Growth consequences of the inhomogeneous organization of the bacterial cytoplasm

In many bacteria, translating ribosomes are excluded from the nucleoid, while amino-acid and energy-supplying metabolic enzymes spread evenly throughout the cytoplasm. Here we show with time-lapse fluorescence microscopy that this inhomogeneous organisation of the cytoplasm can cause single Escherichia coli cells to experience an imbalance between biosynthesis and metabolism when they divide, resulting in cell size-dependent growth rate perturbations. After division, specific growth rate and ribosome concentration correlates negatively with birthsize, and positively with each other. These deviations are compensated during the cell-cycle, but smaller-than-average cells do so with qualitatively different dynamics than larger-thanaverage cells. A mathematical model of cell growth, division and regulation of biosynthetic and metabolic resource allocation reproduces our experimental findings, suggesting a simple mechanism through which long-term growth rate homeostasis is maintained while heterogeneity is continuously generated. This work shows that the life of single bacterial cells is intrinsically out-of-steady-state, dynamic and reliant on cytoplasmic organization. Popular summaryClassical, population-level studies of the metabolism and growth of bacteria indicate that the average cell in a growing population operates at steady state and can be viewed as an homogeneous bag of enzymes. Here we show that this view does not capture the lives of single cells. At birth, they are perturbed from the steady state of their mother cell after which they need their entire cell cycle to return to this state by active regulation. Then they divide and their daughters are perturbed again; a never ending cycle that is inescapable and akin to a Sisyphean task. This behaviouremerges from the delicate interplay of the intrinsic randomness of (uneven) cell division, the inhomogeneous localisation of metabolic and ribosomal proteins in the cell, unbalanced metabolism, and compensatory steering of gene expression.