Staining of Escherichia coli for flow cytometry: influx and efflux of ethidium bromide.

In an attempt to develop procedures for nucleic acid staining of bacteria for clinical routine assays, the uptake of ethidium bromide (EB) in wild-type Escherichia coli was studied using flow cytometry. Phosphate-buffered saline (PBS) containing EDTA or Tris significantly increased the net uptake of EB compared to PBS only. However, in the majority of the cells, the net uptake reached a constant level that was only a few percent of that of fully permeabilized cells, apparently due to the activity of a metabolically driven efflux pump.