Intermittent flow paths in biofilms grown in a microfluidic channel

Biofilms, complex microbial communities embedded in an extracellular matrix, are significantly influenced by flow-induced shear stress, which creates a competition between biofilm growth and detachment. In this study, biofilms of Pseudomonas fluorescens were grown in a microfluidic channel and exposed to aqueous flow which includes nutrients at varying velocities. Real-time observations using transmitted-light microscopy coupled with a camera revealed that biofilms can adapt to their conditions and grow accordingly. In some cases, intermittent flow-path regimes emerged, maintaining a dynamic balance with biofilm growth. This balance was observed within certain flow velocity ranges, corresponding shear forces, nutrient availability, and biofilm cohesiveness.

• At very low nutrient velocities, biofilm growth was inhibited due to nutrient limitations. However, when nutrient concentration was increased, growth occurred briefly without intermittency, likely because the biofilm adapted to low-shear conditions by forming a highly permeable and porous structure. 
• When the mean velocity was sufficiently high for a given nutrient concentration, biofilm growth resumed. Under these conditions, the biofilm adapted to the challenging environment, withstanding shear forces and enabling the formation of intermittent flow paths.
• Adding pore bodies to the flow channel introduced regions of lower shear stress. The biofilm adapted to these low-shear conditions, and grow in the pore bodies but could not survive in the channel, highlighting its adaptability to varying shear environments. 
• As the mean velocity of nutrient flow increased further, the frequency of flow paths initially rose but eventually disrupted the dynamic balance by exceeding the critical shear stress. This led to higher detachment rates and ultimately inhibited biofilm growth.

As a result, the intermittent flow-path regime, in dynamic balance with biofilm growth, is defined within specific ranges of flow velocity, nutrient availability, and the ratio of shear stress to the biofilm’s ability to resist these forces, which we also confirm by comparison to a numerical model.