Single-cell level based approach to investigate bacterial metabolism during batch industrial fermentation

Escherichia coli fermentations have been studied for decades, but most results are based on average measurements of the whole populations of cells, whilst averaged data can mask the distribution of activities at the sub-population or single-cell level. A population of genetically identical cells can exhibit different phenotypes under specific environmental conditions that show significant differences in physiological parameters from the population average. However, studies concerning segregation of populations into metabolically diversified subpopulations are scarce.

Acetate is a product of Escherichia coli overflow metabolism when the bacteria are grown under aerobic conditions and glucose is present in excessive concentrations. Acetate accumulation is of the utmost importance in batch fermentation processes as it is an undesirable byproduct that negatively affects growth, physiology, and performance of Escherichia coli. An insight into glucose and acetate fate on the level of individual cell can provide the type of information which are valuable for the understanding of bacterial metabolism in fermentation process and can shed more light on the differentiation of isogenic fermenting populations into subpopulations expressing metabolically different profiles.

The goal of this study was to observe and quantify in situ metabolic response of Escherichia coli at single-cell level and determine activity distribution of glucose and acetate uptake during batch fermentation processes. Pure culture of Escherichia coli MG1655 was used to investigate glucose and acetate metabolism at single-cell level during different stages of glucose batch fermentation process. Uptake of the substrates was observed and measured in situ by quantitative microautoradiography.

Sub-populations of Escherichia coli cells expressing different activity levels for the uptake of glucose or acetate were observed. The distribution of these uptake activities changed along the batch fermentation process. The results based on the observation of single cells indicate that heterogeneity exists within bacterial populations and is a result of metabolic diversification of individual cells.