An Active Fluid Model of Expansion and Sporulation in Wild-isolate Bacterial Biofilms

Many species of bacteria, including Bacillus subtilis, can undergo a transition to a non-growing spore state in response to nutrient limitation. Spores have been observed to arise in biofilms late in colony development, after nutrients have been exhausted. However, we find that sporulation emerges early in biofilms formed by wild isolates of B. subtilis, coupling sporulation to the biofilm expansion process. Particularly, the wild strain forms smaller biofilms with matrix-producing cells at the edge and spores in the center, while the lab strain forms larger biofilms with mostly matrix-producing cells. To understand how wild-isolate cells produce structurally different biofilms, we develop an active fluid model with diffusing nutrients. The model incorporates two cell types – matrix-producing cells and spores that are connected by a sporulation rate. There is also a parameter that takes into account the effect of matrix production. Through varying these parameters and investigating traveling fronts of biofilm edge and spore activities, we've found that matrix production can affect biofilm sizes, while nutrient induced sporulation reproduces experimentally observed patterns with cells at the front and spores in the center. The model also shows how these parameters can affect biofilm expansion speed and spore fraction, which can be measured by experiments. In general, our model can be useful for understanding spatial pattern and expansion properties of growing media.