Extracellular enzyme production and cheating in Pseudomonas fluorescens depend on diffusion rates

Area of Science:

• Microbial Ecology
• Evolutionary Biology
• Bacterial Genetics

Background:

• Bacteria secrete extracellular enzymes to degrade complex substrates for nutrient acquisition.
• This cooperative strategy is susceptible to exploitation by non-producing 'cheater' cells.
• Cheating can impact the stability and dynamics of microbial populations.

Purpose of the Study:

• To investigate the impact of bacterial cheating on extracellular enzyme production.
• To determine how environmental conditions (well-mixed vs. spatially structured) influence the prevalence of cheating.
• To understand the evolutionary dynamics between enzyme-producing and cheater bacterial genotypes.

Main Methods:

• Co-culturing of Pseudomonas fluorescens producer and cheater strains in liquid and on agar media.
• Monitoring of protease expression levels and genotype frequencies over time.
• DNA sequencing to identify genetic basis of cheating and adaptation.

Main Results:

• In well-mixed liquid cultures, protease production rapidly declined to zero, with cheaters dominating.
• Cheater mutants appeared to arise spontaneously in producer-only cultures, leading to population-wide cheating.
• Spatially structured conditions (agar plates) maintained a diversity of enzyme production strategies, preventing cheater sweeps.

Conclusions:

• Well-mixed environments favor the proliferation of bacterial cheater strains, leading to loss of public goods like enzymes.
• Spatially structured environments promote coexistence and diversification of cooperative and cheating strategies.
• Environmental diffusion rates likely influence the balance between cooperation and cheating in natural microbial communities.