PT - JOURNAL ARTICLE AU - Gabriel Rosser AU - Ruth E. Baker AU - Judith P. Armitage AU - Alexander G. Fletcher TI - Modelling and analysis of bacterial tracks suggest an active reorientation mechanism in <em>Rhodobacter sphaeroides</em> AID - 10.1101/001917 DP - 2014 Jan 01 TA - bioRxiv PG - 001917 4099 - http://biorxiv.org/content/early/2014/01/19/001917.short 4100 - http://biorxiv.org/content/early/2014/01/19/001917.full AB - Most free-swimming bacteria move in approximately straight lines, interspersed with random reorientation phases. A key open question concerns varying mechanisms by which reorientation occurs. We combine mathematical modelling with analysis of a large tracking dataset to study the poorly-understood reorientation mechanism in the monoflagellate species Rhodobacter sphaeroides. The flagellum on this species rotates counterclockwise to propel the bacterium, periodically ceasing rotation to enable reorientation. When rotation restarts the cell body usually points in a new direction. It has been assumed that the new direction is simply the result of Brownian rotation. We consider three variants of a self-propelled particle model of bacterial motility. The first considers rotational diffusion only, corresponding to a non-chemotactic mutant strain. A further two models also include stochastic reorientations, describing ‘run-and-tumble’ motility. We derive expressions for key summary statistics and simulate each model using a stochastic computational algorithm. We also discuss the effect of cell geometry on rotational diffusion. Working with a previously published tracking dataset, we compare predictions of the models with data on individual stopping events in R. sphaeroides. This provides strong evidence that this species undergoes some form of active reorientation rather than simple reorientation by Brownian rotation.