Adhesion of Biodegradative Anaerobic Bacteria to Solid Surfaces

In order to exploit the ability of anaerobic bacteria to degrade certain contaminants for bioremediation of polluted subsurface environments, we need to understand the mechanisms by which such bacteria partition between aqueous and solid phases, as well as the environmental conditions that influence partitioning. We studied four strictly anaerobic bacteria, Desulfomonile tiedjei, Syntrophomonas wolfei, Syntrophobacter wolinii, and Desulfovibrio sp. strain G11, which theoretically together can constitute a tetrachloroethylene- and trichloroethylene-dechlorinating consortium. Adhesion of these organisms was evaluated by microscopic determination of the numbers of cells that attached to glass coverslips exposed to cell suspensions under anaerobic conditions. We studied the effects of the growth phase of the organisms on adhesion, as well as the influence of electrostatic and hydrophobic properties of the substratum. Results indicate that S. wolfei adheres in considerably higher numbers to glass surfaces than the other three organisms. Starvation greatly decreases adhesion of S. wolfei and Desulfovibrio sp. strain G11 but seems to have less of an effect on the adhesion of the other bacteria. The presence of Fe³⁺ on the substratum, which would be electropositive, significantly increased the adhesion of S. wolfei, whereas the presence of silicon hydrophobic groups decreased the numbers of attached cells of all species. Measurements of transport of cells through hydrophobic-interaction and electrostatic-interaction columns indicated that all four species had negatively charged cell surfaces and that D. tiedjei and Desulfovibrio sp. strain G11 possessed some hydrophobic cell surface properties. These findings are an early step toward understanding the dynamic attachment of anaerobic bacteria in anoxic environments.     

Solid–Aqueous Phase Partitioning of Radionuclides by Complexing Compounds Excreted by Subsurface Bacteria

Radionuclides are present in numerous aerobic and anaerobic subsurface environments due to nuclear weapons testing, leakage from process and storage facilities, and discharge of radioactive waste. The partitioning of radionuclides between liquid and solid phases by complexing compounds excreted by subsurface bacteria was studied. The solid–aqueous phase partitioning of pico- to submicromolar amounts of ⁵⁹Fe, ¹⁴⁷Pm, ²³⁴Th, and ²⁴¹Am was analyzed in the presence of quartz sand and exudates from three species of subsurface bacteria: Pseudomonas fluorescens, Pseudomonas stutzeri, and Shewanella putrefaciens. All were grown under aerobic conditions, and P. stutzeri and S. putrefaciens were grown under anaerobic conditions as well. The supernatants of the aerobic and anaerobic cultures were collected and radionuclide was added. Quartz sand, with a Brunauer, Emmett, and Teller (BET) surface area of 0.1 m² g^–1, was added to the supernatant radionuclide mix, and the pH was adjusted to approximately 8. After centrifuging, the amount of radionuclide in the liquid phase of the samples and controls was analyzed using scintillation. Relative to the control, aerobic supernatants maintained more than 50% of the added ⁵⁹Fe, ²³⁴Th, and ²⁴¹Am. The highest amount of metal present in the liquid phase of the anaerobic supernatants was found in the case of ²⁴¹Am, with 40% more ²⁴¹Am in samples than in controls. Both aerobic and anaerobic supernatants tested positive for complexing compounds when analyzed using the Chrome Azurol S assay. The great amounts of radionuclides in the liquid phases of samples were likely due to complexation with such compounds. Bacterially excreted complexing compounds hence seem able to influence the solid–aqueous phase partitioning of radionuclides. This could influence the mobility of radionuclides in contaminated subsurface environments.     

