Kuhn, A.; Krueger, T.; Schuettler, M.; Engstler, M.; Fischer, S. C.

In vitro colonies of the flagellated parasite Trypanosoma brucei grow in characteristic fingering instability patterns. The underlying cause of this this type of collective migration and the associated behavior called social motility remains a topic of intense debate in the scientific community. In order to study these mechanisms in detail, it is crucial to develop quantitative methods to assess and measure social motility patterns at the level of the whole colony beyond qualitative image comparisons. Here, we show a quantification of the growth process based on two scale free metrics designed to quantify the shape of two-dimensional colonies. While initially developed for yeast colonies, we adapted, modified and extended the analysis pipeline for the Trypanosoma system. Combining the quantitative measurements with colony growth simulations based on the Eden model, we discovered two distinct growth phases in social motility showing colonies: In the first phase, the colonies shown mainly circular expansion and later switch to an almost exclusive finger growing phase. These phases are robust when increasing the number of cells as well as upon partial inhibition of the finger formation. A newly developed anisotropy index shows that upon partial inhibition the anisotropy in the colony increases over time. Our results provide objective measurements that help advance the understanding of social motility of trypanosomes. Furthermore, our approach is suitable as a blueprint for investigations of other colony forming cells such as yeast or bacteria.