Microbial Enhanced Oil Recovery: Mechanisms of Microbial Movement in Subsurface Materials
Efficient and economical treatment of contaminated subterranean formations by in situ bioremediation process is under investigation. These processes involve injection of specific nutrients into the subsurface to stimulate bacterial populations involved in transformation and/or mineralization of potentially toxic compounds. The success of such processes depends on a fundamental understanding of the factors that influence the propagation, growth, and movement of bacteria within porous subsurface formations. In flow systems, bacterial transport through porous media has been largely described as a function of the porous entrance size and, hence, adequately simulated by a filtration model. Under static conditions, however, it is known that a relationship between the bacterial penetration rate and the subsurface permeability exists. Furthermore, a penetration mechanism based entirely on motility has previously been derived which successfully predicts penetration times for a motile bacillus strain growing within Berea sandstone.
Although motility was an important mechanism for bacterial penetration in this case, it was unknown whether other characteristics of motility, i.e., random or nonrandom movement, were also important. Directional or nonrandom movement is usually thought to occur only in chemotactic strains, since non-chemotactic mutants are known to move randomly. Nonmotile bacteria have also been shown to penetrate consolidated sandstone cores, although their penetration rates were considerably slower than that found in motile bacteria. The mechanisms by which nonmotile bacteria penetrate porous media were not known. The aims of this study were to determine the biological factors which govern the movement of motile and nonmotile bacteria through unconsolidated porous media. To answer these questions, we compared the penetration times of Eschericia coli mutants defective in chemotaxis, flagellar synthesis, and gas production under anaerobic conditions.
For motile strains, the penetration rate decreased with increasing galactose concentrations in the core and with decreasing inoculum sizes. Also, motile strains with the faster growth rates had faster penetration rates. These results imply that, for motile bacteria, the penetration rate is regulated by the in situ bacterial growth rate. A sigmoidal relationship was found between the specific growth rates of all of the motile bacteria used in this study and the penetration rates through cores saturated with galactose-peptone medium.
This research was supported under contract CR-813559 from the U.S. Environmental Protection Agency, under the supervision of Professor Michael McInerney at the University of Oklahoma, USA.