Fitness Costs and Benefits of Resistance to Phage Lambda in Experimentally Evolved Escherichia coli*

Fitness tradeoffs play important roles in the evolution of organisms and communities. One such tradeoff often occurs when bacteria become resistant to phage at the cost of reduced competitiveness for resources. Quantifying the cost of phage resistance has frequently relied on measuring specific traits of interest to industrial applications. In an evolutionary context, however, fitness encompasses the effects of all traits relevant to an organism’s survival and reproductive success in a particular environment. Therefore, measurements of the net reproduction and survival of alternative genotypes offer greater power for predicting the fate of different genotypes in complex and dynamic communities. In this study, we measured the fitness of experimentally evolved, λ-resistant Escherichia coli isolates relative to their sensitive progenitor in both the absence and presence of phage. We also characterized certain phage-related phenotypes and obtained complete genome sequences of the bacteria. All of the evolved bacteria exhibited tradeoffs, such that they were more fit than the ancestor in the presence of phage, but less fit than the ancestor in the absence of phage. The fitness benefit of evolved resistance in the presence of abundant phage was generally much larger than the cost, and these effects appear to be driven by only a few resistance mutations. This asymmetrical benefit-tocost relation is consistent with the observation that sensitive cells did not persist in the experimental communities. Quantifying fitness effects in both the presence and absence of phage may thus provide a useful approach for predicting evolutionary outcomes in both natural and engineered microbial communities.