Derick G. Brown

Theoretical prediction of collision efficiency between adhesion-deficient bacteria and sediment grain surface

Abstract Our earlier results concerning bacterial transport of an adhesion-deficient strain Comamonas sp. (DA001) in intact sediment cores from near South Oyster, VA demonstrated that grain size is the principle factor controlling bacterial retention, and that Fe and Al hydroxide mineral coatings are of secondary importance. The experimentally determined collision efficiency ( α ) was in the range of 0.003–0.026 and did not correlate with the Fe and Al concentration. This study attempts to theoretically predict α , and identifies factors responsible for the observed low α . The modified Derjaguin–Landau–Verwey–Overbeek (DLVO) theory was used to calculate the total intersurface potential energy as a function of separation distance between bacterial and sediment surfaces and to provide insights into the relative importance of bacterial and sediment grain surface properties in controlling magnitude of α . Different models for calculating theoretical α were developed and compared. By comparing theoretical α values from different models with previously published experimental α values, it is possible to identify a suitable model for predicting α . When DA001 bacteria interact with quartz surfaces, the theoretical α best predicts experimental α when DA001 cells are reversibly attached to the secondary minimum of the energy interaction curve and α depends on the probability of escape from that energy well. No energy barrier opposes bacterial attachment to clean iron oxide surface of positive charge at sub-neutral pH, thus the model predicts α of unity. When the iron oxide is equilibrated with natural groundwater containing 5–10 ppm of dissolved organic carbon (DOC), its surface charge reverses, and the model predicts α to be on the order of 0.2. The theoretical α for DA001 in the natural sediments from South Oyster, VA was estimated by representing the surface potential of the sediment as a patch-wise binary mixture of negatively charged quartz ( ζ =−60 mV) and organic carbon coated Fe–Al hydroxides ( ζ =−2 mV). Such a binary mixing approach generates α that closely matches the experimental α . This study demonstrates that it is possible to predict α from known bacterial and grain surface properties.

Electrostatic Behavior of the Charge-Regulated Bacterial Cell Surface

The electrostatic behavior of the charge-regulated surfaces of Gram-negative Escherichia coli and Gram-positive Bacillus brevis was studied using numerical modeling in conjunction with potentiometric titration and electrophoretic mobility data as a function of solution pH and electrolyte composition. Assuming a polyelectrolytic polymeric bacterial cell surface, these experimental and numerical analyses were used to determine the effective site numbers of cell surface acid-base functional groups and Ca(2+) sorption coefficients. Using effective site concentrations determined from 1:1 electrolyte (NaCl) experimental data, the charge-regulation model was able to replicate the effects of 2:1 electrolyte (CaCl(2)), both alone and as a mixture with NaCl, on the measured zeta potential using a single Ca(2+) surface binding constant for each of the bacterial species. This knowledge is vital for understanding how cells respond to changes in solution pH and electrolyte composition as well as how they interact with other surfaces. The latter is especially important due to the widespread use of the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory in the interpretation of bacterial adhesion. As surface charge and surface potential both vary on a charge-regulated surface, accurate modeling of bacterial interactions with surfaces ultimately requires use of an electrostatic model that accounts for the charge-regulated nature of the cell surface.

Variation in Bacterial ATP Level and Proton Motive Force Due to Adhesion to a Solid Surface

ABSTRACT Bacterial adhesion to natural and man-made surfaces can be beneficial or detrimental, depending on the system at hand. Of vital importance is how the process of adhesion affects the bacterial metabolic activity. If activity is enhanced, this may help the cells colonize the surface, whereas if activity is reduced, it may inhibit colonization. Here, we report a study demonstrating that adhesion of both Escherichia coli and Bacillus brevis onto a glass surface resulted in enhanced metabolic activity, assessed through ATP measurements. Specifically, ATP levels were found to increase two to five times upon adhesion compared to ATP levels in corresponding planktonic cells. To explain this effect on ATP levels, we propose the hypothesis that bacteria can take advantage of a link between cellular bioenergetics (proton motive force and ATP formation) and the physiochemical charge regulation effect, which occurs as a surface containing ionizable functional groups (e.g., the bacterial cell surface) approaches another surface. As the bacterium approaches the surface, the charge regulation effect causes the charge and pH at the cell surface to vary as a function of separation distance. With negatively charged surfaces, this results in a decrease in pH at the cell surface, which enhances the proton motive force and ATP concentration. Calculations demonstrated that a change in pH across the cell membrane of only 0.2 to 0.5 units is sufficient to achieve the observed ATP increases. Similarly, the hypothesis indicates that positively charged surfaces will decrease metabolic activity, and results from studies of positively charged surfaces support this finding.

