Alexander J. B. Zehnder

Bacterial adhesion: A physicochemical approach.

The adhesion of bacteria to solid surfaces was studied using a physicochemical approach. Adhesion to negatively charged polystyrene was found to be reversible and could be described quantitatively using the DLVO theory for colloidal stability, i.e., in terms of Van der Waals and electrostatic interactions. The influence of the latter was assessed by varying the electrolyte strength. Adhesion increased with increasing electrolyte strength. The adhesion Gibbs energy for a bacterium and a negatively charged polystyrene surface was estimated from adhesion isotherms and was found to be 2–3 kT per cell. This low value corresponds to an adhesion in the secondary minimum of interaction as described by the DLVO theory. The consequences of these findings for adhesion in the natural environment are discussed.

Dehalobacter restrictus gen. nov. and sp. nov., a strictly anaerobic bacterium that reductively dechlorinates tetra-and trichloroethene in an anaerobic respiration

Abstract The highly enriched anaerobic bacterium that couples the reductive dechlorination of tetrachloroethene to growth, previously referred to as PER-K23, was obtained in pure culture and characterized. The bacterium, which does not form spores, is a small, gram-negative rod with one lateral flagellum. It utilized only H2 as an electron donor and tetrachloroethene and trichloroethene as electron acceptors in an anaerobic respiration process; it could not grow fermentatively. Acetate served as a carbon source in a defined medium containing iron as the sole trace element, the two vitamins thiamine and cyanocobalamin, and the three amino acids arginine, histidine, and threonine. The cells contained menaquinones and b-type cytochromes. The G+C content of the DNA was 45.3 ± 0.3 mol%. The cell wall consisted of type-A3γ peptidoglycan with ll-diaminopimelic acid and one glycine as an interpeptide bridge. The cells are surrounded by an S-layer; an outer membrane was absent. Comparative sequence analysis of the 16S rRNA sequence showed that PER-K23 is related to gram-positive bacteria with a low G+C content of the DNA. Based on the cytological, physiological, and phylogenetic characterization, it is proposed to affiliate the isolate to a new genus, Dehalobacter, with PER-K23 as the type strain of the new species Dehalobacter restrictus.

Reversibility and mechanism of bacterial adhesion

The reversibility and mechanisms of adhesion of various pseudomonads and coryneform bacteria having different hydrophobicities and negative cell surface charges on negatively charged Teflon and glass were studied. Adhesion at an ionic strength of 0.1 M was irreversible and corresponded to activation Gibbs energies for detachment higher than 5 kT for 19 out of 20 combinations of bacterial strains and surfaces. The data further demonstrate the importance of two groups of interactions: (i) the electrostatic and van der Waals interactions as described by the DLVO model, and (ii) the interactions between the outer cell surface macromolecules and the solids (steric interactions). At an ionic strength of 0.1 M, steric interactions control adhesion for all but two bacterium/substratum combinations tested. These interactions are attractive for seven moderately to strongly hydrophobic strains on Teflon and prevented detachment upon decreasing the ionic strength to less than 0.0001 M and also after applying shear forces. In contrast, steric interactions inhibited adhesion for more hydrophilic bacterium/substratum combinations for which detachment occurred upon reducing the ionic strength to less than 0.0001 M and/or after applying shear. The importance of the interactions included by the DLVO model is demonstrated by the following. (i) Two hydrophobic strains adhere irreversibly on glass by strong van der Waals attraction in a secondary DLVO minimum at an ionic strength of 0.1 M and detach when the ionic strength is reduced to less than 0.0001 M. (ii) Electrostatic repulsion inhibits deposition at lower ionic strength. The practical implications of these findings are discussed.

