TRANSFER OF MULTIPLE-DRUG RESISTANCE PLASMIDS BETWEEN BACTERIA OF DIVERSE ORIGINS IN NATURAL MICROENVIRONMENTS

Plasmids harboring multiple antimicrobial-resistance determinants (R p lasmids) were transferred in simulated natural microenvironments from various bacterial pathogens of human, animal, or fish origin to suscep tible strains isolated from a different ecological niche. R plasmids i n a strain of the human pathogen Vibrio cholerae O1 El Tor and a bovin e Escherichia coli strain were conjugated to a susceptible strain of t he fish pathogenic bacterium Aeromonas salmonicida subsp. salmonicida in marine water. Conjugations of R plasmids between a resistant bovine pathogenic E. coli strain and a susceptible E. coli strain of human o rigin were performed on a hand towel contaminated w ith milk from a co w with mastitis. A similar conjugation event between a resistant porci ne pathogenic E. coli strain and a susceptible E. coli strain of human origin was studied in minced meat on a cutting board. Conjugation of R plasmids between a resistant strain of the fish pathogenic bacterium A. salmonicida subsp. salmonicida and a susceptible E. coli strain of human origin was performed in raw salmon on a cutting board. R plasmi ds in a strain of A. salmonicida subsp. salmonicida and a human pathog enic E. coli strain mere conjugated to a susceptible porcine E. coli s train in porcine feces. Transfer of the different R plasmids was confi rmed by plasmid profile analyses and determination of the resistance p attern of the transconjugants. The different R plasmids were transferr ed equally well under simulated natural conditions and under controlle d laboratory conditions, with median conjugation frequencies ranging f rom 3 x 10(-6) to 8 x 10(-3). The present study demonstrates that conj ugation and transfer of R plasmids is a phenomenon that belongs to the environment and can occur between bacterial strains of human, animal, and fish origins that are unrelated either evolutionarily or ecologic ally, even in the absence of antibiotics. Consequently, the contaminat ion of the environment with bacterial pathogens resistant to antimicro bial agents is a real threat not only as a source of disease but also as a source from which R plasmids can easily spread to other pathogens of diverse origins.