Summer Research

* Student(s): Jennifer Henkle , Gemma Kirkwood , Mollie Wright
Advisor: Michael McCormick

Title: Role of Outer Membrane Protein OmcB on Adhesion of the Dissimilative Iron- Respiring Bacteria Geobacter sulfurreducens to Goethite


Dissimilative iron-reducing bacteria (DIRB) are a recently discovered class of organisms that use ferric oxide minerals as a terminal electron acceptor during cellular respiration. The mechanism of electron transfer from the cytoplasmic membrane of DIRB to the surface of iron oxide minerals is unknown. It has been proposed that the outer membrane protein (OmcB) of the iron-reducing bacteria Geobacter sulfurreducens may be involved in this process. As direct contact between the cell and mineral is required for this species to respire iron oxides, we hypothesized that OmcB may also play a role in cell adsorption and adhesion. To test this hypothesis, we are comparing a mutant strain of G. sulfurreducens deficient in the gene that encodes for OmcB with wild-type cells for their relative affinity for adsorption and adhesion to the iron oxide goethite (-FeOOH).

Goethite was produced synthetically in the lab then characterized with regard to crystallography and morphology by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Cell adsorption isotherms were collected for both strains in uniform aqueous suspensions of goethite. The findings were inconclusive but indicated a complex relationship between cell density and concentration of colloidal goethite particles. At high concentrations of cells and goethite particles, evidence of particle flocculation was evident.

In complement to the cell-adsorption studies, preparations were made to use the newly developed technique of Biological Force Microscopy (BFM) to measure adhesive forces between single cells of G. sulfurreducens and the surface of goethite. BFM involves measuring the nano-Newton forces of attraction and repulsion between a cantilever mounted mineral and a monolayer of cells as they move incrementally toward and away from each other on a piezoelectric stage. To permit the use of BFM under anaerobic conditions, an anaerobic chamber was constructed to house a BFM apparatus. Protocols for the preparation of monolayers of G. sulfurreducens cells and cantilever mounted goethite crystals were developed and successfully tested.

Student stipend support for JH and GK provided by the Dean's Summer Research Fund; stipend support for MW provided by the Ralph E. Hansmann Science Student Support Fund.