Staphylococcus aureus forms spreading dendrites that have characteristics of active motility

Staphylococcus aureus is historically regarded as a non-motile organism. More recently it has been shown that S. aureus can passively move across agar surfaces in a process called spreading. We re-analysed spreading motility using a modified assay and focused on observing the formation of dendrites: branching structures that emerge from the central colony. We discovered that S. aureus can spread across the surface of media in structures that we term 'comets', which advance outwards and precede the formation of dendrites. We observed comets in a diverse selection of S. aureus isolates and they exhibit the following behaviours: (1) They consist of phenotypically distinct cores of cells that move forward and seed other S. aureus cells behind them forming a comet 'tail'; (2) they move when other cells in the comet tail have stopped moving; (3) the comet core is held together by a matrix of slime; and (4) the comets etch trails in the agar as they move forwards. Comets are not consistent with spreading motility or other forms of passive motility. Comet behaviour does share many similarities with a form of active motility known as gliding. Our observations therefore suggest that S. aureus is actively motile under certain conditions.