Potential application of aromatic plant extracts to prevent cheese blowing

This study aimed to inhibit the growth of Escherichia coli and Clostridium tyrobutyricum, common bacteria responsible for early and late cheese blowing defects respectively, by using novel aqueous extracts obtained by dynamic solid–liquid extraction and essential oils obtained by solvent free microwave extraction from 12 aromatic plants. In terms of antibacterial activity, a total of 13 extracts inhibited one of the two bacteria, and only two essential oils, Lavandula angustifolia Mill. and Lavandula hybrida, inhibited both. Four aqueous extracts were capable of inhibiting C. tyrobutyricum, but none were effective against E. coli. After extracts’ chemical composition identification, relationship between the identified compounds and their antibacterial activity were performed by partial least square regression models revealing that compounds such as 1,8 cineole, linalool, linalyl acetate, β-phellandrene or verbene (present in essential oils), pinocarvone, pinocamphone or coumaric acid derivate (in aqueous extracts) were compounds highly correlated to the antibacterial activity.     

Downstream processing of extracellular phytase from Aspergillus niger: Chromatography process vs. aqueous two phase extraction for its simultaneous partitioning and purification

The application of single step aqueous two-phase extraction (ATPE) for the downstream processing of phytase from Aspergillus niger NCIM 563, produced under solid state fermentation, has been studied and compared with the traditional multi-step procedure involving salt precipitation and column chromatography. High phytase recovery (98.5%) within a short time (3 h) and improved thermostability was attained by ATPE in comparison to 20% recovery in 96 h by chromatography process. The ATPE method, therefore, seems to be an interesting alternative for simultaneous partitioning and purification of phytase. The influence of system parameters; such as, phase forming salts, polymer molecular weight and system pH on the partitioning behavior of phytase was evaluated. The ATPE system consisting of combination of polyethylene glycol (PEG) 6000 and 8000 (10.5%) and sodium citrate (20.5%) resulted in one-sided partitioning of phytase in bottom phase with a purification factor of 2.5. This is the first report on phytase purification using liquid–liquid extraction and the results are likely to be beneficial in the poultry feed industry.     

Phosphonium ionic liquids for degradation of phenol in a two-phase partitioning bioreactor

Six ionic liquids (ILs), which are organic salts that are liquid at room temperature, were tested for their biocompatibility with three xenobiotic-degrading bacteria, Pseudomonas putida, Achromobacter xylosoxidans, and Sphingomonas aromaticivorans. Of the 18 pairings, seven were found to demonstrate biocompatibility, with one IL (trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl) amide) being biocompatible with all three organisms. This IL was then used in a two-phase partitioning bioreactor (TPPB), consisting of 1 l aqueous phase loaded with 1,580 mg phenol and 0.25 l IL, inoculated with the phenol degrader P. putida. This initially toxic aqueous level of phenol was substantially reduced by phenol partitioning into the IL phase, allowing the cells to utilize the reduced phenol concentration. The partitioning of phenol from the IL to the aqueous phase was driven by cellular demand and thermodynamic equilibrium. All of the phenol was consumed at a rate comparable to that of previously used organic-aqueous TPPB systems, demonstrating the first successful use of an IL with a cell-based system. A quantitative ³¹P NMR spectroscopic assay for estimating the log P values of ILs is under development.     

Biodegradation by an Arthrobacter Species of Hydrocarbons Partitioned into an Organic Solvent

An Arthrobacter strain mineralized naphthalene and n-hexadecane dissolved in 2,2,4,4,6,8,8-heptamethylnonane. The extent of mineralization increased with greater volumes of solvent. Measurements under aseptic conditions of the partitioning of naphthalene into the aqueous phase from the solid phase or from heptamethylnonane showed that the rates were rapid and did not limit mineralization. The rate of mineralization of hexadecane was rapid, although partitioning of the compound into aqueous solution was not detected. The Arthrobacter sp. grown in media with or without heptamethylnonane did not excrete products that increased the aqueous solubility of naphthalene and hexadecane. Measurements of the number of cells in the aqueous phase showed that the Arthrobacter sp. attached to the heptamethylnonane-water interface, but attachment was evident even without a substrate in the heptamethylnonane. Tests with small inocula of the Arthrobacter sp. demonstrated that at least a portion of naphthalene or hexadecane dissolved in heptamethylnonane was degraded by cells attached to the solvent-water interface. The cells did not adhere in the presence of 0.1% Triton X-100. The surfactant prevented mineralization of the hexadecane initially dissolved in heptamethylnonane, but it increased the rate and extent of mineralization of naphthalene initially dissolved in heptamethylnonane. The data show that organic solvents into which hydrophobic compounds partition affect the biodegradation of those compounds and that attachment of microorganisms to the organic solvent-water interface may be important in the transformation.     

