Doctoral Dissertations

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Life Sciences

Major Professor

Jennifer Morrell-Falvey

Committee Members

Alison Buchan, Gladys Alexandre, Dale A. Pelletier, Mitchel J. Doktycz


Biological interactions occur on multiple length scales, ranging from molecular to population wide interactions. This work describes the study of two specific areas of biological interactions in microbial systems: intracellular protein-protein interactions and cell-to-cell interactions. The implementation of optical and atomic force microscopy and the methodologies developed during this study proved to be invaluable tools for investigating these systems.

Identifying and characterizing protein interactions are fundamental steps toward understanding complex cellular networks. We have developed a unique methodology which combines an imaging-based protein interaction assay with a fluorescence recovery after photobleaching technique (FRAP). Protein interactions are readily detected by co-localization of two proteins of interest fused to green fluorescent protein (GFP) and DivIVA, a cell division protein from Bacillus subtilis. We demonstrate that the modified co-localization assay is sensitive enough to detect protein interactions over four orders of magnitude. FRAP data was analyzed using a combination of various image processing techniques and analytical models. This combined approach made it possible to estimate cell morphology parameters such as length, diameter, the effective laser probe volume, as well as to the mobile protein concentration in vivo, the number of bound molecules at the cellular poles, and the biophysical parameter koff.

Cells not only utilize molecular interactions in the intracellular environment, but also express proteins, polysaccharides and other complex molecules to mediate interactions with the surrounding extracellular environment. In Azospirillum brasilense, cell surface properties, including exopolysaccharide production, are thought to play a direct role in promoting cell-to-cell interactions. Recently, the Che1 chemotaxis-like pathway from A. brasilense was shown to modulate flocculation, suggesting an associated modulation of cell surface properties. Using atomic force microscopy, distinct changes in the surface morphology of flocculating A. brasilense Che1 mutant strains were detected. Further analyses suggest that the extracellular matrix differs between the cheA1 and the cheY1 deletion mutants, despite similarity in the macroscopic floc structures. Collectively, these data indicate that disruption of the Che1 pathway is correlated with distinctive changes in the extracellular matrix, which likely result from changes in surface polysaccharides structure and/or composition.

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