Rheological transitions from bacterial suspensions to biofilms: A perspective for space environments

Human crews aboard the International Space Station (ISS) rely on cutting-edge technologies for resource reuse and system operation units. However, despite this great achievement, ISS maintenance still relies on Earth proximity, which continuously supports the station and provides help in the case of systems failures. For future missions beyond Earth orbit, such support will not be feasible, and any malfunction at the life support systems must be anticipated and proactively prevented. One of the recurring problems aboard the ISS is the uncontrolled growth of a three-dimensional structure made of micro-organism suspended in an extracellular polysaccharide matrix, known as a biofilm. This phenomenon has caused the biofouling and contamination of valves, filters, and reservoirs, with biofilm growth at solid–liquid interfaces progressively clogging components and impairing fluid circulation. Microgravity plays a not well-understood role on bacteria growth and biofilm morphology under flow. From a rheological point of view, bacterial biofilms are viscoelastic active matter with dynamic properties, meaning that they can adapt their structure in response to external stresses. In this framework, understanding their rheological features is fundamental to link their mechanical response to the resilience often mentioned in the literature. In this Perspective, we first discuss the rheology of bacterial suspensions and how gravity can have an influence, highlighting activity-driven viscosity changes and motility-induced structuring. Then, we explore the mechanical development of biofilms, considering how adhesion, extracellular matrix production, and flow interact to shape viscoelastic properties. Particular attention is given to the influence of gravity on structural organization. Finally, we summarize current rheological models of biofilms, distinguishing between different kinds of biofilm structures; identify how rheology can provide useful tools to understand biofilm mechanical properties; and try to mitigate this phenomenon both on space and on Earth.