Fluid dynamics in the mouth shape how biofilms grow and survive
By J. Christopher McInnes, Ph.D.
Fluid motion is an integral force forming and constantly reshaping our surroundings and us. Water falls from the sky and feeds rivers that gouge valleys through mountains, eventually carrying the land to the sea. Ocean waves pound against beaches and bluffs, reshaping our coastlines. Winds from hurricanes or tornadoes flatten objects within their path. Humans have harnessed these fluid forces — fluids turn turbines to generate electricity, and streams of water pressure are used to wash roofs and cut through materials in industry.
Moving fluid also provides a means of delivery and transportation. Fluids course through the veins of living organisms delivering nutrients to cells through a complex network of tubes, pumps, and diffusion surfaces. Organisms such as birds, fish, and motile bacteria that live within a fluid environment adapt so as to be able to transport themselves from one location to the next. Humans also have developed sophisticated fluid transportation systems for moving themselves and material such as waste and irrigation water.
Though not always visible to the human eye, similar fluid delivery systems and forces are influencing life at biofilm size. Biofilms utilize channels and voids to direct nutrients into their interior regions. They adapt to the force of moving fluids in their environment, even streamlining themselves to maximize surface area. As with many biofilms, dental plaque is subjected to fluid forces, in this case saliva, and uses the moving fluids to acquire nutrients for continued growth.
Within a moving fluid, shear forces are present wherever there is a velocity gradient, a region in which adjacent fluid particles are moving at different velocities. Velocity gradients may exist within areas of turbulence or opposing flows, or at the interface between fluids and solids such as along the walls of a pipe or on the surface of teeth or gums.
Fluid in direct contact with a solid boundary has the same velocity as the boundary itself. Thus, in the case of a stationary channel, the water velocity is zero at the channel surface. However, because there remains a bulk flow of water through the channel, the velocity of water is greater away from the channel's surface. The region between the stationary surface and the full velocity is referred to as "the boundary layer." Within this boundary layer there is a velocity gradient creating shear forces. The thickness of the boundary layer depends on characteristics of the fluid itself, the fluid velocity, and the position along the channel.
Objects extending from a stationary object into a fluid flow are subject to forces associated with the moving fluid. A common example is a tree exposed to a windstorm. The moving fluid (air) acts upon the tree, which is attached to the stationary object (the ground). The air generates sufficient force to sway the tree. Under typical wind conditions the tree bends, but returns to its upright position after the wind stops; forces within the tree restore it to its original orientation. Under extreme wind conditions, however, the restoring forces within the tree may not be sufficient to overcome the fluid-induced forces; therefore the tree shears off, or breaks free, from the stationary object and moves with the fluid.
Similar forces act on biofilms in fluid environments. Under moderate flow conditions, fluid forces are insufficient to dislodge the biofilm. Instead, the biofilm deforms in reaction to the fluid flow and may even grow in a streamlined profile, allowing the biofilm to better survive the forces of the moving fluid as well as increase its surface area, thereby creating a larger surface to receive nutrients from the fluid. However, when the fluid velocity is greater than the restoring forces within the biofilm, portions of the biofilm break free and head downstream, either to be flushed way or potentially to colonize a new site.
Whether or not portions of a biofilm exposed to fluid forces break free from a surface depend on both the characteristics of the flow and the characteristics of the biofilm itself. A discrete value cannot be assigned to the force at which a biofilm is removed from a surface because the biofilm structure is a complex, heterogeneous three-dimensional arrangement of bacterial species in various stages of life cycles in addition to the bacterial by-products they produce. Loosely adherent bacteria on the surface of the biofilm structure may break free at a lower force than bacteria embedded in the biofilm matrix. Likewise, elongated biofilm structures extending into the flow pattern may break free at a lower force than flat structures close to the surface.
Other variables also affect the stress put on biofilms. Bubbles and objects within the flow, such as solid particles, influence the flow pattern and can create direct impact forces on the biofilm. Multidirectional, turbulent, or oscillatory flow stresses a biofilm in multiple directions, resulting in a reduced ability of the biofilm to adapt to the flow.
Biofilm-restoring forces include bacteria-to-bacteria interactions as well as bacteria-to-surface interactions. Bacteria hold onto each other and to surfaces via a host of mechanisms including both specific and nonspecific interactions of cell wall components or surface appendages such as fimbriae and pili, both of which are thin hairlike structures serving as "sticky" arms for surface adhesion. Additionally, bacteria bind to surfaces by creating a sticky extracellular environment composed of a polysaccharide matrix. The surface's chemical composition, surface coatings, and total surface area and smoothness also influence the interaction of the biofilm to a surface.