Adhesion of an Amylolytic Arthrobacter sp. to Starch-Containing Plastic Films

Cells of the amylolytic bacterium KB-1 (thought to be an Arthrobacter sp.) adhered (~70%) to the surface of plastic films composed of starch-poly (methylacrylate) graft copolymer (starch-PMA), but did not adhere (<10%) to films composed of polymethylacrylate (PMA), polyethylene (PE), carboxymethyl cellulose, or a mixture of PE plus poly (ethylene-coacrylic acid) (EAA), starch plus PE, or starch plus PE and EAA. About 30% of the cells adhered to gelatinized insoluble starch. Dithiothreitol (5 mM), EDTA (5 mM), and soluble starch (1%, wt/vol) had little effect on the adhesion of KB-1 cells to starch-PMA films. However, glutaraldehyde-fixed cells, azide-treated cells, and heat-killed cells did not bind to starch-PMA plastic, suggesting that the observed adhesion required cell viability. Culture supernatant from 5-day-old KB-1 cultures contained a proteolytic enzyme that inhibited cell adhesion to starch-PMA plastics. Trypsin-treated KB-1 cells also lost their ability to bind to starch-PMA plastic. When washed free of trypsin and suspended in fresh medium, trypsin-treated bacteria were able to recover adhesion activity in the absence, but not in the presence, of the protein synthesis inhibitor chloramphenicol. These results suggested that adhesion of KB-1 to starch-PMA plastic may be mediated by a cell surface protein. Although KB-1 bacteria bound to starch-PMA plastic, they did not appear to degrade starch in these films. Evidence of starch degradation was observed for starch-PE-EAA plastics, where <10% of the bacteria was bound, suggesting that cell adhesion may not be a prerequisite for degradation of some starch-containing plastics.     

LPS–protein aggregation influences protein partitioning in aqueous two-phase micellar systems

Lipopolysaccharide endotoxins (LPS) are the most common pyrogenic substances in recombinant peptides and proteins purified from Gram-negative bacteria, such as Escherichia coli. In this respect, aqueous two-phase micellar systems (ATPMS) have already proven to be a good strategy to purify recombinant proteins of pharmaceutical interest and remove high LPS concentrations. In this paper, we review our recent experimental work in protein partitioning in Triton X-114 ATPMS altogether with some new results and show that LPS–protein aggregation can influence both protein and LPS partitioning. Green fluorescent protein (GFPuv) was employed as a model protein. The ATPMS technology proved to be effective for high loads of LPS removal into the micelle-rich phase (%REM_LPS?>?98 %) while GFPuv partitioned preferentially to the micelle-poor phase (K _GFPuv?<?1.00) due to the excluded-volume interactions. However, theoretically predicted protein partition coefficient values were compared with experimentally obtained ones, and good agreement was found only in the absence of LPS. Dynamic light scattering measurements showed that protein–LPS interactions were taking place and influenced the partitioning process. We believe that this phenomenon should be considered in LPS removal employing any kind of aqueous two-phase system. Nonetheless, ATPMS can still be considered as an efficient strategy for high loads of LPS removal, but being aware that the excluded-volume partitioning theory available might overestimate partition coefficient values due to the presence of protein–LPS aggregation.     

