Hydrating the Pseudomonas aeruginosa periplasm under desiccating conditions
Reduced matric water potential external to the cell has a desiccating effect on bacteria in dry environments. To facilitate hydration, cells must regulate their internal water potential. Accumulating small compatible solutes reduce the cytosolic water potential, though it is unknown how the periplasm of gram negative bacteria is hydrated. As this compartment houses many processes, hydration is important. Linear and cyclic osmoregulated periplasmic glucans are known to accumulate under water-replete (hypo-osmotic) conditions reducing the periplasmic water-potential relative to the cytosol, limiting swelling of the cytoplasmic compartment. Interestingly, we observed a greater accumulation of linear glucans in biofilm grown Pseudomonas aeruginosa cells on matric stress media, strongly suggesting a role hydrating the periplasm under low-water-content conditions. Additionally, deficiency of cyclic, but not linear glucans reduced survival during matric stress conditions, supporting the importance of glucans hydrating the periplasm. Mutants deficient in producing linear or cyclic glucans experienced increased envelope stress during growth on matric stress conditions as shown by the over-expression of the alginate biosynthesis operon - part of the AlgU/T dependent envelope stress response - resulting in a mucoid colony phenotype. Overexpression of the linear glucan biosynthesis gene is able to rescue cyclic glucan deficient mutants from alginate over-expression, suggesting some functional redundancy between the glucans. Overall, our findings suggest that accumulating periplasmic glucans moderate the envelope stress experienced by the cell under matric stress conditions by hydrating the periplasm.