Antibiotic-resistant Escherichia coli in market pigs in 1956-1979: the emergence of organisms with plasmid-borne trimethoprim resistance.

Surveys conducted since 1956 on the antibiotic resistance of the Escherichia coli in faecal specimens from pigs entering Chelmsford Market have revealed that despite the implementation of the Swann Report in 1971 pigs are still an enormous reservoir of tetracycline-resistant E. coli with conjugative ability. Increasingly large amounts of E. coli resistant to streptomycin and sulphonamides were found in specimens examined in recent years until in 1979 the amounts present approached those of tetracycline-resistant organisms. E. coli resistant to chloramphenicol, ampicillin, neomycin, furazolidone or spectinomycin were present, usually in low concentration, in a considerable proportion of the specimens at each yearly examination but the concentration and incidence of these organisms showed no obvious sign of increasing with time. Much of this resistance, except to furazolidone, was of the transferable type. Until 1979 the incidence of faecal specimens containing trimethoprim-resistant E. coli was very low. It increased significantly in that year, most of the resistance being plasmid-, or possibly transposon-determined. The results of surveys performed in a Cambridgeshire market in 1978 and 1979, which showed that a high proportion of faecal specimens contained low concentrations of trimethoprim-resistant E. coli, in general resembled those of the corresponding Chelmsford surveys, suggesting that all the Chelmsford surveys may have accurately reflected the position in the national pig herd.