Novel components of Pseudomonas putida biofilm exopolymeric matrix and a transcriptome analysis of the effects of osmotic and matric stress
Bacteria have the reputation of being `simple' life forms based on their single cell nature and lack of a nucleus. However, no other domain on earth contains species that are so diverse in their metabolic and physiological capabilities allowing them to persist in environmental conditions inhospitable to many plants and animals. The key ability to persist in these environments lies in how cells are able to sense and respond to changes in their abiotic conditions. Nutrient abundance, pH, temperature, oxygen, redox state, and water availability are six major factors that are essential for all life. In soil habitats the availability of water is of particular importance since these habitats become routinely or periodically dehydrated by drought or drainage events. The availability of water is measured in terms of its water potential (y) and is influenced by both the concentration of solutes (solute potential) and the physical sorption of water to surrounding surfaces (matric potential), which increases as habitats dry. The metabolic capabilities of soil microbes can influence global events such as nitrogen and carbon cycling, since metabolic and physiological competence of bacterial cells are influenced by solute and matric potentials. One way to diminish water loss from cells is to form surface adhered aggregates of cells that are enmeshed within exopolymeric substance of their own making, formally known as biofilms. Some materials within in the biofilm exopolymeric matrix, such as exopolysaccharides (EPS), are hygroscopic and absorb water to help maintain cell hydration. Other matrix components may function in biofilm development and stabilization of the biofilm matrix, aiding to long-term preservation of the biofilm lifestyle.
Progress towards understanding the biofilm properties and matrix components utilized during low water potential conditions is necessary for full appreciation of the factors that influence bacterial survival and function within terrestrial environments. Recent advances in whole genome transcriptomics allow us to explore the effects of reductions in solute and matric potential on Pseudomonas putida gene expression that was unavailable to us a decade ago. Also, our understanding of biofilm architecture and molecular interactions within the biofilm matrix has surpassed original perceptions that biofilms are simply a disordered assemblage of cells. Knowing that there are particular functions related to the types and interactions of exopolymeric substances underscores our need to expand our current understanding of the biofilm matrix and its relationship to water stress physiology. To this end, we explored the types of exopolysaccharides and a novel adhesin, purli, produced by P. putida mt2 and speculate on their roles in biofilm formation. Microarray analysis of P. putida gene expression changes following solute or matric stress shock conditions identified genes that may produce products important for biofilm formation and water stress tolerance. Together, a firm understanding of the complex genetic and physical traits involved in biofilm formation and water stress physiology will equip us to better understanding bacteria and fitness traits they employ during a variety environmental conditions.