Ecological memory preserves phage resistance mechanisms in bacteria

Area of Science:

• Evolutionary biology
• Microbial ecology
• Molecular biophysics

Background:

• Bacterial resistance to bacteriophage (phage) infections, including CRISPR-mediated immunity, incurs growth costs.
• These costly defenses can be lost when phage are absent, posing an evolutionary challenge for maintaining immunity.
• Variable pathogen levels and competition between bacterial strains complicate the persistence of these defenses.

Purpose of the Study:

• To investigate how bacteria maintain molecular defenses against phages despite associated costs.
• To understand the evolutionary stability of defense mechanisms under fluctuating pathogen pressures.
• To explore the role of ecological dynamics in the maintenance of bacterial immunity.

Main Methods:

• Developed a multilevel model integrating molecular binding biophysics, host-pathogen population dynamics, and ecological dynamics.
• Employed game theory and non-linear dynamical systems analysis.
• Simulated interactions across numerous independent ecological territories.

Main Results:

• A non-zero defense failure rate is crucial for maintaining sufficient pathogen levels.
• This strategy selects against the loss of costly defenses.
• The 'resistance switching' strategy is evolutionarily stable and prevents 'cheater' strains from dominating.

Conclusions:

• Bacterial defense mechanisms can be maintained through a dynamic balance with pathogens, not just complete elimination.
• Resistance switching is a key evolutionary strategy that sustains host-pathogen interactions and co-evolution.
• This model provides a framework for understanding the persistence of costly traits in variable environments.