A water environment presents unique challenges for the bacterial cell. Water lacks the nutrients found in other environments and provides an extreme hypo-osmotic condition. To survive for long periods of time the organism must be able to deal with the metabolic challenges resulting from lack of substrates normally used for maintenance and growth and must be able to adapt to low osmolarity. The fact that this environment requires special adaptations is illustrated by our observation that E. coli, S. aureus and notably, B. mallei, were unable to survive in this environment for prolonged periods.
A variety of factors contribute to the ability of B. pseudomallei to survive in the environment. These include a versatile metabolic capacity which allows the use of a variety of carbon sources for growth, the ability to live inside other microorganisms including protoza and fungi and the ability to tolerate a wide range of environmental conditions including a variety of soils and water environments such as those associated with rice farming and seasonal flooding. The ability of B. pseudomallei to survive in aqueous environments is a major factor in the survival and persistence of this organism in environments where meliodosis remains endemic.
We tested a number of hypotheses which we thought might contribute to B. pseudomallei survival in water. These included an examination of growth phase, PHB, capsule and LPS and the product of one gene that was shown to be up-regulated in water via microarray analysis. We examined cells grown in log and stationary phase hypothesizing that perhaps stationary phase cells had made the necessary modifications to the cell which would allow survival in water whereby cell in log phase had not. We found that cells taken from either growth phases were equally able to survive in water indicating that cell growth phase was not important or that the cells, in either log or stationary phase, must undergo some adaptation upon entering a water environment.
PHB has been shown to be involved in water survival in Legionella pneumophila  and would seem not unreasonable to play a role in water survival in B. pseudomallei. We found no significant loss in viability in a PHB deficient mutant, RM330, in water suggesting that in B. pseudomallei, PHB may not be critical as an energy source for long term survival. It is possible however, that PHB has an important role in other situations such as intracellular survival in protozoa and fungi and possibly human cells as well.
We found that B. pseudomallei capsule I was not required for survival in water. In contrast, capsule I is essential for B. pseudomallei virulence in hamsters where it serves to evade complement killing. That capsule is not solely required for water survival is further illustrated by the fact that B. mallei, which doesn't survive in water, expresses the same capsule as B. pseudomallei.
The mutant MB301, defective in core LPS oligosaccharide, showed a marked loss of viability by 200 days. This mutation also confers sensitivity to polymyxin B, a cationic antibiotic which is normally ineffective against B. pseudomallei at levels up to 100 mg/ml suggesting a loss of outer membrane integrity as a result of altered outer membrane architecture. LPS oligosaccharide is not essential for water survival suggesting that, like capsule, the molecule probably does little to help maintain the membrane integrity apparently required for long term water survival.
While this study identifies at least one component required for long term survival in water, it is probable that B. pseudomallei undergoes a number of other modifications which allow it to survive for years in nutrient poor, low osmolarity conditions. It is likely that membrane modifications occur as suggested by the up regulation of a putative phosphatidylglycerol phosphatase (BPSL2961). Further, energy metabolism is likely altered as indicated by the up-regulation of a putative cytochrome C related lipoprotein (BPSL1600) and a putative iron-sulfur protein (BPSL1615). In addition, the aqueous environment may trigger a number of cell signals that regulate the various modifications required for long term survival. Understanding how B. pseudomallei is able to survive in water may provide a better understanding of the organisms environmental persistence in endemic areas and may provide public health strategies for those living in these areas.