Mycobacterium smegmatis expands across surfaces by hydraulic sliding

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.