Inhibition of Conjugal Transfer of R Plasmids by Pipemidic Acid and Related Compounds

It was found that pipemidic acid and related compounds have the common property of inhibiting the conjugal transfer of R plasmids carried by Escherichia coli and Pseudomonas aeruginosa. This inhibition was not due to bactericidal activity of the compounds against donors, recipients, or R(+) transconjugants. However, it was correlated inversely with their minimal inhibitory concentrations, depending especially upon sensitivity of donors to the compounds. The inhibition was generally observed in mating systems using various combinations of 14 R plasmids and 9 bacterial strains. The pretreatment of a donor or a recipient strain with pipemidic acid showed only a small inhibitory effect on the transfer of an R plasmid, whereas the presence of the compound during an active mating caused a marked inhibitory effect.