Influence of water limitation on endogenous oxidative stress and cell death within unsaturated Pseudomonas putida biofilms.

Here we examined how water limitation (matric stress) and high osmolarity (solute stress) influence the extent of endogenous oxidative stress and cell death patterns within Pseudomonas putida biofilms. The temporal dynamics and spatial organization of reactive oxygen species (ROS) accumulation and dead cells in biofilms developed under water-replete and solute stress conditions were similar to each other. Arrays of dead cells, typically one cell width in diameter, were distributed throughout the biofilm and occasionally they spanned the entire depth of the biofilm. These arrays of dead cells were not observed under water-limiting conditions, although the extent of ROS accumulation and cell death was substantially greater. Despite the greater death rate under water-limiting conditions, culturable population sizes were transiently maintained at levels comparable to those under water-replete and solute stress conditions. There was greater spatial stratification of dead cells under water-limiting than water-replete conditions with viable cells primarily located at the air interface, which could facilitate cell dispersal following a wetting event. Under water-limiting conditions, ROS accumulation is greater in an DeltaalgD mutant compared with the wild type, suggesting that the exopolysaccharide alginate attenuates the extent of dehydration-mediated oxidative stress. We conclude that endogenous ROS accumulation is correlated with cell death within P. putida biofilms, although mechanisms contributing to their accumulation may differ under water-replete and water-limiting conditions.