Antimicrobial activity in the egg wax of the African cattle tick Amblyomma hebraeum (Acari: Ixodidae)

Eggs of the tick Amblyomma hebraeum Koch (Acari: Ixodidae) inhibited the growth of Escherichia coli and Serratia marcescens (Gram-negative bacteria) in solid culture, but not the growth of Staphylococcus epidermidis, and only marginally the growth of Bacillus subtilis (Gram-positive bacteria). When egg wax was extracted with chloroform/methanol (2:1), the extract contained antibacterial activity, but the denuded eggs did not. When assayed against bacteria in liquid culture, the aqueous phase inhibited the growth of S. epidermidis. However, the activity against E. coli was lost during extraction. The antimicrobial component of the aqueous phase was heat stable (100°C for 10 min), resistant to proteinase K (15 min at 55°C) and to pronase (30 min at 37°C). The antibacterial activity in the aqueous phase increased the permeability of the cell membrane of susceptible bacterial cells within 30 min. However, lysis of the cells was detected by optical density measurements (OD_{600 nm}) only after 1.5 h. The most evident cytological changes observed by transmission electron microscopy were a thickening of the cell wall and the appearance of numerous electron lucent areas within the cytoplasm of treated bacteria. Gené’s organ, the egg-waxing organ in ticks, grew enormously during the first 16 days post-engorgement, and gained antimicrobial activity by day 10 (when oviposition began). This suggests that Gené’s organ is the major source of the antibacterial substance in the egg wax. The vitellogenic hormone in A. hebraeum, 20-hydroxyecdysone, when injected into recently engorged females, did not stimulate growth of Gené’s organ or precocious secretion of antimicrobial activity.     

CTP-controlled liquid–liquid phase separation of ParB

The ParABS system is supposed to be responsible for plasmid partitioning and chromosome segregation in bacteria. ParABS ensures a high degree of fidelity in inheritance by dividing the genetic material equally between daughter cells during cell division. However, the molecular mechanisms underlying the assembly of the partition complex, representing the core of the ParABS system, are still far from being understood. Here we demonstrate that the partition complex is formed via liquid–liquid phase separation. Assembly of the partition complex is initiated by the formation of oligomeric ParB species, which in turn are regulated by CTP-binding. Phase diagrams and in vivo analysis show how the partition complex can further be spatially regulated by parS. By investigating the phylogenetic variation in phase separation and its regulation by CTP, we find a high degree of evolutionary conservation among distantly related prokaryotes. These results advance the understanding of partition complex formation and regulation in general, by confirming and extending recently proposed models.     

Synthesis of octapeptin C4 and biological profiling against NDM-1 and polymyxin-resistant bacteria

The first synthesis of octapeptin C4 was achieved using a combination of solid phase synthesis and off-resin cyclisation. Octapeptin C4 displayed antibiotic activity against multi-drug resistant, NDM-1 and polymyxin-resistant Gram-negative bacteria, with moderate activity against Staphylococcus aureus. The linear analogue of octapeptin C4 was also prepared, which showed reduced activity.     

Determination of stoichiometry of radioiodination of proteins

A sensitive method for the detection of small quantities of hydrophobic antioxidant free radical scavengers such as butylatedhydroxytoluene (BHT) and butylatedhydroxyanisole (BHA) in aqueous samples is described. The procedure involves extraction of the hydrophobic free radical scavenger into an organic solvent phase, followed by the subsequent reaction of an aliquot of this extract with the stable cation radical tris(p-bromophenyl)amminium hexachloroantimonate (TBACA). In experiments with BHT and BHA, the loss of TBACA absorbance at 730 nm was found to be linearly proportional to the amount of antioxidant added, with quantities of BHT as small as 200 pmol being easily detectable. In aqueous suspensions of dimyristoylphosphatidylcholine vesicles, assays of the aqueous BHT concentration showed that BHT partitioned strongly into the membrane phase, achieving very high BHT/phospholipid ratios. For a given concentration of BHT, partitioning into the membrane phase was greater in large, multilamellar liposomes than in either small, single-walled vesicles or in purified rat brain synaptic vesicle membranes. Direct assay of BHT and BHA in phospholipid membranes, however, was complicated by a nonspecific interaction between TBACA and the phospholipid.     

