“…When plasmid transfer occurs in the digestive tract, the resulting transconjugants are generally unable to become established in that indigenous bacterial community (Duval-Iflah et al, 1994). By contrast, the spreading of plasmids which encode resistance to clinically important antibiotics in pathogenic bacteria has been observed often during antimicrobial therapy, which is believed to modify the normal microbiota and to lower host resistance against colonization by pathogenic plasmid-bearing bacteria (Barza et al, 1987). In addition, it should be pointed out that there were R plasmids encoding, simultaneously, resistance, virulence and other features (Sant'Ana et al, 1995).…”