Flower petal patterns formed by dense colony of cross-feeding bacteria growing on hard agar

Metabolic crossfeeding is widely observed in multi-species bacterial communities. To probe the consequences of such crossfeeding in the spatiotemporal context, we studied a colony comprising of two E. coli strains exhibiting a commensal interaction involving a “producer” which can grow on lactose provided in the agar but excretes galactose, and a “consumer” which can grow on galactose but not on lactose. Contrary to expectations formulated based on batch culture measurements, we observed the emergence of a flower petal pattern formed by the two strains in the colony after initial inoculation as a uniform mixture. Through a combination of experiments and simulations, we elucidated key elements of cell growth and metabolic exchanges driving the observed patterns. Further, our results highlight an important role played by colony expansion along the vertical dimension, which is often neglected in the analysis of spatiotemporal dynamics in compact, dense bacterial communities.