The nalidixic acid-resistant strain E. coli K12
14R525 and rifampicin-resistant strain E. coli K12 MV10 RifR were
used as the recipient strains. Aftermating experiments, transconjugants
were selected on Luria–Bertani medium (Difco Laboratories,
Detroit, MI) containing sulfamethoxazole (64mg/L) plus nalidixic
acid (30 mg/L) or rifampicin (100 mg/L). Transconjugantswere also
screened for the presence of conjugative plasmidDNAcontent, class
1 integrons and the spvC gene.