In Vitro and In Vivo Model to Study Bacterial Adhesion to the Vessel Wall Under Flow Conditions

In order to cause endovascular infections and infective endocarditis, bacteria need to be able to adhere to the vessel wall while being exposed to the shear stress of flowing blood.To identify the bacterial and host factors that contribute to vascular adhesion of microorganisms, appropriate models that study these interactions under physiological shear conditions are needed. Here, we describe an in vitro flow chamber model that allows to investigate bacterial adhesion to different components of the extracellular matrix or to endothelial cells, and an intravital microscopy model that was developed to directly visualize the initial adhesion of bacteria to the splanchnic circulation in vivo. These methods can be used to identify the bacterial and host factors required for the adhesion of bacteria under flow. We illustrate the relevance of shear stress and the role of von Willebrand factor for the adhesion of Staphylococcus aureus using both the in vitro and in vivo model.     

Incorporation of 32P and Growth of Pseudomonad UP-2 on n-Tetracosane

Cultures of the marine pseudomonad UP-2 growing on n-tetracosane contained both free cells and cells bound to the solid hydrocarbon. After separation by filtration through a Whatman no. 1 filter, the numbers of free and bound cells were estimated from the amount of ³²P incorporated into each fraction and the determined value of ³²P incorporation per viable cell in the filtrate (free cells). During the early exponential growth phase, over 80% of the cells were bound to large pieces of n-tetracosane; as the culture approached the stationary phase, the number of bound cells remained constant, whereas free cells continued to accumulate. Pulse-labeling experiments indicated that cells grew both on the surface of the solid and in the aqueous medium. During the growth cycle, a portion of the n-tetracosane which was initially nonfilterable was recovered in the filtrate in a form which was largely cell associated. This cell-associated n-tetracosane was preferentially utilized and could completely account for the observed growth of free cells.     

Partitioning of acidic, basic and neutral amino acids into imidazolium-based ionic liquids

In this paper, partitioning behaviors of typical neutral (Alanine), acidic (Glutamic acid) and basic (Lysine) amino acids into imidazolium-based ionic liquids [C₄mim][PF₆], [C₆mim][PF₆], [C₈mim][PF₆], [C₆mim][BF₄] and [C₈mim][BF₄] as extracting solvents were examined. [C₆mim][BF₄] showed the best efficiency for partitioning of amino acids. The partition coefficients of amino acids in ionic liquids were found to depend strongly on pH of the aqueous solution, amino acid and ionic liquid chemical structures. Different chemical forms of amino acids in aqueous solutions were pH dependent, so the pH value of the aqueous phase was a determining factor for extraction of amino acids into ionic liquid phase. Both water content of ionic liquids and charge densities of their anionic and cationic parts were important factors for partitioning of cationic and anionic forms of amino acids into ionic liquid phase. Extracted amino acids were back extracted into phosphate buffer solutions adjusted on appropriate pH values. The results showed that ionic liquids could be used as suitable modifiers on the stationary phase of an HPLC column for efficient separation of acidic, basic, and neutral amino acids.     

Polymer selection for biphenyl degradation in a solid-liquid two-phase partitioning bioreactor

The commercially available thermoplastic polymer Hytrel was selected as the delivery phase for the hydrophobic model compound biphenyl in a solid-liquid two-phase partitioning bioreactor (TPPB), and 2.9 g biphenyl could successfully be degraded in 1-L TPPBs by a pure culture of the biphenyl-degrading bacterium Burkholderia xenovorans LB400 in 50 h and by a mixed microbial consortium isolated from contaminated soil in 45 h. TPPBs consist of an aqueous cell-containing phase and an immiscible second phase that partitions toxic and/or poorly soluble substrates (in this case biphenyl) on the basis of maintaining a thermodynamic equilibrium. This paper illustrates a rational strategy for selecting a suitable solid polymeric substance for the delivery of the poorly water-soluble model compound biphenyl. The partitioning of biphenyl between the selected polymers and water was analogous to partitioning of solutes between two immiscible liquid phases. The partitioning coefficients varied between 180 for Nylon 6.6 and 11,000 for Desmopan, where the later numerical value is comparable to biphenyl partitioning coefficients between water and organic solvents. Employing a solid delivery phase enabled the utilization of a surfactant-producing microbial mixed culture, which could not be cultivated in liquid-liquid TPPBs and thereby extended the range of biocatalysts that can be employed in TPPBs.     

