COPPER-RESISTANT ENTERIC BACTERIA FROM UNITED-KINGDOM AND AUSTRALIAN PIGGERIES

Thirty-three enteric isolates from Australian (Escherichia coli only) and United Kingdom (U.K.) (Salmonella sp., Citrobacter spp., and E. co li) piggeries were characterized with respect to their copper resistan ce. The copper resistance phenotypes of four new Australian E. coli is olates were comparable with that of the previously studied E. coli K-1 2 strain ED8739(pRJ1004), in that the resistance level in rich media w as high (up to 18 MM CuSO4) and resistance was inducible. Copper resis tance was transferable by conjugation from the new Australian isolates to E. coli K-12 recipients. DNA similarity between the new Australian isolates and the pco copper resistance determinant located on plasmid pRJ1004 was strong as measured by DNA-DNA hybridization; however, the copper resistance plasmids were nonidentical as indicated by the pres ence of restriction fragment length polymorphisms between the plasmids . DNA-DNA hybridization and polymerase chain reaction analysis demonst rated DNA homology between the pco determinant and DNA from the U.K. E . coli, Salmonella sp., and Citrobacter freundii isolates. However, th e copper resistance level and inducibility were variable among the U.K . strains. Of the U.K. E. coli isolates, 1 demonstrated a high level o f copper resistance, 4 exhibited intermediate resistance, and 16 showe d a low level of copper resistance; all of these resistances were expr essed constitutively. A single U.K. C. freundii isolate had a high lev el of copper resistance, inducible by subtoxic levels of copper. Trans conjugants from one E. coli and one C. fruendii donor, with E. coli K- 12 strain UB1637 as a recipient, showed copper resistance levels and i nducibility of resistance which differed from that expressed from plas mid pRJ1004. We conclude that closely related resistance determinants in nonidentical plasmids are responsible for copper resistance in ente ric bacteria isolated at separate geographic locations.