UC San Diego

Biofilms are ubiquitous in nature, having dense spatio-temporal structures containing various types of social interactions between different microbial communities. In this study, we use colony growth of E. coli cells on hard agar as a model system for biofilm dynamics. Employing a combination of experimental and modeling techniques, we first quantify the growth characteristics of an E. coli colony starting from a single cell. The agent-based simulations capture spatio-temporal dynamics of important metabolites, with the gradients of these metabolites suggesting physiological differentiation occurring across the colony. To further mimic natural biofilms, we then implemented trophic interactions between a pair of E. coli strains. Confocal microscopy images of a producer-consumer cross-feeding colony show the formation of unique flower-like patterns with the consumer enveloping the producer at the colony frontier. Experimental measurements reveal that the consumer can compensate for its lower growth rate, compared to the producer, with a lower Monod constant, allowing it to coexist with the producer in the initial phase. We find that as colony growth progresses, the consumer, despite having a slower radial expansion rate on one nutrient, can keep up with the producer at the frontier by employing a simultaneous nutrient utilization strategy as opposed to hierarchical for the producer. In the final regime of colony growth, while the producer slows down because of nutrient depletion, the consumer continues to grow, eventually forming the patterns. In conclusion, this work demonstrates the role of physiological diversity and community interactions on bacterial colony growth.