Transfer of ampicillin resistance from Salmonella Typhimurium DT104 to Escherichia coli K12 in food

AIMS: To investigate the transfer of antibiotic resistance from a donor Salmonella Typhimurium DT104 strain to a recipient Escherichia coli K12 strain. METHODS AND RESULTS: Mating experiments were conducted in broth, milk and ground meat (beef) at incubation temperatures of 4, 15, 25 and 37 degrees C for 18 and 36 h. Ampicillin-resistance transfer was observed at similar frequencies in all transfer media at 25 and 37 degrees C (10(-4) to 10(-5) log(10 )CFU ml g(-1), transconjugants per recipient) for 18 h. At 15 degrees C, transfer was observed in ground meat in the recipient strain (10(-6), log10 CFU g(-1), transconjugants per recipient), but not in broth or milk. At 4 degrees C, transfer did not occur in any of the examined mediums. Further analysis of the E. coli K12 nal transconjugant strain revealed the presence of a newly acquired plasmid (21 kbp) bearing the beta-lactamase gene bla(TEM). Transconjugants isolated on the basis of resistance to ampicillin did not acquire any other resistant markers. CONCLUSION: This study demonstrates the transfer of antibiotic resistance in food matrices at mid-range temperatures. SIGNIFICANCE AND IMPACT OF THE STUDY: It highlights the involvement of food matrices in the dissemination of antibiotic-resistant genes and the evolution of antibiotic-resistant bacteria.