Electrodiffusion of ethidium cation into Micrococcus luteus cells

Ethidium bromide fluorescence increased in the presence of Micrococcus luteus cells; this was shown to be due to the interaction of the ethidium cation (Eth) with intracellular nucleic acids. Eth permeation across the cytoplasmic membrane was the rate-limiting step and obeyed first-order kinetics. Both the rate of influx and the amount of Eth in cells depended on respiration and on ATPase activity under aerobic and anaerobic conditions, respectively. The initial rate of uptake positively correlated with the membrane potential and was a linear function of Eth concentration in the range from 2 microM to 1 mM. The data indicate electrodiffusion of Eth into M. luteus.