Mycobacterium smegmatis expands across surfaces using hydraulic sliding

Mycobacterium smegmatis spreads over soft agar surfaces by sliding motility, a form of passive motility in which growth and reduction of surface adhesion enable the bacteria to push each other outwards. Hence, sliding motility is mostly associated with round colonies. However, M. smegmatis sliding colonies can also produce long, pointed dendrites. Round sliding colonies were readily reproduced, but our non-round colonies were different from those seen previously. The latter (named digitate colonies) had centimetre-long linear protrusions, containing a central channel filled with a free-flowing suspension of M. smegmatis and solid aggregates. Digitate colonies had both a surface pellicle and an inner biofilm component surrounding a central channel, which sat in a cleft in the agar. Time-lapse microscopy showed that the expansion of the fluid-filled channel enabled the lengthwise extension of the protrusions without any perceptible growth of the bacteria taking place. These observations represent a novel type of sliding motility, named hydraulic sliding, associated with a specialised colony structure and the apparent generation of force by expansion of a liquid core. As this structure requires pellicle formation without an initial liquid culture it implies the presence of an unstudied mycobacterial behaviour that may be important for colonisation and virulence. Originality-Significance StatementThis study is the first to identify a new form of passive motility in the mycobacteria; hydraulic sliding, in which liquid expansion is the cause of motility. This form of motility has so far never been described in bacteria. The study also reveals new ways mycobacteria can form biofilms and colonize complex three-dimensional substrates, aspects of mycobacterial biology that are important for infection, pathogenesis and vaccine development. Author SummaryMycobacterium smegmatis is used as a non-pathogenic model organism for pathogenic mycobacteria. During growth, M. smegmatis can move passively over soft agar surfaces by a process called sliding motility, whereby colony growth directly pushes cells outwards. Although passive, sliding motility is believed to be important in allowing bacteria to colonise surfaces. Sliding motility however does not fully account for how M. smegmatis produces dendritic colonies. We attempted to generate dendritic colonies but found instead that the cells produced colonies that had larger protrusions radiating from them (digitate colonies). Digitate colonies are a previously unobserved phenomenon, in that the bacteria create a biofilm-lined, fluid-filled, pellicle-covered, deep cleft in the agar and move across the surface by the expansion of the contained liquid core of the protrusions. Given the new structure and the new mechanism of expansion we have termed this set of behaviours hydraulic sliding. These observations are important as it is a new variation in the way bacteria can move, generate biofilms (notably mycobacterial pellicle) and colonize complex three-dimensional substrates.