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The aim of this study was to investigate whether the sulphonamide resistance gene sul2 could be transferred between Escherichia coli in the human gut.Nine volunteers ingested a 109 cfu suspension of sulphonamide-susceptible, rifampicin-resistant E. coli recipients of human origin. Three hours later, they ingested a 107 cfu suspension of a sulphonamide-resistant (MIC >1024 mg/L) E. coli donor of pig origin. Stool samples were collected 24 h prior to ingestion, daily for 7 days and at days 14 and 35. Samples were plated on selective plates and monitored for the acquisition of sulphonamide-resistance by the recipient from the indigenous or administrated donor E. coli. Possible transconjugants were typed by PFGE and tested for the presence of plasmids containing the sul2 gene, which was also sequenced.Concentrations of the human and animal E. coli reached a maximum of 7.5 × 106 cfu/g faeces and colonized for more than 7 days, and 2 × 108 cfu/g for more than 14 days, respectively. On day 2, a transconjugant was detected in one volunteer. This volunteer was colonized with sulphonamide-resistant E. coli at day 0. The transconjugant was sul2-positive, had an MIC >1024 mg/L for sulfamethoxazole and the same PFGE profile as the recipient. The resident E. coli transferred a plasmid (>63 kb), containing the sul2 gene, to the recipient. The sul2 sequence of the transconjugant was identical to that of the volunteer’s own E. coli from day 0, but differed from the animal strain. Co-transfer of ampicillin resistance was also demonstrated.Transfer of sul2 was observed between E. coli bacteria in the human intestine. The transconjugant’s sul2 gene came from the volunteer’s own flora. The origin of the E. coli donor is unknown.