illustration in insert, $N=5004$, average ) and bottom interface (right, schematic illustration in insert, $N=3702$, average ). Proportions of CCW and CW occurrences are displayed in blue and red at the top of each plot. (c) Corresponding chirality indices plotted against azimuthal velocity, near the top (blue) and bottom (red) interfaces. (d) Chiral bias of swimming near the top and bottom interfaces have opposite signs but different amplitudes, under asymmetric conditions.[]{data-label="fig3"}](figure3_7.jpg)

![**Chiral bacterial vortex is collective effect.** A dense ($20\% v/v$) bacterial suspension is confined under asymmetric conditions. Represented chirality indices against azimuthal velocity of (a) individual fluorescent bacteria (schematic illustration in insert) observed under confocal microscopy near the top (left) and bottom (right) interface, (b) fluid flow with tracer particles also observed under confocal microscopy, and (c) collective bacterial motion observed under bright field. Two microwell sizes were considered: $R = \SI{35}{\micro\meter}$ (small blue circles) and $R = \SI{50}{\micro\meter}$. In the larger microwells were considered two regions: the boundary layer within from the lateral boundary (larger blue diamonds), and the bulk that is more than away from the lateral wall (green disks). Sample size and average azimuthal velocity of each case can be found in Fig. S6.[]{data-label="fig4"}](figure4_7.jpg)

As the 