An environmental multiresistance plasmid can be transferred to foodborne pathogens

Introduction : Pathogens resistant to multiple antibiotics are rapidly emerging and can acquire their resistance genes by conjugation from non-pathogenic bacteria inhabiting a multitude of environments. In spite of the concern that foodborne pathogens can become multi-drug resistant by acquiring plasmids from environmental strains, little is known about the success of that natural genetic exchange. This study investigated if the multiresistance plasmid pB10, isolated from a wastewater treatment plant and harbouring amoxicillin, streptomycin, sulfonamides and tetracycline resistance genes, can be transferred to the foodborne pathogens Salmonella and Escherichia coli O157:H7 and how this alters the phenotype of the recipients. Material and methods : The strains used in this study were the donor strain Pseudomonas putida SM1443 (pB10::gfp) and the recipient strains Salmonella spp. (n=10) and E. coli O157:H7 (n=5). The Salmonella spp. strains belonged to the serovars S. Enteritidis (n=3), S. Typhimurium (n=4), S. Virchow (n=1), S. Infantis (n=1) and S. Hadar (n=1). Conjugation experiments were performed by filter mating on 0.22 µm polycarbonate filters. The transfer ratio was determined by plating and by flow cytometry in which gfp was used as reporter gene [1]. Antibiotic resistance profiles were determined for both recipients and transconjugants using the disc diffusion method according to the Clinical and Laboratory Standards Institute (CLSI) guidelines [2]. Results : For 14 of the 15 recipient strains, transconjugants were obtained by plating and/or by flow cytometry. Based on plating, transfer ratios were between 6.8 x 10-9 and 3.0 x 10-2 while using flow cytometry, transfer ratios were between