Saintillan applying fluid mechanics to bacterial suspensions

Bacteria are some of the most abundant life forms in the ecosystem. The biomass of bacteria on Earth is greater than that of plants and animals combined, and there are ten times more bacterial cells than human cells in the human body. In fact, the adult human body carries two to five pounds of bacteria, even though a bacterial cell can be one-hundredth of the size of a human cell. They are necessary for digestion, for synthesizing certain vitamins, and even for defense against other disease-causing bacteria.

MechSE assistant professor David Saintillan specializes in fluid mechanics and has developed algorithms and models for different types of fluid systems. Some of his more recent work has been with “active suspensions” or fluidic suspensions with self-propelled particles—in this case, bacteria.

“When you have a lot of them together, you can see that bacteria affect flows on very large scales,” Saintillan said. “A significant part of the work that I’ve been doing over the past few years is trying to understand why that happens, and what the implications could be.”

Because of the prevalence of bacteria, the implications are broad. Studying these active suspensions could lead to a better understanding of how bacterial infections spread through the body or how microorganisms in the oceans influence the mixing of oxygen and other elements, which has an effect on sea life. In fact, the mixing that goes on in active suspensions is so effective that researchers have attempted to use bacteria in engineering devices to mix fluids more efficiently.

“What we find is that there are several conditions for these large-scale flows to arise,” Saintillan said. “One is that the system needs to be dense enough. We actually calculated a threshold concentration above which the large-scale flows can be created. When it’s dilute, you don’t see very much happening, but as the concentration increases you start seeing interaction between the organisms that lead these large-scale collective motions.”

Other applications of this work involve the influence of fluid dynamics on the growth of bacterial biofilms in the human body. Biofilms are collections of microorganisms (usually bacteria) that cling to each other on a surface. The most prevalent example is dental plaque, which can cause tooth decay and gum disease. Biofilms are also thought to play a role in infectious diseases such as microbial infections and may impair the healing of cutaneous wounds. A better understanding of how they are created may be helpful in finding new ways to prevent infections in the future.

Saintillan and his group are also studying biological polymers, specifically the dynamics of flexible filaments within the cell. Actin and microtubules are flexible rods in a cell that are important for locomotion, cell division, and for the mechanical structure of the cell.

“We’re trying to model and understand what the behavior of these microscopic filaments is under flow,” Saintillan said. “Essentially, we find that these filaments behave much like macroscopic elastic beams, and for instance can be caused to buckle under certain flow conditions. We have developed a model and simulations of these dynamics, and we’re finding for instance that there exists a critical flow strength for this buckling to occur, and that depending on how strong the flow is different buckling modes and shapes can be excited.”

Down the road, this fundamental study may shed insight on the mechanics of the cell, as well technological applications involving other synthetic semiflexible polymers in microfluidic devices.

“There are many complex processes going on inside the cell,” Saintillan said. “What we are essentially doing is stripping these down and looking at the basic mechanics of these flexible polymers from a basic perspective. By characterizing in detail the behavior of these building blocks and their interaction with fluid flows, we hope to eventually improve our understanding of some of these processes.”