Mechanical and Civil Engineering Seminar

                Bacterial infections are one of the greatest risks to public health. For example, one particular species of bacteria, Mycobacterium tuberculosis, is the causative agent of tuberculosis and a leading causes of death worldwide with roughly 1.8 million casualties per year, mostly in the developing world. Bacterial infections also pose a major threat to developed nations like the United States. It has been estimated that bacteria account for an estimated 30% of the 13.8 million foodborne illnesses in the US. However, there are also several bacteria that have tremendous potential to benefit mankind. For example, recently it was discovered that some bacteria have the ability to transfer electrons directly to an electrode. These unique organisms, which we will call electrochemically active bacteria, can be utilized to convert organic fuels directly into electricity in microbial fuel cells. Interestingly, for both pathogens and electrochemical active bacteria, surface bound proteins which constitute the 'soft' layer surrounding the cell play a major role in their unique phenotypes.

In this talk I will present a detailed theoretical model to investigate the effect of 'soft' polyelectrolyte layers on the polarization of bacteria. We model the soft layer of bacteria including dissociation of ionogenic charged groups and specific interactions with the background electrolyte. The fluid flow around the particle is modeled by a modified Stokes equation accounting for permeability of the soft layer.  To demonstrate our model we consider two test cases: fibrillated and unfibrillated Streptococcus salivarius whose electrophoretic mobilities were analyzed theoretically and experimentally by Duval et al. [Langmuir, 2005]. We explore a wide range of bulk electrolyte concentrations to consider both thin and thick double layer limits. The results demonstrate an interesting interplay between soft layer conductivity and double layer conductivity on polarizability, subtleties often neglected when assuming Maxwell-Wagner polarization.

We have recently designed a novel platform for screening bacteria based on their surface polarizability known as three dimensional insulator based dielectrophoresis (3DiDEP). In dielectrophoresis (DEP), microorganisms in a non-uniform electric field experience forces related to their volume and electrical properties. Differences in physical properties can be exploited to distinguish populations of bacteria. To date DEP has been used to sort cells based on morphology (e.g. gram positive vs. gram negative bacteria), size, and viability. 3DiDEP features advantages over previous embodiments of DEP in that it offers high sensitivity, uses low applied voltages, requires minimal fabrication, and is relatively simple. Our recent work has shown that 3DiDEP can be useful to distinguish bacteria with sub-species resolution. In this talk I will discuss our 3DiDEP design and describe exciting results on the characterization of pathogenic and electrochemically active bacteria.