Structural and environmental features of novel mdfA variant and mdfA genes in recombinant regions of Escherichia coli.

Novel mdfA gene variants were identified simultaneously from 3 of 13 positive isolates of PCR amplification in Escherichia coli from patients. These 13 positive isolates showed resistance to chloramphenicol, tetracycline, and erythromycin. The 3 mdfA gene variants were of the same genotype and all the 13 positive isolates were investigated by conjugation experiment, EcoRI restriction, and gene mapping. Conjugation experiments demonstrated that the novel mdfA variant and mdfA genes were located on plasmids that were restricted by EcoRI for ∼8.2 kb-length, which was also validated by gene mapping. Further study indicated three types of genetic structures (A, B, and C) in the recombinant plasmids harboring mdfA and surrounding genes, and structure B was first reported in the article. Structure A comprises two partial-length and six full-length genes, including the mdfA gene variant in the recombinant plasmid; structure B comprises four full-length genes, the mdfA, ybjG, dacC, and ybjI; structure C comprises two full-length genes, the mdfA and dacC. These results suggested that the mdfA gene can function as transporter responsible for multidrug resistance and also mediated the synergistic function with its surrounding genes in conjugative plasmids.