POPULATION HETEROGENEITY AFFECTS TRANSPORT OF BACTERIA THROUGH SAND COLUMNS AT LOW-FLOW RATES

Travel distances of bacteria in groundwater aquifers often exceed pred ictions based on filtration theories. These findings have mostly been ascribed to structural heterogeneities in the subsurface, but variatio ns in the adhesive properties within the microbial populations have be en observed too. In laboratory experiments with Pseudomonas sp, strain B13, we found that only a fraction of the cells was efficiently depos ited in sand columns while the remainder passed a second column identi cal to the first without hindrance. Upon rinsing the columns with deio nized water, between 10 and 35% of the deposited cells were flushed ou t, thus showing that increased electrostatic repulsion between sand an d bacteria partially reverted the deposition. Lipopolysaccharides (LPS ) extending from the cell surface into the medium as well as estimated DLVO-type interaction energy curves indicate that cells were trapped at a distance of more than 20 nm from the sand surface. We hypothesize that differences in the LPS coat were responsible for the fractionati on of the bacterial population. Our results indicate that the high tra vel distances microorganisms might be due not only to the complex stru cture of aquifer material but also to heterogeneity in the adhesion pr operties within the bacterial populations.