  Associative learning is one of the key mechanisms displayed by living
organisms in order to adapt to their changing environments. It was early
recognized to be a general trait of complex multicellular organisms but also
found in "simpler" ones. It has also been explored within synthetic biology
using molecular circuits that are directly inspired in neural network models of
conditioning. These designs involve complex wiring diagrams to be implemented
within one single cell and the presence of diverse molecular wires become a
challenge that might be very difficult to overcome. Here we present three
alternative circuit designs based on two-cell microbial consortia able to
properly display associative learning responses to two classes of stimuli and
displaying long and short-term memory (i. e. the association can be lost with
time). These designs might be a helpful approach for engineering the human gut
microbiome or even synthetic organoids, defining a new class of decision-making
biological circuits capable of memory and adaptation to changing conditions.
The potential implications and extensions are outlined.
