Purpose: This study was aimed to investigate the dynamics of antimicrobial resistance expansion among different lineages and isolates of S . Typhi.
Materials and methods: The S . Typhi isolates were collected from the patients clinically suspected of typhoid fever in Eastern China during 2005–2017. All isolates were tested retrospectively for susceptibility to eight antimicrobials and the genes related to quinolone and ampicillin resistance, including gyrA , ParC , qnrA , qnrB , qnrS , aac(6´)-Ib-cr , qepA  and bla _TEM. The isolates were subtyped by PFGE.
Results: Of 140 isolates, all were susceptible to ciprofloxacin, cefotaxime, chloramphenicol, and trimethoprim-sulfamethoxazole, 95 (68%) were nalidixic acid resistant, and 74 (53%) were ampicillin resistant. The resistance to ampicillin and nalidixic acid was first observed in 2006. Among the 95 nalidixic acid-resistant S . Typhi isolates, 62 possessed S83F mutation in gyrA  and 25 possessed D87Y mutation. All ampicillin-resistant isolates harbored gene bla _TEM-1. PFGE generated 47 distinguishable clonal lineages. Overall, 64% (89/140) belonged to seven prevalent lineages of clustering isolates. PFGE results illustrated the prevalence of nalidixic acid-resistant lineages increased steadily from 19% during 2005–2012 to 50% during 2013–2014, and thereafter to 74% during 2015–2017 and similar development of ampicillin-resistant lineages increased from 6% to 38%, and also to 39%.
Conclusion: The present study indicated the clonal expansion of S . Typhi with ampicillin resistance and reduced ciprofloxacin susceptibility. The findings also suggested that the differential development of antimicrobial resistance to various antimicrobial agents in S . Typhi, showing the rapid increase in ampicillin resistance and reduced ciprofloxacin susceptibility, and the high susceptibility to other traditional antimicrobial agents.
Keywords: ampicillin resistance, reduced ciprofloxacin susceptibility, Salmonella  Typhi, clonal expansion