty-seven isolates of nalidixic acid-resistant  Salmonella  (NARS) showed decreased susceptibility to ciprofloxacin (MICs of 0.125-1 μg/mL). All 75 NARS isolates had a single substitution in the quinolone resistance-determining region (QRDR) of GyrA (Ser83→Phe/Pro/Tyr, or Asp87→Gly/Asn), and 90.7% of these isolates carried the substitution Ser83Phe in GyrA. No mutation was found in the QRDR of  gyrB ,  parC , or  parE . Plasmid mediated quinolone resistance genes including  qnr  and  aac(6')-Ib-cr  were not detected in any isolate. Twenty-two distinct pulsed field gel electrophoresis (PFGE) patterns were observed among  S. typhi . Sixty-four isolates of  S. paratyphi  A belonged to one clone. Eighty-seven investigated inpatients were infected in the community. Six patients infected by  S. paratyphi  A had a travel history before infection.    Conclusions  Nalidixic acid-resistant  S. typhi  and  S. paratyphi  A blood isolates were highly prevalent in Shenzhen, China. PFGE showed the variable genetic diversity of nalidixic acid-resistant  S. typhi  and limited genetic diversity of nalidixic acid -resistant  S. paratyphi  A.

Genetic Diversity of Salmonella enteric serovar Typhi and Paratyphi in Shenzhen, China from 2002 through 2007