Individual-Level Bet Hedging in the Bacterium Sinorhizobium meliloti

The expression of phenotypic variability can enhance geometric mean fitness and act as a bet-hedging strategy in unpredictable environments [1]. Metazoan bet hedging usually involves phenotypic diversification among an individual's offspring [2-6], 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 [7-10]. 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 [11]. 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.