Figure 1. Conditions of various bacterial growth models.

Metabolic heterogeneity arises in bacterial systems due to innate and stochastic effects. However, differentiation can be further promoted by chemical gradients and spatial constraints that result from endogenous structure formation or imposed architectures. In well-mixed liquid cultures (e.g., batch cultures or chemostats) cells with different metabolisms (represented by green and beige coloration) are evenly distributed throughout the suspension. In microfluidic devices such as the one depicted, medium flows across one side of the population, leading to chemical gradient formation and metabolic differentiation along the gradient. When bacterial aggregates are suspended in a provided matrix (such as an agar block or sputum in the lung of a patient with cystic fibrosis), gradients can be present in the external matrix, leading to the differentiation seen in the upper aggregate, but can also form inside aggregates due to the activities of the cells, as seen in the lower aggregate. In biofilms, cells reside in a self-produced matrix and promote gradient formation and thereby metabolic differentiation.