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Stochasitc Modeling of Microbial Chemotaxis
Stochastic Modeling of Microbial Chemotaxis
Christian M. Lastoskie
Department of Chemical Engineering
Michigan State University
East Lansing, MI 48824-1226
Chemotaxis refers to the directed migration of organisms in response to local chemical gradients of chemoattractants. Chemotaxis redirects microorganisms to regions of high chemoattractant concentration at rates much faster than would otherwise be realized through random migration or convective transport alone. A number of different chemical substances serve as bacterial chemoattractants, including nutrients, electron acceptors, and other substances relevant to microbial metabolism. Other chemical species that are potentially toxic to bacterial cells, such as trace metals and metabolism waste products, act as chemorepellents and induce cell migration away from regions of high toxin concentration. Chemotaxis imparts a significant competitive advantage to chemotactic microorganisms over nonchemotactic competitors, in that the chemotactic species are more readily able to secure nutrients needed for cell growth while avoiding toxic substances. The effect of chemotaxis on microbial ecology is important in efforts to treat soil/groundwater contaminant plumes via engineered in situ bioremediation.
In situ bioremediation requires the stimulation of cell growth in a subsurface bacterial population to levels sufficient to degrade organic pollutants. In some cases, in situ bioaugmentation may be implemented; this involves the introduction of a new bacterial species, with special transformational capabilities, to the subsurface to displace the native microbial population. Successful in situ bioremediation operations have been achieved at a number of locations throughout the United States. The denitrifying bacterium pseudomonas stutzeri KC, for example, has been successfully deployed in a contaminated aquifer near Schoolcraft, Michigan to degrade a carbon tetrachloride plume without producing chloroform. Contaminant removal efficiencies of 75-90% have been achieved using the in situ bioaugmentation strategy, at a considerably lower cost than would be incurred through surface treatment of pumped groundwater via air stripping.
Despite these successes, many difficulties remain to be solved before widespread implementation of in situ bioremediation can be realized. Three major problems that hamper in situ bioremediation efforts are the following:
(1) Mass transfer limitations often result in slow dissemination of microbes and/or nutrients from the injection wells into the contaminated regions.
(2) Excessive microbial growth can plug aquifers, further diminishing mass transfer.
(3) Microbial species introduced via bioaugmentation are often ill equipped to compete with indigenous species, resulting in a low concentration of the introduced organism and a low pollutant degradation rate.
Chemotaxis can address the mass transfer problem by speeding dissemination of cells throughout the contaminated zone. More rapid transport of cells away from the injection well also reduces the plugging problem. Also, by increasing competitiveness, the chemotactic trait improves the likelihood that an augmented microbial species will successfully colonize an aquifer and transform the targeted pollutant.
Field sampling and laboratory column measurements indicate that chemotaxis is a major component of the transport of pseudomonas stutzeri KC and other bacterial organisms through porous aquifer media. An understanding of microbial chemotaxis is therefore needed to optimize the placement of wells and the inoculation/feeding timetables for in situ bioremediation. Various experimental assays and mathematical models have been developed for studying bacterial migration in the homogeneous phase, i.e. in aqueous solution. Recently, we have extended these studies to investigate microbial chemotaxis in heterogeneous porous solids, such as those encountered by bacterial organisms in sandy aquifers. We report chemotactic motility measurements for pseudomonas stutzeri KC migration in Schoolcraft aquifer solids in response to chemoattractant gradients of acetate and nitrate. We also describe a stochastic cellular dynamics simulation method that can be used to calculate motility coefficients from first-principles knowledge of the swimming characteristics of a bacterial cell. The cellular dynamics simulation technique is used to investigate the chemotactic transport of bacterial cell populations in confined geometries and in the presence of consumable chemoattractants (i.e. chemoattractants that are utilized as nutrients for cell metabolism).