Alginate production by Pseudomonas putida creates a hydrated microenvironment and contributes to biofilm architecture and stress tolerance under water-limiting conditions

Graduate Program in Microbiology and Department of Plant Pathology, Iowa State University, Ames, Iowa 50011

^* To whom correspondence should be addressed. Email: larryh{at}iastate.edu.

	Abstract

Biofilms exist in a variety of habitats that are routinely or periodically not saturated with water and residents must integrate cues on water abundance (matric stress) or osmolarity (solute stress) into lifestyle strategies. Here we examine this hypothesis by assessing the extent to which alginate production by Pseudomonas putida strain mt-2 as well as other fluorescent pseudomonads occurs in response to water limitations and how its presence in turn influences biofilm development and stress tolerance. Total exopolysaccharide (EPS) and alginate production increased with increasing matric, but not solute, stress severity and alginate was a significant but not the major EPS component. Alginate influenced biofilm architecture resulting in biofilms that were taller, covered less surface area, and had a thicker EPS layer at the air-interface than those formed by an mt-2 algD mutant under water limiting conditions, properties that could contribute to less evaporative water loss. We examined this possibility and show that alginate reduces the extent of water loss from biofilm residents by using a biosensor to quantify the water potential of individual cells and by measuring the extent of dehydration-mediated changes in fatty acid composition following a matric- or solute-stress shock. Alginate deficiency decreased desiccation survival of not only P. putida but also that of Pseudomonas aeruginosa PAO1 and Pseudomonas syringae pv syringae B728a. Our findings suggest that in response to water-limiting conditions Pseudomonads produce alginate which influences biofilm development and EPS physiochemical properties. Collectively these responses may facilitate maintaining a hydrated microenvironment, protecting residents from desiccation stress and increasing survival.