The ability of bacteria to respond to the ever changing environmental conditions they encounter requires the utilization of a broad range of colonization and survival strategies. One of the most important and probably least understood factors determining bacterial activity in terrestrial habitats is water availability. The goal of this dissertation research was to identify traits that contribute to maintaining a functional cell envelope when cells are dehydrated, which will help us to assess the scope of physiological changes necessary for growth in low-water-content habitats. The primary objective was to identify genes that are specifically regulated by dehydration (matric stress), and not by a thermodynamically-equivalent solute (osmotic) stress. We used transposon mutagenesis combined with a screen to identify genes in the soil and rhizosphere colonizing Pseudomonas putida strain mt-2. Although some of the w&barbelow;ater d&barbelow;eprivation-c&barbelow;ontrolled (wdc) genes were regulated by growth phase, temperature, or toluene exposure, most were specifically induced by matric stress, indicating that bacteria respond differently to a matric stress than to a solute stress. The knowledge of the function of these matric stress-regulated wdc genes allowed us to develop a model of tolerance mechanisms necessary for growth and survival in water-limited environments. Moreover, we found that most wdc loci contributed to survival in low-water-content habitats. One of the identified wdc genes encodes a putative periplasmic lysophospholipase, indicating that dehydration stress disrupts membrane integrity. We determined that lysophospholipids (LPL) accumulated in the wild type when grown under matric stress conditions, but not when grown in the absence of stress or under solute stress conditions. In contrast, the putative lysophospholipase-deficient mutant accumulated LPL even in the absence of a matric stress, indicating that this periplasmic lysophospholipase removes LPL. Exogenously supplied LPL was toxic to P. putida and the mutant was more sensitive than the wild type to LPL. Finally, we observed that biofilm cells are more desiccation-tolerant than planktonic cells and that matric stress is more stressful to cells than solute stress.