A microfluidic investigation unveils the role of gravity and shear stress on Pseudomonas fluorescens motility and biofilm growth

Biofilms proliferation in confined environments poses a recurring challenge across several fields, ranging from contamination in biomedical devices to fouling of industrial equipment, to potential system loss of function in long term human space missions. It's crucial to note that surfaces in contact with the fluid, where submerged biofilm develops, experience different bulk stresses resulting from the combination of flow and gravity, a factor often overlooked in biofilm studies. In our research, we aim to quantify the synergistic effect of gravity and shear stress on monotrichous bacteria motility and biofilm growth, considering Pseudomonas fluorescens SBW25 as model organisms. Role of gravity was investigated by comparing top and bottom surfaces of rectangular microfluidic channels under controlled laminar flow. Results proved gravity induces asymmetric distribution of bacterial cells along the channel resulting in different cell density and surface contamination. We report for the first time also the evolution of cell distribution over time during spatial reorganization, providing a detailed quantitative analysis and classification of cell motility under flow. Both bacteria motility and biofilm morphology development are affected by external mechanical stresses, resulting in different biocontamination under flow, depending on flow intensity and direction of gravity vector.