Microbial growth on the edge of desiccation

Summary The availability of water, which can be expressed in terms of water activity ( a w ), is one of the most important determinants for microbial homeostasis and growth on surface to air interfaces. Here we show, using an environmental control chamber containing a precisely controlled temperature/ a w gradient in combination with a mathematical approach, that the environmental a w growth limit of a microorganism can be lower than its intracellular a w limit. This internal limit represents the point at which microbial cells cannot lower their internal a w any further in response to low external a w values without interfering with essential intracellular processes. To grow at external a w values below their internal limit, microbes need to generate more water metabolically than they lose to their environment. This internal a w limit can be calculated by measuring the a w growth limit of an organism at different water vapour diffusivities using barometric pressure as a variable. Fascinating morphological changes, such as rope‐like superstructures formed by B. subtilis , are furthermore observed in response to low external a w values in particular around the calculated intracellular a w limit. The intracellular a w limit of an organism is a decisive parameter for water limitation‐induced adaptations in cellular hydrophilicity and morphogenesis.