Coiled to diffuse: Brownian motion of a helical bacterium

Area of Science:

• Microbiology
• Biophysics
• Fluid Dynamics

Background:

• Spirochaete bacteria, like Leptospira interrogans (LI), possess unique helical shapes.
• Understanding bacterial motion and diffusion is crucial in microbiology and disease transmission.

Purpose of the Study:

• To quantify the translational and rotational diffusion coefficients of fixed, helically shaped Leptospira interrogans.
• To investigate the impact of the bacterium's helical structure on its Brownian motion.
• To compare experimental findings with theoretical models.

Main Methods:

• Real-time three-dimensional confocal microscopy was used to observe the Brownian motion of fixed Leptospira interrogans.
• Translational and rotational diffusion coefficients were extracted from the microscopy data.
• A simple theoretical model was developed and compared to experimental results.

Main Results:

• The helical coiling of Leptospira interrogans significantly increases its rotational diffusion coefficient by a factor of 5 compared to a hypothetical rectified bacterium.
• The helical shape decreases translational diffusion by approximately 1.5 times compared to a rectified bacterium.
• A simple theoretical model accurately reproduced the experimental diffusion coefficients without tunable parameters, outperforming older models that neglected edge effects.

Conclusions:

• The spiral shape of spirochaete bacteria, beyond its role in active motility, enhances passive exploration of the surrounding fluid via Brownian diffusion.
• The helical morphology is advantageous for bacterial dissemination and interaction within fluid environments.
• Theoretical modeling provides valuable insights into the biophysical properties of microorganisms.