Single-Cell Dynamics Reveals Sustained Growth during Diauxic Shifts

Abstract

Stochasticity in gene regulation has been characterized extensively, but how it affects cellular growth and fitness is less clear. We study the growth of E. coli cells as they shift from glucose to lactose metabolism, which is characterized by an obligatory growth arrest in bulk experiments that is termed the lag phase. Here, we follow the growth dynamics of individual cells at minute-resolution using a single-cell assay in a microfluidic device during this shift, while also monitoring lac expression. Mirroring the bulk results, the majority of cells displays a growth arrest upon glucose exhaustion, and resume when triggered by stochastic lac expression events. However, a significant fraction of cells maintains a high rate of elongation and displays no detectable growth lag during the shift. This ability to suppress the growth lag should provide important selective advantages when nutrients are scarce. Trajectories of individual cells display a highly non-linear relation between lac expression and growth, with only a fraction of fully induced levels being sufficient for achieving near maximal growth. A stochastic molecular model together with measured dependencies between nutrient concentration, lac expression level, and growth accurately reproduces the observed switching distributions. The results show that a growth arrest is not obligatory in the classic diauxic shift, and underscore that regulatory stochasticity ought to be considered in terms of its impact on growth and survival.

Hasard et destinée cellulaire

“…Elles disposent de régula-tions génétiques comme l'opéron lactose, leur permettant d'exprimer les enzymes appropriées en fonction de la source de carbone disponible qu'elles rencontrent. Il a été observé chez E. coli [14,15] et Lactococcus lactis [16] qu'une fraction des cellules était capable (Figure 3). Une étude similaire de l'épiderme de souris montre une dynamique plus hiérarchique, entre un type prolifératif et un type différencié, mais toujours suivant des règles de transitions aléatoires [26].…”

Evolving strategies for single-celled organisms in multi-nutrient environments

“…New methods in single-cell observation now make it possible to see what happens in diauxic growth at the level of the individual cell. There is now evidence (Boulineau et al, 2013) that the growth phases and the lag phase at the level of the individual cell are not as well defined as they are at the population level. For example, throughout the (population level) lag phase, some of the individual cells continue to grow without delay while others cease growth for substantial amounts of time before resuming growth well into the second growth phase.…”

Single yeast cells vary in transcription activity not in delay time after a metabolic shift

“…The general interpretation is that a lag phase results from the time that it takes for the microorganism to adapt its metabolism. Several single-cell studies indicate that the cause for the lag phase can also involve stochasticity: either only a small fraction of the population grows under the new condition [2][3][4][5][6] , and the lag phase derives from the time that it takes until those cells reach a significant fraction of the total population; or cells start to grow only after a fluctuation in gene-expression activity has occurred that is large enough to kick-start the induction of the nutrient uptake pathway [7][8][9] . In both these cases, the lag phases of individual cells vary greatly.…”

“…In both these cases, the lag phases of individual cells vary greatly. The fact that many different adaptation mechanisms can occur may explain the great variation in lag phase durations that have been experimentally observed, which range from minutes to days [2][3][4]7,8 .…”

“…Then they have to rely on stochasticity-induced adaptation mechanisms. This applies for instance to E. coli with respect to lactose and for S. cerevisiae in the case of galactose 4,[7][8][9] . In such a case, the organism has to rely on leaky gene expression of dedicated permeases.…”