Characterization of a naturally occurring R plasmid in Salmonella isangi conferring unusually high resistance to ampicillin.

A strain of Salmonella isangi was isolated from HUMANA milk, used a baby food in Bulgarian hospitals. It harbours two stable plasmids, distinguishable by their molecular weight of 21 Mdal for the transfer factor (rRB 1) and 9,3 Mdal for R-factor rRB 2, confering resistance to ampicillin, cephalothin, streptomycin and sulfonamides. Both plasmids are maintained in different copy numbers per chromosome genome equivalent, 8 copies for rRB 1 and 158 copies for rRB 2, associated with an unusual high level of resistance to ampicillin (MIC 65 mg/ml).