Effects of starvation and osmotic stress on viability and heat resistance of Pseudomonas fluorescens AH9

This study addresses the responses to starvation and osmotic stress of Pseudomonas fluorescens isolated from spoiled fish. Culturability and viability of stressed cells were determined. Cells maintaining an active electron transport system were considered to be viable and this activity was assessed by the ability of the cells to reduce the 5‐cyano‐2,4‐ditolyl tetrazolium chloride (CTC) to fluorescent CTC‐formazan. Cells starved of carbon maintained high culturability and a high proportion of the cells were capable of reducing CTC during short‐time (up to 5 d) experiments. ATP concentrations were lower in carbon‐starved than in log‐phase cells but the measured levels suggested that metabolic activity was retained. Carbon‐starved cells developed an increased heat resistance and prolonged starvation resulted in further protection. Viable, but non‐culturable cells were found during heat challenge implying that culture methods underestimate the recovery potential of these cells. Osmotically‐stressed Ps. fluorescens maintained a high viability, whereas culturability was rapidly lost. In contrast to starved cells, no protection against a subsequent heat challenge was found in osmotically‐stressed (4 or 18 h) cells, but an increased salinity of the heating menstruum alone resulted in elevated heat resistance.