  We study the assembly of colloids in a two phase water-water system that
provides an environment that can sustain bacteria, providing a new structure
with rich potential to confine and structure living colloids. The water-water
system, formed via phase separation of a casein and xanthan mixture, forms a
3-D structure of coexisting casein-rich and xanthan-rich phases. Fluorescent
labelling and confocal microscopy reveal the attachment of these living
colloids, including Escherichia coli and Pseudomonas aeruginosa, at the
interface between the two phases. Inert colloids also become trapped at the
interfaces, suggesting that the observed attachment can be attributed to
capillarity. Over time, these structures coarsen and eventually degrade,
illustrating the dynamic nature of these systems. This system lays the
foundation for future studies of the interplay of physicochemical properties of
the fluid interfaces and bulk phases and microbial responses they provoke to
induce complex spatial organization, to study species which occupy distinct
niches, and to optimize efficient microbial cross-feeding or protection from
competitors.
