Bacterial responses to osmotic stress: diverse mechanisms to achieve a common goal

Abstract

Low water activity is used frequently, either on its own or in combination with other stresses, to preserve food from spoilage and from the growth of potential pathogens. Bacteria in their natural niche(s) are often required to survive periods of exposure to low water activity and even desiccation. In consequence bacteria have evolved systems that allow them to adapt to changes in the water activity of the environment. Two major types of system contribute to survival at low water activity: (1) the accumulation of compatible solutes that assist in maintenance of enzyme activity as the cell adjusts its cytoplasmic osmolarity sufficiently to restore turgor, and (2) genetic systems that aid survival. When the growth rate of bacterial cells is slowed expression patterns change, leading to expression of a range of gene products that counter stress. This so called ‘stationary phase response’ is probably one of the most important factors determining the survival by bacteria in conditions of extreme stress. Cells with a recent history of starvation or slow growth are better poised to survive the harsh conditions imposed to preserve foods. In this brief review selected aspects of the survival and growth of bacteria under conditions of low water activity are discussed.