Combining in situ reverse transcriptase polymerase chain reaction, optical microscopy, and X-ray photoelectron spectroscopy to investigate mineral surface-associated microbial activities

A study was undertaken to investigate expression of a gene encoding a c-type cytochrome in cells of the dissimilatory metal reducing bacterium (DMRB) Geobacter sulfurreducens during association with poorly crystalline and crystalline solid-phase Fe(III)-oxides. The gene encoding OmcC (outer membrane c-type cytochrome) was used as a target for PCR-based molecular detection and visualization of omcC gene expression by individual cells and aggregates of cells of G. sulfurreducens associated with ferrihydrite and hematite mineral particles. Expression of omcC was demonstrated in individual bacterial cells associated with these Fe-oxide surfaces by in situ RT-PCR (IS-RT PCR) and epifluorescence microscopy. Epifluorescence microscopy also permitted visualization of total DAPI-stained cells in the same field of view to assess the fraction of the cell population expressing omcC. By combining reflected differential interference contrast (DIC) microscopy and epifluorescence microscopy, it was possible to determine the spatial relationship between cells expressing omcC and the mineral surface. Introduction of the fluorescently labeled lectin concanavalin A revealed extracellular polymeric substances (EPS) extending between aggregations of bacterial cells and the mineral surface. The results indicate that EPS mediates an association between cells of G. sulfurreducens and ferrihydrite particles, but that direct cell contact with the mineral surface is not required for expression of omcC. XPS analysis revealed forms of reduced Fe associated with areas of the mineral surface where EPS-mediated bacterial associations occurred. The results demonstrate that by combining molecular biology, reflectance microscopy, and XPS, chemical transformations at a mineral surface can be related to the expression of specific genes by individual bacterial cells and cell aggregates associated with the mineral surface. The approach should be useful in establishing involvement of specific gene products in a wide variety of surface chemical processes.