Research

Nanoscale Cell-Surface Interactions

Bacterial cells are commonly known to interact with surfaces in a variety of medical, industrial and natural environments. While cells can interact with surfaces individually, many bacteria can form complex communities known as biofilms in which cells are typically surrounded by a protective layer of extracellular polymeric substances (EPS) that mediate cell attachment, protect cells from dislodgement from the surface and provide mechanical stability. Comparatively little is known about the nanoscale material properties (including adhesive and cohesive strengths) of attached biofilms, making it difficult to generally predict how a biofilm may behave. Understanding how bacteria behave while attached to surfaces as part of a biofilm is highly important in medical and industrial applications. Biofilms in these environments can represent a hazardous and costly problem, causing biocorrosion, biofouling, infection and disease. It is therefore important to better understand the properties and characteristics of biofilms in order to develop better control and remediation techniques.

Microbial biofilms have unique mechanical properties that dictate their response to shear stress environments. These viscoelastic properties allow biofilms to survive a variety of externally applied stresses such as turbulent, high-shear fluid flow. This can enable biofilms to persist in high fluid flow and resist even harsh remediation or cleaning methods. Micro-scale rheological studies have shown that biofilms formed by a wide range of microorganisms have certain universal viscoelastic properties that have been associated with biofilm resistance to detachment from surfaces, as well as facilitating certain forms of “rolling migration” that allow biofilms to move along surfaces (Nauman et al., 2007; Musk et al., 2005). Studies have also shown that biofilms grown under high shear conditions are more strongly adhered to surfaces and have increased mechanical strength over those grown under low shear conditions (Oh et al., 2007). These results suggest that biofilms can respond to environmental stresses and that chemical cues in the liquid environment and intercellular signaling can greatly affect biofilm physical/mechanical properties. Hence, a broad, thorough understanding of the physical properties of biofilms as a function of their environment is required for future applications including remediation or cleaning. It is also of interest to monitor and compare the behavior of biofilms to the mechanical response of individual bacterial cells to external signals such as nutrition levels and cell communication.

Our laboratory is currently working on methods to directly measure the biophysical properties of bacterial biofilms using atomic force microscopy (AFM).  By coupling microfluidic fluid control with precision AFM measurement, we hope to better understand the dynamics of cell behavior under varying fluid shear conditions and during exposure to external chemical signals.  In addition to this work, we are also interested in how cells respond to nano and microscale topographical features.  We are currently developing 3D nanomanufacturing strategies to create nanoscale topographical features that can be used for cell adhesion assays.  These experiments will provide insight for the design of low-adhesion surfaces that could resist bacterial adhesion and colonization. 

Cell Signaling in Defined Microenvironments

In addition to understanding how cells interact with surfaces, we are also interested in how bacteria communicate within well-defined microenvironments.  Previous work has shown that many bacteria are capable of emitting and responding to diffusible chemical signals in a behavior known as “quorum sensing.”  In large volume environments these chemical signals diffuse throughout the medium and must reach a critical concentration before downstream responses are observed.  The critical concentration of the signal is usually proportional to the number of cells within the environment.  Therefore, signal response is often linked to the cell density.  Currently, little is known about diffusion-limited environments in which very few cells may produce and respond to chemical cues.  Even less is known about signaling behavior when cells are attached to surfaces.  To better understand cell signaling behavior in microenvironments, we are interested in measuring the minimal distances needed for effective cell-to-cell communication and understanding the affects of cell density for surface attached organisms.  To this end, we have pursued various patterning strategies to attach cells to surfaces in fixed geometric patterns.  To create these patterns, we have used the BioForce Nano eNabler (NeN), a novel surface patterning system that can deposit fluids directly onto solid surfaces.  Using this system, we have been able to directly deposit antibodies onto glass surfaces in well-ordered two-dimensional arrays.  We have also been able to capture live bacterial cells on these antibody arrays and are currently working to optimize our patterning techniques to isolate single cells on individual array elements.  Future studies will include the measurement of signal production and responses using fluorescent and luminescent live-cell reporter assays.  These studies will provide detailed information about the minimum distance required for effective cell signaling.  Understanding this behavior in greater detail could give us the tools to intentionally modify bacterial signaling during bacterial infections and biofouling.   

Biosensor Development

Our work with biosensors involves the development of microfluidic detection systems as well as novel biological detection assays.  Much of this work stems from Prof. Cady’s previous work on microchip-based, real-time PCR systems and quantum dot (QD)-based detection strategies.  We are actively pursuing new optical detection architectures with Prof. Shahedipour-Sandvik (CNSE) and multiplexed Raman spectroscopy-based assays with Prof. Igor Lednev (Dept. of Chemistry). 

References:

Oh, Y. J., Jo, W., Yang, Y., Park, S. 2007. Influence of culture conditions on Escherichia coli O157:H7 biofilm formation by atomic force microscopy. Ultramicroscopy:Article In Press.

Nauman, E. A., C. M. Ott, E. Sander, D. L. Tucker, D. Pierson, J. W. Wilson, and C. A. Nickerson. 2007. Novel quantitative biosystem for modeling physiological fluid shear stress on cells. Appl Environ Microbiol 73:699-705.

Musk, D. J., D. A. Banko, and P. J. Hergenrother. 2005. Iron salts perturb biofilm formation and disrupt existing biofilms of Pseudomonas aeruginosa. Chem Biol 12:789-96.