The minimum inhibitory concentation of cetrimide and chlorhexidine for Escherichia coli is altered when R-plasmid RPI is inserted into it. Using disinfectant-agar plates as a test system, the sensitivity of Esch. coli to these disinfectants and to phenol was found to vary, not only with the presence of RPI, but also with the nutritional status of the inoculum. The different inocula also varied in their ability to bind H i and there was some correlation between this property and chlorhexidine sensitivity.

325The minimal nutritional requirements of Escherichia coli have been quantitatively determined in batch culture for cells with (R+) and without (R-) the R plasmid RPl. In these conditions R+ cells have a greater requirement than R-for several nutrients, particularly Mg2+, K+, Fe2+ and PO: -.The maximum growth rate in a simple salts medium was the same for R+ and R-cells. At low concentrations of phosphate, the specific growth rate of R+ cells differed from that found for R-cells. The R plasmid was stable in simple salts medium, irrespective of the nutrient ultimately depleted by growth, but, on storage, R+ cells survived for a shorter time than R-cells.