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

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.