Evaporation controls contact-dependent bacterial killing during surface-associated growth

Area of Science:

• Microbiology
• Biophysics
• Ecology

Background:

• Bacteria utilize contact-dependent killing mechanisms, such as the type VI secretion system (T6SS), for competitive advantage.
• Physical forces and fluid dynamics can significantly influence microbial interactions and population dynamics.

Purpose of the Study:

• To investigate the role of evaporation-induced fluid flows, specifically the coffee ring effect (CRE), in modulating bacterial contact-dependent killing.
• To determine how the CRE influences the efficacy of the type VI secretion system (T6SS) in inter-bacterial competition.

Main Methods:

• Experimental manipulation of the coffee ring effect (CRE) strength in liquid droplets.
• Quantification of cell-cell contacts between attacking (Vibrio cholerae) and target (Escherichia coli) cells.
• Measurement of bacterial killing and competitive outcomes mediated by the type VI secretion system (T6SS).
• Individual-based computational simulations to model the impact of cell density and CRE on killing.

Main Results:

• The CRE significantly increases the number of contacts between attacking and target bacterial cells.
• Enhanced cell-cell contact due to the CRE leads to increased killing efficacy and competitive success of Vibrio cholerae over Escherichia coli.
• Computational simulations confirmed that higher cell densities at the droplet periphery, driven by the CRE, enhance bacterial killing.
• T6SS firing rate, lethal hit threshold, and lysis delay were identified as critical factors influencing killing under strong CRE conditions.

Conclusions:

• Evaporation-induced fluid flows, exemplified by the CRE, are crucial physical factors that shape bacterial interactions.
• The CRE enhances bacterial killing by increasing cell-cell contacts, thereby influencing competitive outcomes in microbial communities.
• Understanding these physical forces is essential for predicting and controlling bacterial competition and the efficacy of contact-dependent weapons.