ISS: Biofilm growth and architecture in porous media: exploring the effect of gravitational and interfacial forces on biofilm growth patterns

Microbes are ubiquitous in nature and colonize our environments from shallow soils to the deep subsurface, the human body, and engineered systems. Biofilms are aggregates of microorganisms that stick to each other and often also to a surface. They are embedded within a slimy extracellular matrix known as polymeric substances.

Biofilms are alive and have a complex structure that scientists and engineers are still trying to understand. This complex structure protects the biofilms and allows them to thrive – there is strength in numbers, so survival rates improve greatly. Biofilms block the penetration of intruders (e.g. immune cells and antimicrobials), promoting bacterial survival.

Improved understanding of the development and function of biofilms in porous media (e.g. soils and rocks, packed beds, trickling filters, reverse osmosis membranes) is impactful in fields ranging from groundwater remediation, water treatment, and soil and agricultural science, to the vast problem of fouling of mechanical and medical systems and implants. Under unsaturated conditions, meaning when pore spaces are only partially filled with water, capillary forces (as what holds water in a straw or sponge) will dominate fluid flow relative to gravity.

This can have a significant impact on how biofilms survive and thrive since they need water to deliver nutrients and oxygen. The overarching goal of the research is to use the microgravity environment on the International Space Station to study the respective roles that gravity and capillary forces play in the development of biofilms in porous media on Earth.

By conducting biofilm growth experiments in space/microgravity and on Earth, the research team can isolate the effects of gravity and capillarity, forming a better understanding of the role that each plays in the development of biofilms in the absence of the other. This will enable researchers to better understand how biofilm 3D shape and function are affected by either of these forces and will allow better designs of systems that make use of biofilms to work for us, or to prevent biofilms from fouling systems where that is undesirable.

The involvement of research conducted on the International Space Station will provide unique outreach opportunities, both via live-streaming from the International Space Station, and also in terms of opportunities to explore 3D objects as samples are scanned with x-rays upon return to Earth. The visualization capabilities of the x-ray imaging facility are ideally suited to support hands-on learning for young students interested in science, technology, engineering, and math.

The resulting volumetric images can be rendered in 3D, giving observers the impression of “flying through” the object.

This project aims to improve existing theories describing biofilm growth and functional processes by generating data that will first and foremost support the development of a mechanistic and quantitative understanding of biofilm function in porous media. The overarching goal of the research is to use the microgravity environment on the International Space Station to study the respective roles that gravity and capillarity (interfacial forces) play in the development of biofilms in porous media on Earth.

By conducting biofilm growth experiments under saturated conditions in microgravity (microG) and on Earth (1G), the role that gravity plays in the development of biofilm architecture in the absence of capillarity can be assessed. By conducting a complementary set of experiments under unsaturated conditions (in both microG and 1G), the role of the force balance between gravity and capillarity will be studied, along with the effects on both hydrodynamics and associated differences in biofilm growth.

Using 3D imaging, the research team aims to establish a “phase diagram” for biofilm growth along the lines of those used to conceptualize different multi-phase (unsaturated) flow regimes. The hypothesis is that a similar diagram could be established that relates dimensionless numbers (Capillary and Bond numbers, representing variations in the force balance between gravity and capillarity) to different types of biofilm growth and architecture, ranging from sparse to dense, and from flat surface growth to “mushroom” or “column-and-canopy” type architectures.

The research will facilitate comparison among detailed images of biofilms grown in different gravitational environments using high-resolution x-ray tomographic imaging. The data generated will provide unprecedented insight regarding biofilm formation in porous media and reveal the relative significance of gravitational and interfacial forces as dominant mechanisms governing biofilm growth and architecture.

The imaging effort will include high-resolution imaging of biofilms, grown in porous media under saturated or unsaturated conditions, in both 1G and microG. The research will generate 3D images of biofilm distribution and architecture, and allow for measurements such as changes in porosity, permeability, and tortuosity due to clogging of pores. Additional measurements include biofilm volume; surface area; interfacial contact area between biofilm and nutrients (biofilm-fluid) area; and biofilm and porous medium (biofilm-solid) area.

To characterize the structural evolution of the biofilm/pore space, axial distributions of a number of structural and topological measures will also be established. Finally, the research team will simulate single-phase flow before and after biofilm growth to visualize and quantify changes in the flow field and velocity distribution caused by biofilm growth.

All of these measurements will help populate the proposed biofilm growth “phase diagram," and can be used to evaluate existing theories and models, as well as support the development of new models. Thus, the significance of this research consists of contributing new knowledge that can shed light on the mechanisms governing biofilm growth and structural evolution in porous media on Earth.

The involvement of research conducted on the International Space Station will provide unique outreach opportunities, both via live-streaming from the International Space Station, and also in terms of opportunities to explore 3D objects as samples are scanned with x-rays upon return to Earth. The visualization capabilities of the x-ray imaging facility are ideally suited to support hands-on learning for young students interested in science, technology, engineering, and math.

The resulting volumetric images can be rendered in 3D, giving observers the impression of “flying through” the object.

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.