Distribution of florfenicol resistance genes fexA and cfr among chloramphenicol-resistant Staphylococcus isolates.

A total of 302 chloramphenicol-resistant Staphylococcus isolates were screened for the presence of the florfenicol/chloramphenicol resistance genes fexA and cfr and their localization on mobile genetic elements. Of the 114 isolates from humans, only a single Staphylococcus aureus isolate showed an elevated MIC to florfenicol, but did not carry either of the known resistance genes, cfr or fexA. In contrast, 11 of the 188 staphylococci from animal sources were considered florfenicol resistant and carried either cfr (one isolate), fexA (five isolates), or both resistance genes (five isolates). In nine cases we confirmed that these genes were carried on a plasmid. Five different types of plasmids could be differentiated on the basis of their sizes, restriction patterns, and resistance genes. The gene fexA, which has previously been shown to be part of the nonconjugative transposon Tn558, was identified in 10 of the 11 resistant isolates from animals. PCR assays were developed to detect different parts of this transposon as well as their physical linkage. Complete copies of Tn558 were found in five different isolates and shown by inverse PCR to be functionally active. Truncated copies of Tn558, in which the tnpA-tnpB area was in part deleted by the integration of a 4,674-bp segment including the gene cfr and a novel 2,446-bp IS21-like insertion sequence, were seen in a plasmid present in three staphylococcal isolates.