ISS: The Influence of Microgravity on Bacterial Transport and Pellicle Morphogenesis

This award investigates the formation and growth of pellicles, which are membrane-like bacterial communities that accumulate at liquid interfaces. Pellicles are found in many settings and contribute to food spoilage, hospital infections, and environmental contamination. Pellicles formed at air/water interfaces exhibit distinct properties and formation dynamics compared to biofilms formed on solid interfaces, e.g., they undergo complex changes in shape over time and are strongly influenced by oxygen availability. Experiments in microgravity provide a unique opportunity to investigate two unexplored aspects of pellicle growth: (1) the role of gravity-driven mechanisms for bacteria and oxygen transport near an interface, and (2) growth at a spherical interface. Studying pellicle growth patterns at spherical interfaces can yield insights into how soft living materials behave, which is valuable for tissue engineering and biomaterials. By understanding the transport mechanisms underlying pellicle growth, pellicles can be controlled for purposes such as food preservation, infection control, and environmental remediation. Experiments will be conducted using the Ring Shear Drop module aboard the International Space Station, which enables deployment of centimeter-scale drops constrained by surface tension. Drops in microgravity can be used as container-less reactors for studying phenomena at air/liquid interfaces. Two facets of pellicle formation will be investigated: (1) initial transport of bacteria and oxygen to the interface in the absence of gravity-driven convection and sedimentation over short timescales (2-6 hours), and (2) morphogenesis of pellicles at a spherical drop interface over long timescales (48 hours). Numerical modeling will supplement experiments to decipher the individual contributions of specific transport processes responsible for pellicle morphogenesis. By comparing results from microgravity experiments to Earth-based controls, the effects of gravity on pellicle formation can be distinguished. Microgravity is known to significantly affect bacterial biofilms at solid surfaces, and it is expected that microgravity will exert an even greater influence on pellicle growth at elastic fluid interfaces. Exploring this influence will enable valuable insights into mechanisms responsible for initial adhesion, growth, and proliferation of pellicles. This award will also characterize pellicle morphogenesis at the drop air/water interface, focusing on the out-of-plane buckling transitions that occur due to interfacial instabilities and compressive stresses generated by growth. Pellicle dynamics on three-dimensional curved shapes with approximately spherical boundaries can serve as a model system that mimics natural transitions observed in soft living materials. Because microgravity enables deployment of centimeter scale droplets, this award will be the first to observe pellicle morphogenesis at spherical interfaces. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.