wet active matter [@Koch2011; @Saintillan2013].

Collective motion in bacteria has been extensively studied in dilute [@Soni2003; @Dombrowski2004; @Gachelin2014] and dense [@Mendelson1999; @Wu2006; @Sokolov2007; @Sokolov2009; @Cisneros2011; @Sokolov2012; @Wensink2012a; @Dunkel2013] suspensions. These studies reveal the following sequence of dynamical states. At very low densities, bacterial suspensions appear featureless and disordered [@Wu2006; @Gachelin2014]. At higher, yet still sufficiently low densities, collective motion sets in on the scale of the system. In this state, bacterial motion takes the form of large-scale jets and vortices with typical speeds that are larger than the swimming speeds of individual organisms [@Soni2003; @Dombrowski2004; @Gachelin2014]. At significantly higher densities, there emerges a typical lengthscale of the vortices, which is comparable to about $5-10$ times the bacterial size [@Sokolov2012; @Dunkel2013; @Ryan2013]. Although this sequence of dynamical states has never been simultaneously observed in a single systematic bulk experiment, with the exception of Sokolov *et al.* [@Sokolov2009], the transition scenario is supported by computer simulations of self-propelled particles interacting through various forms of long-ranged hydrodynamic fields and short-ranged steric repulsion [@Hernandez-Ortiz2005; @Saintillan2007; @Wolgemuth2008; @Underhill2008; @Hernandez-Ortiz2009; @Lushi2013; @Lushi2014; @Krishnamurthy2015; @Wioland2016; @Saintillan2012; @Stenhammar2017; @Theillard2017; @Schwarzendahl2018; @Bardfalvy2019; @Theillard2019].

Bulk experiments with *E.coli* [@Gachelin2014] and *B.subtilis* [@Dombrowski2004] show that the transition to collective motion occurs 