Potassium Leakage as a Lethality Index of Phenol and the Effect of Solute and Water Activity

Continuous monitoring of K + leakage from Serratia marcescens on exposure to phenol in aqueous and solute‐containing suspensions was followed by a K + selective electrode. At each phenol concentration, the suspension reached an equilibrium between the internal and external K + as the result of cell leakage. The K + concentration at equilibrium was constant for each cell suspension and independent of the phenol concentration that induced it. Phenol concentration affected only the rate of K + release and the time ‘ t Kmax’to reach the equilibrium state. The cell number affected the maximum K + concentration at equilibrium and the rate of K + leakage in such a way that ‘ t Kmax’was constant at a given phenol concentration. Therefore, the ratio t Kmax/ D value was constant at phenol concentrations below 1.2%. When the water activity of the phenol solutions was reduced by addition of sucrose and glycerol. the kinetics of K + leakage compared consistently with that of lethality by phenol. Consequently, the concentration coefficient ‘ n ’derived from the log‐log plot of t Kmax. and phenol concentrations was influenced by the solutes and the resulting water activity similarly as that derived from D values. Thus, ‘ n ’increased in the presence of suerose and decreased in the presence of glycerol progressively with their concentration. It was concluded that t Kmax could be used as a lethality parameter with predominantly membrane active antimicrobial agents.