Experimental evolution of cell shape in bacteria

Cell shape is a fundamental property in bacterial kingdom. MreB is a protein that determines rod-like shape, and its deletion is generally lethal. Here, we deleted the mreB homolog from rod-shaped bacterium Pseudomonas fluorescens SBW25 and found that ΔmreB cells are viable, spherical cells with a 20% reduction in competitive fitness and high variability in cell size. We show that cell death, correlated with increased levels of elongation asymmetry between sister cells, accounts for the large fitness reduction. After a thousand generations in rich media, the fitness of evolved ΔmreB lines was restored to ancestral levels and cells regained symmetry and ancestral size, while maintaining spherical shape. Using population sequencing, we identified pbp1A, coding for a protein involved in cell wall synthesis, as the primary target for compensatory mutations of the ΔmreB genotype. Our findings suggest that reducing elongasome associated PBPs aids in the production of symmetric cells when MreB is absent