  Real-life bacteria often swim in complex fluids, but our understanding of the
interactions between bacteria and complex surroundings is still evolving. In
this work, rod-like \textit{Bacillus subtilis} swims in a quasi-2D environment
with aqueous liquid-liquid interfaces, i.e., the isotropic-nematic coexistence
phase of an aqueous chromonic liquid crystal. Focusing on the bacteria motion
near and at the liquid-liquid interfaces, we collect and quantify bacterial
trajectories ranging across the isotropic to the nematic phase. Despite its
small magnitude, the interfacial tension of the order of 10 $\mathrm{\mu N/m}$
at the isotropic-nematic interface justifies our observations that bacteria
swimming more perpendicular to the interface have a higher probability of
crossing the interface. Our force-balance model, considering the interfacial
tension, further predicts how the length and speed of the bacteria affect their
crossing behaviors. We also find, as soon as the bacteria cross the interface
and enter the nematic phase, they wiggle less, but faster, and that this occurs
as the flagellar bundles aggregate within the nematic phase.
