Evolution of Penicillin-Binding Protein 2 Concentration and Cell Shape during a Long-Term Experiment with Escherichia coli

Peptidoglycan is the major component of the bacterial cell wall and is involved in osmotic protection and in determining cell shape. Cell shape potentially influences many processes, including nutrient uptake as well as cell survival and growth. Peptidoglycan is a dynamic structure that changes during the growth cycle. Penicillin-binding proteins (PBPs) catalyze the final stages of peptidoglycan synthesis. Although PBPs are biochemically and physiologically well characterized, their broader effects, especially their effects on organismal fitness, are not well understood. In a long-term experiment, 12 populations of Escherichia coli having a common ancestor were allowed to evolve for more than 40,000 generations in a defined environment. We previously identified mutations in the pbpA operon in one-half of these populations; this operon encodes PBP2 and RodA proteins that are involved in cell wall elongation. In this study, we characterized the effects of two of these mutations on competitive fitness and other phenotypes. By constructing and performing competition experiments with strains that are isogenic except for the pbpA alleles, we showed that both mutations that evolved were beneficial in the environment used for the long-term experiment and that these mutations caused parallel phenotypic changes. In particular, they reduced the cellular concentration of PBP2, thereby generating spherical cells with an increased volume. In contrast to their fitness-enhancing effect in the environment where they evolved, both mutations decreased cellular resistance to osmotic stress. Moreover, one mutation reduced fitness during prolonged stationary phase. Therefore, alteration of the PBP2 concentration contributed to physiological trade-offs and ecological specialization during experimental evolution.