The adhesion of a well-characterized Escherichia coli bacterial strain to quartz sediment grains in the presence of repulsive electrostatic interactions is systematically examined. An increase in the ionic strength of the pore fluid results in an increase in bacterial attachment, despite DLVO calculations indicating a sizable electrostatic energy barrier to deposition. Bacterial deposition is likely occurring in the secondary energy minimum, which DLVO calculations indicate increases in depth with ionic strength. A decrease in the ionic strength of the pore fluid--thereby eliminating the secondary energy minimum--resulted in release of the majority of previously deposited bacteria, suggesting that these cells were deposited reversibly in the secondary minimum. Additionally, bacterial attachment to a quartz surface in a radial stagnation point flow system was absent at ionic strengths less than 0.01 M and resulted in attachment efficiencies over an order of magnitude lower than in the packed-bed column experiments at higher ionic strengths. Because of the hydrodynamics in the radial stagnation point flow system, this observation supports our conclusion that the majority of bacterial deposition in the packed bed occurs in a secondary energy minimum.

The influence of bacterial surface lipopolysaccharides (LPS) on cell transport and adhesion has been examined by use of three mutants of Escherichia coli K12 with well-characterized LPS of different lengths and molecular composition. Two experimental techniques, a packed-bed column and a radial stagnation point flow system, were employed to investigate bacterial adhesion kinetics onto quartz surfaces over a wide range of solution ionic strengths. Although the two systems capture distinct deposition (adhesion) mechanisms because of their different hydrodynamics, similar deposition kinetics trends were observed for each bacterial strain. Bacterial deposition rates were directly related to the electrostatic double layer interaction between the bacteria and quartz surfaces, in qualitative agreement with classic Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. However, DLVO theory does not fully explain the deposition behavior for the bacterial strain with the lengthy, uncharged O-antigen portion of the LPS. Neither the length nor the charge characteristics of the LPS molecule directly correlated to deposition kinetics, suggesting a complex combination of cell surface charge heterogeneity and LPS composition controls the bacterial adhesive characteristics. It is further suggested that bacterial deposition behavior is determined by the combined influence of DLVO interactions, LPS-associated chemical interactions, and the hydrodynamics of the deposition system.

The transport and fate of microbial particles in subsurface environments is controlled by their capture (natural filtration) by sediment grains. Typically, filtration models used to describe microbe removal in porous media predict exponential decrease in microbial particle concentration with travel distance. However, a growing body of laboratory-scale column experiments suggests that the retained microbial particle profiles decay nonexponentially. The observed behavior may be attributed to the heterogeneity in the interactions between microbial particles and sediment grains, most likely due to the inherent variability in the microbial particles. This factor can be incorporated into classical colloid filtration (deposition) theory by inclusion of a distribution in the deposition rate coefficient. We show that certain distributions of the deposition rate coefficient (i.e., log-normal, bimodal, and power-law distributions) give rise to nonexponential deposition patterns. Comparisons of model predictions to experimental data indicate that the observed nonexponential deposition behavior of bacteria and virus particles may be attributed to a broad range (i.e., a power-law distribution) of microbial deposition rates. Other mechanisms such as particle release and blocking by previously deposited microbial particles are also shown to be potential sources of deviation from the classical filtration theory. Our results further suggest that monitoring fluid-phase particle concentration is insufficient for accurate characterization of the deposition and transport behavior of microbial particles in saturated porous media. Rather, the shape of the microbial particle retention profile is shown to be a key indicator of the mechanisms controlling microbial deposition and transport.

The influence of bacterial growth stage and the evolution of surface macromolecules on cell adhesion have been examined by using a mutant of Escherichia coli K-12. To better understand the adhesion kinetics of bacteria in the mid-exponential and stationary growth phases under flow conditions, deposition experiments were conducted in a well-controlled radial stagnation point flow (RSPF) system. Complementary cell characterization techniques were conducted in combination with the RSPF experiments to evaluate the hydrophobicity, electrophoretic mobility, size, and titratable surface charge of the cells in the two growth phases considered. It was observed that cells in stationary phase were notably more adhesive than those in mid-exponential phase. This behavior is attributed to the high degree of local charge heterogeneity on the outer membranes of stationary-phase cells, which results in decreased electrostatic repulsion between the cells and a quartz surface. The mid-exponential-phase cells, on the other hand, have a more uniform charge distribution on the outer membrane, resulting in greater electrostatic repulsion and, subsequently, less adhesion. Our results suggest that the macromolecules responsible for this phenomenon are outer membrane-bound proteins and lipopolysaccharide-associated functional groups.

Elevated levels of enterococci bacteria, an indicator of fecal pollution, are routinely detected in the surf zone at Huntington State and City Beaches in southern California. A multidisciplinary study was carried out to identify sources of enterococci bacteria landward of the coastline. We find that enterococci bacteria are present at high concentrations in urban runoff, bird feces, marsh sediments, and on marine vegetation. Surprisingly, urban runoff appears to have relatively little impact on surf zone water quality because of the long time required for this water to travel from its source to the ocean. On the other hand, enterococci bacteria generated in a tidal saltwater marsh located near the beach significantly impact surf zone water quality. This study identifies a potential tradeoff between restoring coastal wetlands and protecting beach water quality and calls into question the use of ocean bathing water standards based on enterococci at locations near coastal wetlands.