The physiological origin of bacterial growth rate variations

Growth is imperative to microbial life as it drastically shapes cells, their surroundings, and their competition within microbial communities. Decades of work have elucidated how metabolic and biosynthetic processes, in concert, promote efficient and fast growth, with protein synthesis playing a pivotal role. However, the variation of growth rates observed across different environments remains puzzling. It is well known, for example, that the model organism E. coli grows fast on glucose, while growth on many other sugars and carbon sources is substantially slower. This is striking as carbon sources are often chemically very similar and thus require the utilization of similar metabolic pathways. To better understand the cell-physiological origin of this growth variation, we systematically quantified the growth behavior of different Escherichia coli strains, going well beyond the commonly studied K12 strains like MG1655. While the genomes of all strains are very similar, with the sequences of core metabolic and biosynthetic genes often exactly matching, their growth rate behavior varies strongly with no clear dependence on growth conditions. Glucose, for example, is not always a "good" carbon source promoting fast growth. Utilizing biochemical assays and mass-spec methods, we show that this variation is not due to differences in the overall ability of strains to promote protein synthesis and biomass accumulation. Instead, differences in growth emerge from the differential synthesis of a range of different proteins, which deplete resources away from the synthesis of those proteins required for growth.