Pectinatus frisingensis, a Gram-negative and strictly anaerobic beer spoilage bacterium is sensitive to nisin. An increase in nisin concentration (0 to 1100 IU ml-1) added to the culture medium prolonged the lag phase, and decreased the growth rate of the bacterium. In addition, late exponential cells of P. frisingensis exposed to low concentrations of nisin lost immediately a part of their intracellular K+. Presence of Mg2+ up to 15 mmol l-1 did not protect P. frisingensis from nisin-induced loss of viability and K+ efflux. Potassium leaks were also measured in P. frisingensis late exponential phase cells exposed to combined effects of nisin addition (100-500 IU ml-1), 10 min mild heat-treatment (50 degrees C) or rapid cooling (2 degrees C), and pH (4.0 and 6.2). Net K+ efflux from both starving and glucose-metabolizing cells, was more important at pH 6.2, whatever the temperature treatment and nisin addition. Reincubation at 30 degrees C of P. frisingensis glucose-metabolizing cells exposed to a preliminary combination of nisin addition and mild heat or cooling down treatment, showed that cells exposed to rapid cooling reaccumulated more K+ than heat-treated cells, whatever the pH conditions. A combination of nisin and mild heat-treatment could thus be of interest to prevent P. frisingensis growth in beers