# Mycobacterium smegmatis Expands Across Surfaces by Hydraulic Sliding

## TL;DR

Mycobacterium smegmatis spreads across surfaces using a new type of passive movement called hydraulic sliding, where fluid expansion propels the bacteria forward.

## Contribution

The study introduces hydraulic sliding, a novel form of passive motility in mycobacteria driven by liquid expansion.

## Key findings

- Digitate colonies of M. smegmatis feature centimeter-long protrusions with a fluid-filled central channel.
- Expansion of the fluid core drives protrusion extension without visible bacterial growth.
- A pellicle forms without a liquid-air interface, suggesting new mycobacterial behaviors for colonization and virulence.

## Abstract

Passive sliding motility allows
Mycobacterium smegmatis
to spread over soft agar surfaces. Usually, bacterial growth and reduced surface adhesion push individual bacteria outwards, resulting in circular colonies; however, more complex, dendritic colonies have also been reported. Although we could readily reproduce the circular morphotype, our non‐circular colonies (hereafter digitate colonies) differed from dendritic colonies. Digitate colonies were characterised by centimetre‐long, linear protrusions consisting of surface pellicle and inner biofilm components surrounding a central channel that was filled with a free‐flowing suspension of
M. smegmatis
and aggregates. Time‐lapse microscopy showed that the expansion of the fluid‐filled channel resulted in lengthwise extension of the protrusions without any perceptible bacterial growth. These observations reveal a novel type of sliding motility (named hydraulic sliding) associated with a distinct colony structure and the apparent generation of force by expansion of a liquid core. The presence of a pellicle (a floating biofilm) generated without an initial liquid‐air interface suggests that a previously unknown mycobacterial behaviour that could be important for colonisation and virulence has been discovered.

We present a new form of passive motility in mycobacteria, named hydraulic sliding, in which liquid expansion is the cause of motility. This study also reveals new ways mycobacteria can form biofilms and colonise 3D substrates, aspects of mycobacterial biology that are important for understanding infection, pathogenesis and vaccine development.