R-factor-mediated antibiotic resistance in E. coli strains isolated from piglets in Sweden.

E. coli strains, isolated from piglets over the periods 1964-1968 and 1974-1975, were investigated and compared with respect to antibiotic resistance. The frequency of monoresistant strains decreased from 50% in 1964-68 to 27% in 1974-75, while that of strains showing double or multiple resistance increased from 13% to 22%. The proportions of the different resistance determinants were as follows (figures for 1964-68 within parentheses): Tetracycline 23% (49%), sulphaisodimidine 38% (24%), streptomycin 9% (3%), ampicillin 6% (1%), chloramphenicol 6% (0.5%) and neomycin 4% (0%). All the strains were sensitive to trimethoprim. In 1974-75 the resistance determinants were R factor-carried to the percentage numbers as follows: Tetracycline 53%, sulphaisodimidine 64%, streptomycin 77%, neomycin 65%, ampicillin 95% and chloramphenicol 90%. These determinants were with the exception of tetracycline and sulphaisodimidine predominantly present in multiply resistant strains.