Effects of porous media preparation on bacteria transport through laboratory columns

Bacterial and colloid transport experiments related to environmental systems are typically performed in the laboratory, with sand often used as the porous media. In order to prepare the sand, mechanical sieving is frequently used to tighten the sand grain size distribution. However, mechanical sieving has been reported to provide insufficient repeatability between identical colloidal transport experiments. This work examined the deficiencies of mechanical sieving with respect to bacterial transport through sand columns. It was found that sieving with standard brass sieves (1) contaminates the sand with copper and zinc as a linear function of sieving time and (2) inefficiently sizes sand grains below 300 microm (the largest size examined in this study) due to rapid clogging of the sieves. A procedure was developed that allows utilization of brass sieves for sizing the sand grains and removes the metal contamination introduced from the sieves. Bacterial transport experiments utilizing this column preparation procedure gave repeatable breakthrough curves. Further examination of the effects of these treatments on bacterial transport showed interesting results. First, it was found that the metal contamination did not affect the clean-bed bacterial transport. Second. it was found that variations of the column flushing procedure did not alter the clean-bed breakthrough of the bacteria, but did alter the inter-particle blocking. Finally, it was found that the shape of the sand grains (oblong vs. rounded) significantly alters the bacterial transport. with the transport being dominated by the smallest dimension of the oblong grains.

Alteration of bacterial surface electrostatic potential and pH upon adhesion to a solid surface and impacts to cellular bioenergetics

In our previous study [Hong Y, Brown DG (2009) Appl Environ Microbiol 75(8):2346–2353], the adenosine triphosphate (ATP) level of adhered bacteria was observed to be 2–5 times higher than that of planktonic bacteria. Consequently, the proton motive force (Δp) of adhered bacteria was approximately 15% greater than that of planktonic bacteria. It was hypothesized that the cell surface pH changes upon adhesion due to the charge‐regulated nature of the bacterial cell surface and that this change in surface pH can propagate to the cytoplasmic membrane and alter Δp. In the current study, we developed and applied a charge regulation model to bacterial adhesion and demonstrated that the charge nature of the adhering surface can have a significant effect on the cell surface pH and ultimately the affect the ATP levels of adhered bacteria. The results indicated that the negatively charged glass surface can result in a two‐unit drop in cell surface pH, whereas adhesion to a positively charged amine surface can result in a two‐unit rise in pH. The working hypothesis indicates that the negatively charged surface should enhance Δp and increase cellular ATP, while the positively charged surface should decrease Δp and decrease ATP, and these results of the hypothesis are directly supported by prior experimental results with both negatively and positively charged surfaces. Overall, these results suggest that the nature of charge on the solid surface can have an impact on the proton motive force and cellular ATP levels. Biotechnol. Bioeng. 2010;105: 965–972.

Partitioning of phenanthrene into surfactant hemi-micelles on the bacterial cell surface and implications for surfactant-enhanced biodegradation

Recent studies have suggested that the ability of a surfactant to enhance the bioavailability of hydrophobic organic compounds (HOC) requires the formation of surfactant hemi-micelles on the bacterial cell surface and subsequent partitioning of HOC into the hemi-micelles. However, the studies did not provide direct evidence of HOC partitioning into surfactant hemi-micelles on the bacterial cell surface. In this study, direct evidence is provided to demonstrate that the nonionic surfactant Brij 30 forms hemi-micelles on the bacterial cell surface and that phenanthrene sorption at the bacterial surface is enhanced by the surfactant. These results are in agreement with the current theory describing surfactant-enhanced HOC bioavailability. This enhanced bioavailability is put into context with microbial kinetics and system partitioning processes, and it is demonstrated that the addition of surfactant can enhance, have no effect, or inhibit HOC biodegradation depending upon surfactant concentration and microbial growth rate. Understanding these non-linear relationships between surfactant-enhanced HOC bioavailability, biodegradation kinetics, and system partitioning will assist in the design and implementation of surfactant-enhanced bioremediation programs.

