Distribution of the multidrug resistance gene cfr in Staphylococcus species isolates from swine farms in China

Abstract

A total of 149 porcine Staphylococcus isolates with florfenicol MICs of ≥ 16 μg/ml were screened for the presence of the multiresistance gene cfr, its location on plasmids, and its genetic environment. In total, 125 isolates carried either cfr (16 isolates), fexA (92 isolates), or both genes (17 isolates). The 33 cfr-carrying staphylococci, which included isolates of the species Staphylococcus cohnii, S. arlettae, and S. saprophyticus in which the cfr gene has not been described before, exhibited a wide variety of SmaI pulsed-field gel electrophoresis patterns. In 18 cases, the cfr gene was located on plasmids. Four different types of cfr-carrying plasmids--pSS-01 (n = 2; 40 kb), pSS-02 (n = 3; 35.4 kb), pSS-03 (n = 10; 7.1 kb), and pBS-01 (n = 3; 16.4 kb)--were differentiated on the basis of their sizes, restriction patterns, and additional resistance genes. Sequence analysis revealed that in plasmid pSS-01, the cfr gene was flanked in the upstream part by a complete aacA-aphD-carrying Tn4001-like transposon and in the downstream part by a complete fexA-carrying transposon Tn558. In plasmid pSS-02, an insertion sequence IS21-558 and the cfr gene were integrated into transposon Tn558 and thereby truncated the tnpA and tnpB genes. The smallest cfr-carrying plasmid pSS-03 carried the macrolide-lincosamide-streptogramin B resistance gene erm(C). Plasmid pBS-01, previously described in Bacillus spp., harbored a Tn917-like transposon, including the macrolide-lincosamide-streptogramin B resistance gene erm(B) in the cfr downstream region. Plasmids, which in part carry additional resistance genes, seem to play an important role in the dissemination of the gene cfr among porcine staphylococci.

Fig 1

Dendrogram and associated SmaI PFGE patterns of 32 cfr-carrying Staphylococcus isolates and the localization of cfr and fexA gene. (A) S. arlettae; (B) S. saprophyticus; (C) S. cohnii; (D) S. sciuri. The single S. aureus strain with chromosomally located genes cfr and fexA was not included.

Fig 2

Schematic presentation of the genetic environment of the cfr gene in plasmids pSS-01, pSS-02, pSS-03, and pBS-01. The arrows indicate the positions and directions of the transcription of the genes. The regions of >96% homology are marked using gray shading. The direct target site duplication is boxed. Δ indicates a truncated gene. A distance scale in kilobases is displayed in the upper right corner.