Microbial growth on the edge of desiccation

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.