Hydrophobic and electrostatic parameters in bacterial adhesion

Recently it has been shown that the initial stages of bacterial adhesion to a model-surface of sulphated polystyrene can best be described using hydrophobic and electrostatic parameters. In the present study it is tested whether these parameters can generally be applied to predict bacterial adhesion by using (i) glass, as a model for hydrophilic and natural surfaces of silicates and oxides, (ii) polystyrene coated with proteins, as a model for a surface coated with an organic layer, and (iii) river Rhine sediment, as an example of a natural surface. Adhesion to glass was dominated by electrostatic interaction, whereas adhesion to polystyrene coated with various types of proteins depended on the surface characteristics of the bacteria and the type of protein. By relating Van der Waals interactions to hydrophobicity of the interacting species, the adhesion of bacteria to the various surfaces including the river Rhine sediments could be interpreted in terms of the DLVO-theory. It is therefore concluded that the conceptual principles of the DLVO-theory (interplay of Van der Waals and electrostatic interactions) are suitable to describe, at least qualitatively, the initial processes of bacterial adhesion to a wide range of surfaces.

DLVO and steric contributions to bacterial deposition in media of different ionic strengths

The deposition of eight bacterial strains on Teflon and glass in aqueous media with ionic strengths varying between 0.0001 and 1 M was measured and interpreted. Two types of interactions were considered: (1) those described by the DLVO theory, which comprise van der Waals attraction and electrostatic repulsion (bacteria and surfaces are both negatively charged); and (2) steric interactions between the outer cell surface macromolecules and the substrata. As a trend, at low ionic strength (<0.001 M), deposition is inhibited by DLVO-type electrostatic repulsion, but at high ionic strength (≥0.1 M) it is dominated by steric interactions. The ionic strength at which the transition from the DLVO-controlled to the sterically controlled deposition occurs, is determined by the extension of the macromolecules into the surrounding medium, which varied between 5 and 100 nm among the bacterial strains studied. The steric interactions either promote deposition by bridging or inhibit it by steric repulsion. Between Teflon and hydrophobic bacteria, bridging is generally observed. The surface of one bacterial strain contains amphiphilic macromolecules that form bridges with Teflon but induce steric repulsion on glass. The presence of highly polar anionic polysaccharide coatings on the cell impedes attachment on both glass and Teflon. For practice, the general conclusion is that the deposition of most bacteria is: (1) strongly inhibited by DLVO-type electrostatic repulsion in aqueous environments of low ionic strength such as rain water, streams and lakes; (2) controlled by DLVO and/or steric interactions in systems as domestic waste waters and saliva; and (3) determined by steric interactions only in more saline environments as milk, urine, blood and sea water.

Water scarcity, pricing mechanism and institutional reform in northern China irrigated agriculture

Abstract With water scarcity becoming an increasing constraint to food production in northern China, pricing mechanism has been given a high priority in dealing with the problem. Using selected irrigation districts in northern China as a case study, this paper probes the effectiveness of pricing-based water policies in addressing challenges facing irrigated agriculture under China’s current water management institutions. The examination shows that the rapid increase in irrigation cost during the past decade has failed to generate a force for water conservation. Over-exploitation of groundwater resources has even intensified with the shift to higher value-added but often more water intensive crops. Based on a normative analysis of water demand curves, the logic behind the reluctance for water authorities and farmers to conserve water is elaborated. The result suggests that pricing irrigation alone is not a valid means of encouraging water conservation under the current irrigation management institutions. Clearly defined and legally enforceable water rights and responsibilities for water operators and users in the irrigation system are the foundation underlying the incentives for conserving water and improving the irrigation efficiency.

The isoelectric point of bacteria as an indicator for the presence of cell surface polymers that inhibit adhesion.

Abstract The use of the isoelectric point (IEP) of a bacterium as a measure of the ability of bacterial surface polymers to inhibit adhesion was tested. This inhibition is attributed to repulsive steric interactions and not to electrostatic repulsion as accounted for by the DLVO theory of colloid stability. IEP values were compared with literature data on cell wall composition and with adhesion results, obtained at pH 7 and an ionic strength of 0.1 M. The literature data demonstrate that an IEP ⩽ 2.8 indicates the presence of significant amounts of cell surface polysaccharides containing negatively charged phosphate and/or carboxyl groups. The experimental results showed that these polymers inhibit adhesion onto both hydrophilic (glass) and hydrophobic (Teflon) surfaces. The coryneform Rhodococcus strain C125 with an IEP of 3.0 possesses amphiphilic cell surface components which inhibit adhesion onto glass and promote deposition onto Teflon. Bacteria with an IEP ⩾ 3.2 appear to be free from polymer coatings that inhibit adhesion. They adhere in large amounts onto Teflon and in slightly lower amounts onto glass. Our findings therefore indicate that the IEP is a suitable parameter complementary to hydrophobicity in predicting the affinity of bacterial surface polymers for substrata with different hydrophobicities.

