Population heterogeneity affects transport of bacteria through sand columns at low flow rates

Travel distances of bacteria in groundwater aquifers often exceed predictions based on filtration theories. These findings have mostly been ascribed to structural heterogeneities in the subsurface, but variations in the adhesive properties within the microbial populations have been observed too. In laboratory experiments with Pseudomonas sp, strain B13, we found that only a fraction of the cells was efficiently deposited in sand columns while the remainder passed a second column identical to the first without hindrance. Upon rinsing the columns with deionized water, between 10 and 35% of the deposited cells were flushed out, thus showing that increased electrostatic repulsion between sand and 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 fractionation of the bacterial population. Our results indicate that the high travel distances microorganisms might be due not only to the complex structure of aquifer material but also to heterogeneity in the adhesion properties within the bacterial populations.