Epidemiologic relationship between fluoroquinolone-resistant Salmonella enterica Serovar Choleraesuis strains isolated from humans and pigs in Taiwan (1997 to 2002)

Abstract

The emergence of ciprofloxacin-resistant Salmonella enterica serovar Choleraesuis in recent years has become an important public health issue in Taiwan. The resistant strains that cause human infections are considered to be from pigs. In this study, we characterized 157 swine and 42 human Salmonella serovar Choleraesuis isolates by pulsed-field gel electrophoresis (PFGE) and drug susceptibility testing to investigate the epidemiologic relationship among the isolates. By PFGE analyses, two major clusters (clusters GA and GB) were identified. Isolates in cluster GA were of both human and swine origins, while those in cluster GB were from pigs only. Among the various genotypes identified, genotype gt-1a was the most prevalent, which was found in 71% (30 of 42) and 48% (76 of 157) of human and swine isolates, respectively. The susceptibility tests for the 106 gt-1a isolates identified 44 susceptibility profiles and showed that 73% of human isolates and 34% of swine isolates were resistant to three fluoroquinolones (ciprofloxacin, enrofloxacin, and norfloxacin). Our findings indicate that a clonal group of Salmonella serovar Choleraesuis may have been circulating in human and swine populations in Taiwan for years and that the fluoroquinolone-resistant Salmonella serovar Choleraesuis strains most likely evolved from a gt-1a clone that emerged in 2000 and that then caused widespread infections in humans and pigs. Nevertheless, it is still debatable whether those Salmonella infections in humans are caused by isolates derived from pigs, on the basis of the higher fluoroquinolone and other antimicrobial resistance percentages in human isolates than in pig isolates.

FIG. 1.

Dendrogram and PFGE patterns of XbaI-digested chromosomal DNA of S. enterica serovar Choleraesuis and the numbers of isolates of pig and human origin. The dendrogram was constructed by use of the UPGMA algorithm and the Dice similarity coefficient by using BioNumerics software with 3% optimization and 1% position tolerance.

FIG. 2.

Dendrogram and profiles of susceptibility to 12 antimicrobials for the 106 gt-1a S. enterica serovar Choleraesuis isolates and the numbers of isolates of pig and human origin. Am, ampicillin; C, chloramphenicol; S, streptomycin; SxT, trimethoprim-sulfamethoxazole; Te, tetracycline; Cip, ciprofloxacin; Na, nalidixic acid; Nor, norfloxacin; Eno, enrofloxacin; Cf, cephalothin; Gm, gentamicin; F/m, nitrofurantoin; black box, resistant; grid box, intermediate; gray box, susceptible.