Bioremediation of organic compounds — putting microbial metabolism to work

Microorganisms can metabolize many aliphatic and aromatic organic contaminants, either to obtain carbon and/or energy for growth, or as co-substrates, thus converting them to products such as carbon dioxide, water, chloride and biomass. These biotransformations can be exploited for treatment of contaminated soils and ground water.

Determination of the total charge in the cell walls of Gram-positive bacteria

The charge in the bacterial wall originates from the dissociation of acidic groups such as carboxyl, phosphate and amino groups. The degree of dissociation of these chargeable groups is a function of the pH and the activity of the surrounding electrolyte solution. In this study the cell wall charge density of Gram-positive bacterial strains, including four coryneforms and a Bacillus brevis, is assessed by proton titrations of whole bacterial cells and isolated cell walls at different electrolyte concentrations. At neutral pH rather high values, between 0.5 and 1.0 C m−2, for the cell wall surface charge density are found. The titration curves for the isolated cell walls are free of hysteresis allowing a rigorous (thermodynamic) analysis. For the coryneform bacteria these curves have a common intersection point between pH 3 and 4, which is identified as the point of zero charge. The carboxyl and phosphate groups are titrated in distinct pH regions, allowing accurate estimation of their numbers. These numbers compare very well with those based on a chemical analysis of the isolated cell walls. The uncertainty in the estimated number of amino groups is somewhat higher, because these groups are only partly titrated within the pH range accessible by proton titrations. At electrolyte concentrations below 0.01 M maximum expulsion of co-ions from the cell walls already occurs at relatively low charge densities. At these low electrolyte concentrations the compensating countercharge predominantly consists of counterions that penetrate into the porous cell wall matrix and to a much lower and constant extent by the exclusion of co-ions.

Community Analysis of Ammonia and Nitrite Oxidizers during Start-Up of Nitritation Reactors

ABSTRACT Partial nitrification of ammonium to nitrite under oxic conditions (nitritation) is a critical process for the effective use of alternative nitrogen removal technologies from wastewater. Here we investigated the conditions which promote establishment of a suitable microbial community for performing nitritation when starting from regular sewage sludge. Reactors were operated in duplicate under different conditions (pH, temperature, and dilution rate) and were fed with 50 mM ammonium either as synthetic medium or as sludge digester supernatant. In all cases, stable nitritation could be achieved within 10 to 20 days after inoculation. Quantitative in situ hybridization analysis with group-specific fluorescent rRNA-targeted oligonucleotides (FISH) in the different reactors showed that nitrite-oxidizing bacteria of the genus Nitrospira were only active directly after inoculation with sewage sludge (up to 4 days and detectable up to 10 days). As demonstrated by quantitative FISH and restriction fragment length polymorphism (RFLP) analyses of the amoA gene (encoding the active-site subunit of the ammonium monooxygenase), the community of ammonia-oxidizing bacteria changed within the first 15 to 20 days from a more diverse set of populations consisting of members of the Nitrosomonas communis and Nitrosomonas oligotropha sublineages and the Nitrosomonas europaea-Nitrosomonas eutropha subgroup in the inoculated sludge to a smaller subset in the reactors. Reactors operated at 30°C and pH 7.5 contained reproducibly homogeneous communities dominated by one amoA RFLP type from the N. europaea-N. eutropha group. Duplicate reactors at pH 7.0 developed into diverse communities and showed transient population changes even within the ammonia oxidizer community. Reactors at pH 7.5 and 25°C formed communities that were indistinguishable by the applied FISH probes but differing in amoA RFLP types. Communities in reactors fed with sludge digester supernatant exhibited a higher diversity and were constantly reinoculated with ammonium oxidizers from the supernatant. Therefore, such systems could be maintained at a higher dilution rate (0.75 day−1 compared to 0.2 day−1 for the synthetic wastewater reactors). Despite similar reactor performance with respect to chemical parameters, the underlying community structures were different, which may have an influence on stability during perturbations.