AimsTo investigate how the growth stage of Enterococcus faecalis affects its photoinactivation in clear water. Methods and ResultsEnterococcus faecalis were grown in batch cultures to four different growth stages or grown in chemostats set at four different dilution rates, then harvested and exposed to full spectrum or UVB-blocked simulated sunlight. Experiments were conducted in triplicate in clear water with no added sensitizers. Decay curves were shoulder-log linear and were generally not statistically different in experiments conducted under full spectrum light. Shoulders were longer and first order inactivation rates smaller when experiments were seeded with cells grown to stationary as compared to exponential phase, and for slower growing cells when experiments were done under UVB-blocked light. Chemostat-sourced bacteria generally showed less variability among replicates than batch-sourced cells. ConclusionsThe physiological state of cells and the method via which they are being generated may affect the photoinactivation experimental results. Significance and Impact of the StudyPhotoinactivation experiments conducted with exponential phase cells may overestimate the photoinactivation kinetics in the environment, particular if UVB-independent mechanisms predominate. Chemostat-sourced cells are likely to provide more consistent experimental results than batch-sourced cells.