Construction and Analysis of Fractional Multifactorial Designs To Study Attachment Strength and Transfer of Listeria monocytogenes from Pure or Mixed Biofilms after Contact with a Solid Model Food

The aim of this study was to establish which of seven factors influence the adhesion strength and hence bacterial transfer between biofilms containing Listeria monocytogenes (pure and two-species biofilms) and tryptone soya agar (TSA) as a solid organic surface. The two-species biofilms were made of L. monocytogenes and one of the following species of bacteria: the nonpathogenic organisms Kocuria varians, Pseudomonas fluorescens, and Staphylococcus sciuri and CCL 63, an unidentified gram-negative bacterium isolated from the processing plant environment. We used biofilms prepared under conditions simulating open surfaces in meat-processing sites. The biofilm's adhesion strength and population were evaluated by making 12 contacts on a given whole biofilm (4.5 cm²), using a new slice of a sterilized TSA cylinder for each contact, and plotting the logarithm CFU · cm⁻² detached by each contact against the contact number. Three types of detachment kinetics were observed: biphasic kinetics, where the first slope may be either positive or negative, and monophasic kinetics. The bacteria that resisted a chlorinated alkaline product and a glutaraldehyde- and quaternary ammonium-based disinfectant had greater adhesion strengths than those determined for untreated biofilms. One of the four non-Listeria strains studied, Kocuria varians CCL 56, favored both the attachment and detachment of L. monocytogenes. The stainless steel had smaller bacterial populations than polymer materials, and non-Listeria bacteria adhered to it less strongly. Our results helped to evaluate measures aimed at controlling the immediate risk, linked to the presence of a large number of CFU in a foodstuff, and the delayed risk, linked to the persistence of L. monocytogenes and the occurrence of slightly contaminated foods that may become dangerous if L. monocytogenes multiplies during storage. Cleaning and disinfection reduce the immediate risk, while reducing the delayed risk should be achieved by lowering the adhesion strength, which the sanitizers used here cannot do at low concentrations.     

Investigation of energy gene expressions and community structures of free and attached acidophilic bacteria in chalcopyrite bioleaching

In order to better understand the bioleaching mechanism, expression of genes involved in energy conservation and community structure of free and attached acidophilic bacteria in chalcopyrite bioleaching were investigated. Using quantitative real-time PCR, we studied the expression of genes involved in energy conservation in free and attached Acidithiobacillus ferrooxidans during bioleaching of chalcopyrite. Sulfur oxidation genes of attached A. ferrooxidans were up-regulated while ferrous iron oxidation genes were down-regulated compared with free A. ferrooxidans in the solution. The up-regulation may be induced by elemental sulfur on the mineral surface. This conclusion was supported by the results of HPLC analysis. Sulfur-oxidizing Acidithiobacillus thiooxidans and ferrous-oxidizing Leptospirillum ferrooxidans were the members of the mixed culture in chalcopyrite bioleaching. Study of the community structure of free and attached bacteria showed that A. thiooxidans dominated the attached bacteria while L. ferrooxidans dominated the free bacteria. With respect to available energy sources during bioleaching of chalcopyrite, sulfur-oxidizers tend to be on the mineral surfaces whereas ferrous iron-oxidizers tend to be suspended in the aqueous phase. Taken together, these results indicate that the main role of attached acidophilic bacteria was to oxidize elemental sulfur and dissolution of chalcopyrite involved chiefly an indirect bioleaching mechanism.     

