An expanding bacterial colony forms a depletion zone with growing droplets

Area of Science:

• Microbiology
• Biophysics
• Fluid Dynamics

Background:

• Bacteria spread on surfaces using various mechanisms.
• Pseudomonas aeruginosa produces rhamnolipids, a type of biosurfactant.
• Colony expansion involves complex physical and biological processes.

Purpose of the Study:

• To investigate the expansion dynamics of Pseudomonas aeruginosa on agar gels.
• To understand the role of rhamnolipids in colony morphology and spread.
• To elucidate the fluid dynamic behaviors within bacterial colonies.

Main Methods:

• Observing Pseudomonas aeruginosa colony expansion on agar gel with minimal evaporation.
• Analyzing the formation and spread of a depletion zone.
• Quantifying rhamnolipid concentration within the colony.
• Investigating the effect of adding rhamnolipids on depletion zone expansion.
• Studying expansion on surfaces with killed bacteria.

Main Results:

• A depletion zone, characterized by a thinner bacterial film, was observed within expanding colonies.
• This depletion zone was colocalized with high concentrations of rhamnolipids.
• Rhamnolipids were found to drive the expansion of depletion zones, even on surfaces with dead bacteria.
• Bacterial growth, secretion, osmotic swelling, and fluid dynamics (Marangoni and capillary flows) contribute to observed phenomena.

Conclusions:

• Rhamnolipids play a crucial role in the fluid dynamics and expansion of Pseudomonas aeruginosa colonies.
• Depletion zones are a key feature of colony expansion, driven by biosurfactant activity.
• The study provides insights into bacterial surface motility and biofilm formation mechanisms.