Fluoroquinolone resistance in clinical isolates of Streptococcus pneumoniae: contributions of type II topoisomerase mutations and efflux to levels of resistance

Abstract

We report on amino acid substitutions in the quinolone resistance-determining region of type II topisomerases and the prevalence of reserpine-inhibited efflux for 70 clinical isolates of S. pneumoniae for which the ciprofloxacin MIC is >/=4 microgram/ml and 28 isolates for which the ciprofloxacin MIC is </=2 microgram/ml. The amino acid substitutions in ParC conferring low-level resistance (MICs, 4 to 8 microgram/ml) included Phe, Tyr, and Ala for Ser-79; Asn, Ala, Gly, Tyr, and Val for Asp-83; Asn for Asp-78; and Pro for Ala-115. Isolates with intermediate-level (MICs, 16 to 32 microgram/ml) and high-level (MICs, 64 microgram/ml) resistance harbored substitutions of Phe and Tyr for Ser-79 or Asn and Ala for Asp-83 in ParC and an additional substitution in GyrA which included either Glu-85-Lys (Gly) or Ser-81-Phe (Tyr). Glu-85-Lys was found exclusively in isolates with high-level resistance. Efflux contributed primarily to low-level resistance in isolates with or without an amino acid substitution in ParC. The impact of amino acid substitutions in ParE was minimal, and no substitutions in GyrB were identified.

FIG. 1

Percentage of clinical isolates with amino acid substitutions in ParC only (dotted bars), ParC and GyrA (solid bars), neither ParC nor GyrA (bars with horizontal lines), and ParE (bars with diagonal lines) for each ciprofloxacin resistance group (high-level, intermediate-level, and low-level resistance and susceptible). The percentage of isolates for which the ciprofloxacin MIC decreased by fourfold or greater when grown in the presence of reserpine is also shown (cross-hatched bars).