Single-cell data reveal heterogeneity of resource allocation across a bacterial population

Ribosomes are responsible for the synthesis of proteins, the major component of cellular biomass. Classical experiments have established a linear relationship between the fraction of resources invested in ribosomal proteins and the rate of balanced growth of a microbial population. We extended the study of ribosomal resource allocation from populations to single cells, using a combination of time-lapse fluorescence microscopy and statistical inference. We found a large variability of ribosome concentrations and growth rates in conditions of balanced growth of the model bacterium Escherichia coli. Moreover, the ribosome concentrations and growth rates of individual cells are uncorrelated, contrary to what would be expected from the population-level growth law. A similar large heterogeneity was found during the transition of the bacteria from a poor to a rich growth medium. Whereas some cells immediately adapt ribosomal resource allocation to the new environment, others do so only gradually. Our results thus reveal distinct strategies for investing resources in the molecular machines at the heart of cellular self-replication. This raises the interesting question whether the observed variability is an intrinsic consequence of the stochastic nature of the underlying biochemical processes or whether it improves the fitness of Escherichia coli in its natural environment.