Surfactant-Enhanced Biodegradation of a PAH in Soil Slurry Reactors

This study focuses on finding operational regimes for surfactant-enhanced biodegradation. Biodegradation of phenanthrene as a model poly cyclic aromatic hydrocarbon (PAH) was studied in soil slurry reactors in the presence and absence of a Triton N-101 surfactant solution. Results showed that the presence of surfactant slowed the initial biodegradation rate of phenanthrene, but increased the total mass of phenanthrene degraded over a four day period by 30%. A mathematical model was developed which simulates the biodegradation of low solubility hydrocarbons in the presence of soils and surfactants by accounting for the hydrocarbon bioavailability in different phases of the system. The model was able to simulate the experimental results using parameters and rate coefficients that were obtained through independent experiments. The model was used to investigate the effect of different operating conditions on the overall biodegradation of phenanthrene. Simulation results showed that there is a system-specific ...

Impact of the Charge-Regulated Nature of the Bacterial Cell Surface on the Activity of Adhered Cells

Since the first reported observations by Zobell in 1943, it has been recognized that the metabolic activity of adhered bacteria can differ from that of their planktonic counterparts. Many studies have been performed and the overwhelming evidence is that bacterial adhesion to surfaces can result in changes to cellular metabolic activity and that the changes are a function of the surface properties of both the bacterial and adhering surfaces. However, the mechanism that results in these observations has remained elusive. The authors have approached this problem by focusing on cellular bioenergetics, which describes how bacteria obtain, store and use energy, and how adhesion can affect this process. In a series of experimental and numerical studies, the authors developed a hypothesis linking cellular bioenergetics to the physicochemical charge-regulation effect, which causes variations in surface electrostatic properties as a surface containing acid/base functional groups (e.g., the bacterial cell surface) approaches another surface. The purpose of this paper is to synthesize these prior studies and provide a cohesive presentation of the hypothesis. If this hypothesis is ultimately shown to be true, it will provide a fundamental basis for the engineered design of surface materials and coatings that can enhance or inhibit bacterial activity and colonization, depending on the requirements of the system at hand.

Enhancement of Pervious Concrete Pile Subjected to Uplift Load Using Microbial Induced Carbonate Precipitation

Microbial induced carbonate precipitation (MICP) is considered an environmental-friendly soil improvement technique and laboratory studies have demonstrated that it can improve the soil shear strength and stiffness. In addition to the lab-scale tests, a limited number of field scale tests have been performed for mass soil stabilization using MICP. However, the improvement of soil at the field-scale has encountered practical difficulties including bio-clogging and heterogeneity of CaCO₃ precipitation, which limits the precipitated CaCO₃ to a small zone around the injection points. This paper focuses on verifying limited zone improvement surrounding permeable piles to enhance their axial capacity and improvement of the soil-pile interaction using MICP. To verify this, two pervious concrete pile tests subjected to uplift loading were conducted with and without MICP treatment. The same pervious concrete pile was used and embedded in a small soil box filled with poorly graded sand for the tests. For the MICP-treated test, the treatment solution was percolated from the top of the pile to induce CaCO₃ precipitation at the soil-pile interface. After the treatment, the pile was subjected to an increasing uplift load. The load and displacement at the pile top were measured and the responses from the MICP-treated and untreated tests are compared in this paper.

Measurement of Bonding Strength between Glass Beads Treated by Microbial-Induced Calcite Precipitation (MICP)

Microbial induced calcite precipitation (MICP) is a new soil improvement technique that shows an enhancement of the shear strength and shear wave velocity. However, the mechanical property of the calcite bond at particle-scale level, which controls macroscopic responses of soil, remains unexplored. This paper focuses on the measurement of calcite tensile and shear strength between two glass beads (simulating two sand particles) treated by MICP. The glass beads were mounted on separate movable stages attached by a displacement actuator. Then, MICP treatment was introduced to induce calcite precipitation on the glass beads. After the treatment, the stage was slowly moved sideward or upward to generate tensile or shear force on calcite bond between the two glass beads. The measured ultimate tensile and shear forces were 0.02 N and 1.94 N, respectively. The maximum tensile and shear strength in the calcite bond were 41.1 kPa and 616.5 kPa.