Transfer of erythromycin resistance from poultry to human clinical strainsof Staphylococcus aureus

The transfer of ermA and ermC genes, the two most common resistance determi nants of erythromycin resistance, was studied with Luria-Bertani broth in t he absence of additional Ca2+ or Mg2+ ions, Fifteen human and five poultry isolates of Staphylococcus aureus, which were resistant to erythromycin but carried different genetic markers for erythromycin resistance, were used f or conjugation, Since both the donors (Amp(s)-Tet(r)) and recipients (Amp(r )-Tet(s)) were resistant to erythromycin, the transconjugants were initiall y picked up as ampicillin- and tetracycline-resistant colonies. The resista nce transfer mechanisms of the chromosomally located erythromycin rRNA meth ylase gene ermA and the plasmid-borne ermC gene were monitored by a multipl ex PCR and gene-specific internal probing assay. Four groups of transconjug ants, based upon the transfer of the ermA and/or ermC gene, were distinguis hed from each other by the use of this method, Selective antibiotic screeni ng revealed only one type of transconjugant that was resistant to ampicilli n and tetracycline, A high frequency of transfer (4.5 x 10(-3)) was observe d in all of the 23 transconjugants obtained, and the direction of tetracycl ine and erythromycin resistance marker transfer was determined to be from p oultry to clinical isolates. The transfers of the ermA and ermC genes were via transposition and transformation, respectively.