Effect of subinhibitory concentrations of ampicillin on the R plasmid transfer in Escherichia coli.

The effects of subinhibitory concentrations (sub-MICs) of ampicillin on R plasmid transfer in Escherichia coli were studied. Each donor strain culture was separated into two parts; one was mixed with a recipient strain culture for mating, the other was treated with the sub-MIC of ampicillin and then mixed with the recipient strain culture. In both cases the R plasmid transfer frequency was determined at 30, 60, 90 and 120 min of mating. Results showed that there exists a general decrease in the transfer frequencies under sub-MIC treatment (two plasmids did not transfer at all). The proportion of aggregates and the number of cells that compose them were not affected by the sub-MIC of ampicillin. Our study supports the idea that the changes induced in E. coli by sub-MICs of ampicillin did not affect the function of the surface structures responsible for aggregation but did affect the proteins implicated in DNA transfer, situated on the cell surface.