Smile, and the bacterial world smiles with you
The surfaces of teeth provide a friendly environment for the adherence of biofilms; the environment is rich in nutrients from regular dietary intake and is kept moist by the salivary glands that may produce up to a liter of saliva per day. The complex geometry of the oral cavity and dentition also provide plenty of physical niches for biofilm to develop relatively protected from outside forces, such as between teeth or below the gum line. The biological complexity of the oral cavity with the tooth structure penetrating the oral mucosa also provides a wide range of environmental niches, including areas of varying oxygenation and nutrient richness, for different species of bacteria to colonize.
Not all forces in the oral cavity are conducive to biofilm growth. The natural production of saliva helps wash away nonadherent or loosely adherent bacteria. Fluids commonly introduced into the oral cavity through dietary intake, which may provide nourishment for biofilms, also act to dislodge and wash them away. Water and toothpaste act to dislodge biofilms.
The fluid forces are aided by mechanical action. The tongue, cheeks, and lips continuously rub against the tooth surface, abrading attached biofilm. During the process of mastication, the impact of food particles scraping across teeth helps limit biofilm development. The mechanical forces of oral hygiene aid these biological forces, whether from a toothbrush, pick, or floss. These actions may not totally eliminate the biofilm from the exposed surfaces, but they do contribute to keeping the biofilm development in check. However, these forces may also help overall bacteria growth by weeding out the less adaptable bacteria in favor of microorganisms that bind more firmly to the oral surface. Mechanical forces may also flatten the biofilm, making it more difficult to remove, or force it into sheltered areas such as in between teeth or below the gum line.
Recent research at Eastman Dental Institute for Oral Health Care Sciences at University College, London and at the Center for Biofilm Engineering at Montana State University has shown that dynamic fluid motion generated by oral hygiene devices, such as a power toothbrush with high bristle tip velocities, generates sufficient forces to dislodge a portion of biofilm from model dental surfaces. Continued study of biofilm morphology and behavior will elucidate the nature of biofilms' interaction with the fluid environment. Such understanding has the potential to revolutionize the means to treat conditions in which biofilms can have negative impacts. The future of oral hygiene may very well build on the current technology and take advantage of the fluid in the oral cavity to penetrate areas traditionally not reached by mechanical cleaning methods.
J. Christopher McInnes, Ph.D., is principal scientist at Philips Oral Healthcare. His current research includes investigations of fluid dynamic effects on surfaces.
More to explore
Biofilms on Oral Surfaces: Implications for Health and Disease. 14th International Conference on Oral Biology. Advances in Dental Research, Vol. 11, Number 1; April 1997.
Images and information about measuring biofilms under fluid stress can be found at the Eastman Dental Institute for Oral Health Care Sciences at University College, London, at www.eastman.ucl.ac. uk/~microb/flowcell.html.
Life in Moving Fluids: The Physical Biology of Flow (Second Edition). Steven Vogel. Princeton University Press, 1996
Measuring Fluid Shear
By Paul Stoodley
Very little is known about the material properties of dental biofilms. Unlike conventional materials like plastics, which can be molded into uniform test pieces, biofilms are nonuniform, microscopically small, and attached to surfaces. Removal from the surface will inevitably disrupt the sample, and it is difficult to reproduce in the lab the varying and complex physical forces existing in the mouth, so testing remains a challenge.
In our laboratory at the Center for Biofilm Engineering at Montana State University, we have developed methods for testing the material properties of biofilms using fluid shear as the deforming force. By measuring the deformation to biofilms caused by long- and short-term exposure to elevated fluid shear, we found that various pure and mixed-species aerobic and anaerobic biofilms grown in glass flow cells were in fact viscous fluids that behaved elastically over short loading time periods (seconds or less), but could flow like viscous fluids when the load was sustained. Also, biofilms grown at higher shear were more firmly attached and cohesively stronger than those grown at lower shear.
This has a number of implications. Because the mouth has an incredibly wide range of shear and normal stresses, we might expect that the biofilms will also exhibit a wide range of cohesive and adhesive strengths depending on the local growth environment in the mouth. The material properties of dental plaque will also likely change with time. As calcification occurs, the plaque will be expected to become more rigid and solid-like and behave less like a fluid. In this case, instead of flowing it may fracture in response to an applied physical force. Also, because biofilms can flow, albeit slowly, it is likely that the action of chewing or movement of the tongue may actually smear biofilm from one place to another. By looking at biofilms from a materials standpoint and refining our methods, we can begin to design new technologies to address their control.