What limits biofilm growth on agar plates?

A common observation in microbiology is that the growth of bacterial biofilms on agar plates slows dramatically and seems to halt completely. The dramatic slowing of growth is typically attributed to nutrient depletion. However, a typical agar plate has more than enough nutrients to sustain colony growth across the entire plate, and a recent paper (Bravo, et al., PNAS 2023) demonstrated that vertical growth halts even when nutrients are not depleted. Thus, through detailed experiments, we aim to determine precisely what mechanisms cause colony growth to slow,  and to quantify the role of each of these factors in this phenomenon. We will consider the roles of multiple interacting factors, nutrient depletion, nutrient diffusion, and evaporation-induced drying of the agar, among others. The drying of agar is expected to result in multiple changes, from an increase in frictional forces between the colony and substrate to a decrease in nutrient diffusion rates to a decrease in the size of the agar pad itself. Although agar plates represent an artificial growth environment, understanding the limits of biofilm expansion under these conditions is important for interpreting a wide range of laboratory studies. Moreover, this knowledge provides ecological insight into how biofilm growth in natural settings may be constrained by factors other than nutrient availability in an environment.