Selection for Cell Yield Does Not Reduce Overflow Metabolism in Escherichia coli

Area of Science:

• Microbial Physiology and Metabolism
• Evolutionary Biology
• Systems Biology

Background:

• Overflow metabolism is common in microbes, often linked to high growth rates but low biomass yield.
• This strategy is typically viewed as beneficial primarily under nutrient competition conditions.
• The evolutionary drivers and benefits of overflow metabolism in non-competitive environments remain less understood.

Purpose of the Study:

• To investigate the evolutionary consequences of overflow metabolism in Escherichia coli under conditions lacking nutrient competition.
• To determine if overflow metabolism confers a fitness advantage when cells are not competing for scarce resources.
• To explore the relationship between overflow metabolism, cell size, and numerical cell yield.

Main Methods:

• Experimental evolution of Escherichia coli using millions of parallel microfluidic batch cultures.
• Cultures were initiated from single cells in water-in-oil emulsion droplets.
• A mathematical model was employed to analyze experimental data and simulate metabolic outcomes.

Main Results:

• Overflow metabolism (acetate production) persisted in evolved E. coli populations even without nutrient competition.
• A significant increase in numerical cell yield was observed during the consumption of accumulated acetate.
• This enhanced yield resulted from a reduction in average cell size, not a change in overflow metabolism itself.

Conclusions:

• Overflow metabolism, coupled with subsequent overflow metabolite consumption, can enhance numerical cell yield and fitness.
• Reduced cell size is a key adaptation that optimizes numerical yield in this scenario.
• This study presents an evolutionary framework where overflow metabolism can be advantageous even in the absence of direct nutrient competition.