[Paper Review] Local polar order controls mechanical stress and triggers layer formation in developing Myxococcus xanthus colonies

Endao Han, Chenyi Fei+5 more|PubMed|Aug 1, 2023

Micro and Nano Robotics37 references4 citations

TL;DR

This study reveals that local polar order in Myxococcus xanthus colonies, driven by stochastic cell reversals, generates anomalous stress fluctuations that trigger mechanical layering and fruiting body formation. By tuning reversal frequency in response to starvation, cells control polar order to switch from monolayer spreading to 3D multicellular development.

ABSTRACT

Colonies of the social bacterium <i>Myxococcus xanthus</i> go through a morphological transition from a thin colony of cells to three-dimensional droplet-like fruiting bodies as a strategy to survive starvation. The biological pathways that control the decision to form a fruiting body have been studied extensively. However, the mechanical events that trigger the creation of multiple cell layers and give rise to droplet formation remain poorly understood. By measuring cell orientation, velocity, polarity, and force with cell-scale resolution, we reveal a stochastic local polar order in addition to the more obvious nematic order. Average cell velocity and active force at topological defects agree with predictions from active nematic theory, but their fluctuations are anomalously large due to polar active forces generated by the self-propelled rod-shaped cells. We find that M. xanthus cells adjust their reversal frequency to tune the magnitude of this local polar order, which in turn controls the mechanical stresses and triggers layer formation in the colonies.

Motivation & Objective

• To understand the mechanical mechanisms triggering multicellular layer formation in Myxococcus xanthus during starvation-induced development.
• To investigate the role of local polar order—beyond nematic order—in generating mechanical stresses and collective dynamics.
• To determine how cellular reversal frequency modulates polar order and thereby controls morphological transitions in bacterial colonies.
• To quantify the impact of polar order on traction forces, velocity fluctuations, and defect-mediated cell fluxes in active nematic systems.

Proposed method

• Imaged live M. xanthus colonies on agar surfaces using time-lapse bright-field and fluorescence microscopy to track cell orientation, velocity, and polarity.
• Mapped topological defects (±1/2) in the nematic order parameter field using cell orientation data and analyzed their role in cell flux and accumulation.
• Measured cell traction forces with high spatial and temporal resolution using a deformable substrate and image correlation techniques.
• Compared reversing wild-type and non-reversing ΔfrzE mutant strains to isolate the effect of reversal frequency on polar order and mechanical stress.
• Calculated temporal autocorrelation functions of cell polarity (p) and velocity (v) to quantify correlation times and infer local polar order strength.
• Analyzed power spectral densities (PSDs) of traction forces to identify distinct power-law behaviors (f⁻¹.¹ vs. f⁻²) in reversing vs. non-reversing populations.

Experimental results

Research questions

• RQ1 How does local polar order contribute to mechanical stress fluctuations and layer formation in M. xanthus colonies?
• RQ2 What is the role of cell reversal frequency in tuning local polar order and triggering multicellular morphogenesis?
• RQ3 How do polar and nematic order coexist and interact in shaping collective cell dynamics and defect-mediated flows?
• RQ4 Why do non-reversing mutants exhibit enhanced layer formation despite similar speeds to reversing cells?
• RQ5 What is the origin of the distinct power-law scaling in traction force spectra in reversing versus non-reversing colonies?

Key findings

• Non-reversing ΔfrzE mutants formed significantly more multi-layer regions than reversing wild-type cells under nutrient-rich conditions, indicating that long reversal times enhance local polar order.
• The correlation times of cell polarity (τₚ) and velocity (τᵥ) increased from ~0.8 min in reversing cells to ~3.7 min in non-reversing cells, confirming stronger local polar order in the latter.
• Despite similar average speeds, non-reversing colonies generated more persistent flows and stronger mechanical stress fluctuations, with traction force distributions showing longer tails at low frequencies.
• Power spectral density (PSD) analysis revealed that reversing cells exhibited a f⁻¹.¹ power law, while non-reversing cells approached f⁻² at low frequencies, indicating enhanced long-wavelength stress fluctuations.
• Topological defects of +1/2 charge induced net cell influx, promoting layer formation, while -1/2 defects caused cell outflux and hole formation, consistent with active nematic theory.
• The study identifies cell reversal time as a tunable biological knob that controls polar order, mechanical stress, and morphological transitions from monolayers to 3D fruiting bodies.