Response to Phosphate Starvation and Osmotic Homeostasis

Phosphorus is one of the important nutrients for bacterial growth. Mostly, phosphorus is available in low concentrations in natural environment. Therefore, bacteria and all other micro-organisms have developed mechanism to cope up with phosphorus limitation. Upon exposure to phosphorus limited conditions, there is upregulation of a large number of genes belonging to phosphorus transport and its metabolism, and these are coded by a unique Pho regulon. Pho regulon is under the control of a two-component system PhoBR which is unique in the sense that it functions through an ABC transporter involved in phosphorus transport. This chapter describes how PhoBR two-component system regulates Pho regulon as a function of phosphorous availability. Bacteria respond to external osmotic conditions rapidly to maintain their shape and integrity. When cell is subjected to high osmotic pressure, it triggers osmohomeostatic response to protect cell from collapse by counter balancing external pressure. Likewise, in extreme dilute solutions, it maintains ionic balance in the cell through operation of special mechanosensitive channels. Osmoadaptation studies show that it is a two-step process with one transient and fast response followed by delayed long-lasting adaptation. Different adaptations by bacteria to constant osmotic stress are discussed along with signal transduction. Further, differential regulation of outer membrane porins is also explained which is not a very specific response as it is observed during a variety of environmental stresses.