Molecular characterization of ciprofloxacin-resistant Salmonella enterica serovar Typhi and Paratyphi A causing enteric fever in India

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Abstract
Objectives: To define the genetic characteristics and resistance mechanisms of clinical isolates of Salmonella enterica serovar Typhi (S. Typhi) and S. enterica serovar Paratyphi A (S. Paratyphi A) exhibiting high-level fluoroquinolones resistance. Methods: Three S. Typhi and two S. Paratyphi A ciprofloxacin-resistant isolates (MICs > 4 mg/L) were compared with isolates with reduced susceptibility to ciprofloxacin (MICs 0.125–1 mg/L) by PFGE, plasmid analysis, presence of integrons and nucleotide changes in topoisomerase genes. Results: In S. Typhi and Paratyphi A, a single gyrA mutation (Ser-83→Phe or Ser-83→Tyr) was associated with reduced susceptibility to ciprofloxacin (MICs 0.125–1 mg/L); an additional mutation in parC (Ser-80→Ile, Ser-80→Arg, Asp-69→Glu or Gly-78→Asp) was accompanied by an increase in ciprofloxacin MIC (≥ 0.5 mg/L). Three mutations conferred ciprofloxacin resistance: two in gyrA (Ser-83→Phe and Asp-87→Asn or Asp-87→Gly) and one in parC. This is the first report of parC mutations in S. Typhi. Ciprofloxacin-resistant S. Typhi and S. Paratyphi A differed in their MICs and mutations in gyrA and parC. Moreover S. Typhi harboured a 50 kb transferable plasmid carrying a class 1 integron (dfrA15/aadA1) that confers resistance to co-trimoxazole and tetracycline but not to ciprofloxacin. PFGE revealed undistinguishable XbaI fragment patterns in ciprofloxacin-resistant S. Typhi as well as in S. Paratyphi A isolates and showed that ciprofloxacin-resistant S. Typhi have emerged from a clonally related isolate with reduced susceptibility to ciprofloxacin after sequential acquisition of a second mutation in gyrA. Conclusions: To our knowledge this is the first report of molecular characterization of S. Typhi with full resistance to ciprofloxacin. Notably, the presence of a plasmid-borne integron in ciprofloxacin-resistant S. Typhi may lead to a situation of untreatable enteric fever.

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