Motion Anisotropy of Individual, Unconfined Rod-Like-Shaped Salmonella Typhimurium Bacteria

With the continuous advancement of microscopic imaging techniques, the motion of micro- and nanosized particles can be followed with increasing spatial and temporal accuracy. Analyzing the tracks of single particles enables one to address a broad range of scientific questions, e.g., the dynamics of active and passive (thermally driven) motion or the transport of particles in complex environments. While anisotropic motion, which is exhibited by the vast majority of biologically relevant particles (e.g., bacteria and viruses), has been amply studied in theoretical works, only little experimental data with single-particle resolution have been reported so far. In this study, fluorescence microscopy is used to follow the unconstrained motion of green-fluorescent protein (GFP)-labeled Salmonella enterica serovar Typhimurium (S. Typhimurium) in bulk. By decomposing the motion of individual bacteria into contributions parallel and perpendicular to the bacterial long axis, we introduce new scores that quantify the degree of motile activity and alignment with high throughput. We find that S. Typhimurium possesses a surprisingly broad spectrum of opening angles that range from full alignment of the bacterial long axis with the direction of translational motion to no alignment at all. Applied to mutants, in which motility-related proteins (the flagellins FljB and FliC) have been deleted, we find that wild-type (WT) and FliC-expressing S. Typhimurium exhibit very similar motility but differ significantly in the fraction of actively moving bacteria.