Individual-Level Bet Hedging in the Bacterium Sinorhizobium meliloti

Abstract

The expression of phenotypic variability can enhance geometric mean fitness and act as a bet-hedging strategy in unpredictable environments. Metazoan bet hedging usually involves phenotypic diversification among an individual's offspring, such as differences in seed dormancy. Virtually all known microbial bet-hedging strategies, in contrast, rely on low-probability stochastic switching of a heritable phenotype by individual cells in a clonal group. This is less effective at generating within-group diversity when group size is small. Here we describe a novel microbial bet-hedging behavior that resembles individual-level metazoan bet hedging. Sinorhizobium meliloti stores carbon and energy in poly-3-hydroxybutyrate (PHB) as a contingency against carbon scarcity. We show that, when starved, dividing S. meliloti bet hedge by forming two daughter cells with different phenotypes. These have high and low PHB levels and are suited to long- and short-term starvation, respectively. The low-PHB cells have greater competitiveness for resources, whereas the high-PHB cells can survive for over a year without food, perhaps until a legume host is next available.