Elimination of plasmidic resistance to ampicillin and of enterotoxinogenicity in certain enteric gram-negative bacteria after incubation with high concentrations of amprolium.

We have observed that treatment with high concentrations of a thiamine analog (amprolium) can lead to the elimination of the plasmidic resistance to ampicillin and the production of enterotoxin in wild Escherichia coli strains and in E coli and Salmonella typhimurium strains which had received the pKM101 plasmid through bacterial conjugation. By computer analysis, we also have determined that there is a highly significant (P less than 0.01) synergism between ampicillin and amprolium which reduces considerably the growth of certain enteric bacterial strains which have a plasmidic resistance to ampicillin and which were not markedly affected by amprolium alone, in our experimental conditions. Our data indicate that the rate of loss of the plasmid pKM101 after treatment of R+ bacterial strains with amprolium can be increased.