Graduation date: 2010
This research details laboratory assessment of bacterial hydrophobicity, variations in bacterial adhesion properties with growth and extended starvation as well as extension of this assay to colloidal hydrophobicity measurement. Hydrophobicity is an integral component of surface interactions and has been studied extensively for its role in a number of engineering and applied fields. A simple-to-do and quick experimental technique known as the Microbial Adhesion to Hydrocarbons (MATH) test is the focus of this work. This method is based on determination of microbial hydrophobicity by differential partitioning at an aqueous-hydrocarbon interface and the results yield the hydrocarbon interaction affinity of the microbes. Though very popular, this assay still suffers from the lack of a standard protocol. As a first step in that direction, the effects of various operational parameters on MATH hydrophobicity measurements were studied. Some of the previously unexplored parameters like, absorbance wavelength, hydrocarbon saturation of aqueous media and suspension medium were found to affect the results. Application of a high concentration of a lyotrope, ammonium sulfate, was shown to enhance the MATH hydrophobicity of bacteria. Additionally, cell size measurements revealed that the affinity of sulfate ions for water molecules is the primary cause of this increase rather than cell agglomeration. Increased hydrophobicity can be beneficially employed for oral care, prevention of urinary tract infection and mixed microbial community analysis.
Bacteria are often subjected to nutrient variations in different environments which significantly alter their adhesion to surfaces thereby affecting biofilm development and have an important bearing on biofouling. An in-depth study of variations in some of the more common adhesion determinants (biomass, cell size and hydrophobicity) demonstrated a decrease in hydrophobicity from log-growth to stationary growth phases. Short term starvation (up to 7 days) led to significant variations in measured parameters. Starved cells were also more susceptible to hydrocarbon exposure and exhibited smaller cell sizes than growth cultures.
Colloids, the larger particle family of which bacteria are a sub-class, lack a simple assay for hydrophobicity measurement. The MATH test was successfully extended to colloidal domain (non-biological particles) and the hydrophobicity results were verified against the more traditional method- water contact angle measurement. We observed similar hydrophobicity trends, as measured by the MATH test and contact angle measurements, for most of the colloids tested in this work. This dissertation research is expected to enhance our understanding of hydrophobicity in the context of bacteria and colloids, through simple experiments, and reinforce our knowledge of the dynamic nature of bacteria.