  Bacteria exist in a free-swimming state or in a sessile biofilm state. The
transition from free-swimming to sessile mode is characterized by changes in
gene expression which alter, among others,the mechanics of flagellar motility.
In this paper, we propose an idealized physical model to investigate the
effects of flagellar activity on the hydrodynamic interactions among a
population of microswimmers. We show that decreasing flagellar activity induces
a hydrodynamically-triggered transition in confined microswimmers from
turbulent-like swimming to aggregation and clustering. These results suggest
that the interplay between flagellar activity and hydrodynamic interactions
provides a physical mechanism for coordinating collective behaviors in confined
bacteria, with potentially profound implications on biofilm initiation.
