using PIVlab based on MATLAB software, and its grid size was $\times$. Acquired velocity fields were further smoothed by averaging over 1 sec.

In addition, polystyrene tracer particles with red fluorescence (Molecular probes) were used to track the flow field (Fig. 4(b) in main text). The tracer particles were dispersed at a low density of $0.026\% v/v$, where individual particles could be tracked inside the bacterial suspension. Recording of the trajectories of red-labeled bacteria and red tracer particles was done with a confocal microscope (IX73 inverted microscope from Olympus, and confocal scanning unit CSU-X1 from Yokogawa Electric Cor. Ltd., iXon-Ultra EM-CCD camera from Andor Technologies) under red fluorescence channel. All the recordings were done at 30 frames per second.

Experimental details
====================

Preponderance and stability of CCW bacterial vortex
---------------------------------------------------

![**Preponderance and stability of chiral bacterial vortex.** (a) A droplet of a dense bacterial suspension confined in a microwell ($R = \SI{20}{\micro\meter}$) between *asymmetric fluidic (top) and solid (bottom)* interfaces. (b) Control experiment for the effect of top/bottom interfaces on the directionality of the bacterial vortex. A droplets ($\SI{15}{\micro\meter}\leq R\leq\SI{35}{\micro\meter}$) of an emulsion of dense bacterial suspension in oil between *symmetric solid (top)/solid (bottom)* interfaces. (c) Distributions of vorticity averaged 