Aqueous biphasic systems composed of ionic liquids and sodium carbonate as enhanced routes for the extraction of tetracycline

Aqueous biphasic systems (ABS) using ionic liquids (ILs) offer an alternative approach for the extraction, recovery, and purification of biomolecules through their partitioning between two aqueous liquid phases. In this work, the ability of a wide range of ILs to form ABS with aqueous solutions of Na₂CO₃ was evaluated. The ABS formed by IL + water + Na₂CO₃ were determined at 25°C, and the respective solubility curves, tie‐lines, and tie‐line lengths are reported. The studied ILs share the common chloride anion, allowing the IL cation core, the cation isomerism, the presence of functionalized groups, and alkyl side chain length effects to be evaluated. An increase in the cation side alkyl chain length leads to a higher ability for liquid–liquid demixing whereas different positional isomers and the presence of an allyl group have no major influence in the phase diagrams behavior. Quaternary phosphonium‐ and ammonium‐based fluids are more able to form an ABS when compared with imidazolium‐, pyridinium‐, pyrrolidinium‐, and piperidium‐based ILs. Moreover, the presence of an aromatic cation core has no major contribution to the formation of ABS when compared to the respective nonaromatic counterparts. Finally, to appraise on the systems applicability in downstream processing, selected systems were used for the partitioning of tetracyclines (neutral and salt forms) — a class of antibiotics produced by bacteria fermentation. Single‐step extraction efficiencies for the IL‐rich phase were always higher than 99% and confirm the great potential of ILs to be applied in the biotechnological field. © 2013 American Institute of Chemical Engineers Biotechnol. Prog., 29:645–654, 2013     

Dispersal dynamics of groundwater bacteria

Dispersal of bacteria in saturated, porous soils can be characterized by the partitioning of cells between the aqueous and solid phases, as a result of the physical and chemical nature of the soil and water and cell surface modifications. The purpose of this work is to understand variations in partitioning as a consequence of the nutrient conditions and to use this information in mathematical models to predict the dispersal rate of bacteria in aquifer material. Two different models were used to describe dispersal: an advective-dispersive-sorptive model with a first order kinetic sink term to account for irreversible cell reactions, such as death and sorption; and a two-site reaction model, in which the retardation was assumed to be determined by two types of sites, one characterized by instantaneous equilibrium sorption reactions and the other by kinetic nonequilibrium reactions. Water-saturated sand columns were used as continuous-flow groundwater microcosms to test the models under different nutrient regimes. Two strains of indigenous groundwater bacteria were isolated from aquifer material and labelled with³H-alanine,¹⁴C-pyruvic acid,³H-glucose, and³H-adenosine for different measurements of sorption and dispersal, which were estimated from breakthrough curves. Both experimental data and model variables showed that dispersal of bacteria was a dynamic nonequilibrium process, possibly shaped by two subpopulations, one strongly, even irreversibly, adsorbing to the solid particles, and one with very slow adsorption kinetics. The cell surfaces were modified in response to the growth conditions, which was demonstrated by hydrophobic and electrostatic interaction chromatography. Cell surface hydrophobicity was about eight times higher in groundwater than in eutrophic lake water. The partition coefficient varied between 12.6 in the groundwater and 6.4 in the lake water, indicating the prime importance of hydrophobic binding for attachment in low nutrient conditions. The partitioning was also sensitive to the hydrodynamics of the system and the oxygen supply, as demonstrated by comparison of sorption in agitated test tubes, gently shaken vials, and air-flushed bottles. Sorption kinetics were demonstrated in a continuous flow cell. About 45% of a population was associated with sand particles with a continuous flow of pure groundwater and as little as 20% in lake water. However, more than 50% of the bacteria in the aqueous phase were associated with suspended material of less than 60 μm in diameter. This association may enhance dispersal for example, by size exclusion of the colloidal material in the interstitial pores.