Influence of oral antibiotics on resistance and enterotoxigenicity of Escherichia coli.

Three groups of five piglets were formed and 1390 Escherichia coli isolates were obtained during the 45-day period of observation. One of the groups received feed without antibiotic whereas the second received feed containing 100 ppm neomycin and the third feed with 100 ppm neomycin plus 100 ppm tetracycline. Rectal swabbings for bacterial isolation were repeated ten times, twice during an adaptation period and eight times during the treatment period. Resistance among the isolates to tetracycline, streptomycin and triple sulfas remained high throughout this experiment whereas resistance to neomycin, chloramphenicol and ampicillin were found to increase significantly under the influence of antibiotic supplemented feed. This increase of antibiotic resistance was associated with an increase of the percentage of isolates harboring an R. factor. When comparing the ability of strains harboring an R factor to receive the plasmid Ent from the E. coli K12 (P155) with isolates not harboring such a plasmid, no significant difference was observed in their ability to receive the Ent plasmid.