Biotransformation is controlled by the biochemical activity of microorganisms and the mass transfer of a chemical to the microorganisms. A generic mathematical concept for bioavailability is presented taking both factors into account. The combined effect of mass transfer of a substance to the cell and the intrinsic activity of the cell using the substance as primary substrate, is quantified in a bioavailability number (Bn). The concept can easily be extended to secondary substrates. The approach has been applied to explain the observed kinetics of the biotransformation of organic compounds in soil slurries and in percolation columns. The model allowed us to predict threshold concentrations below which no biotransformation is possible. Depending on the environmental system and the chemical involved, predicted threshold concentrations span a range of 11 orders of magnitude from nanograms to grams per liter and match with published experimental data. Mass transferand not the intrinsic microbial activityis in most cases the critical factor in bioremediation.

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 of 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.

Organic pollutants in soil can be removed by biotechnological treatment. A limitation of this technology is the efficiency of biodegradation. In many cases, the bulk of the pollution can be removed but residual pollutants remain and biodegradation rates are slower than expected from laboratory trials. Low biodegradation rates are often a result of limited accessibility of the pollutants. Major reasons for the reduced bioavailability are the unequal spatial distribution of microorganisms and pollutants and the retardation of substrate diffusion by the soil matrix. Mechanical mixing and the addition of surfactants are possible approaches to improve the bioavailability of pollutants during bioremediation. The application of flow-stop-flow techniques may be of help to overcome the limitations resulting from advective-diffusive transport mechanisms during pump-and-treat remediation of contaminant plumes.

SummaryNeisseria meningitidis is a common and usually harmless inhabitant of the mucosa of the human nasopharynx, which, in rare cases, can cross the epithelial barrier and cause meningitis and sepsis. Biofilm formation favours the colonization of the host and the subsequent carrier state. Two different strategies of biofilm formation, either dependent or independent on extracellular DNA (eDNA), have been described for meningococcal strains. Here, we demonstrate that the autotransporter protease NalP, the expression of which is phase variable, affects eDNA-dependent biofilm formation in N. meningitidis. The effect of NalP was found in biofilm formation under static and flow conditions and was dependent on its protease activity. Cleavage of the heparin-binding antigen NhbA and the a-peptide of IgA protease, resulting in the release of positively charged polypeptides from the cell surface, was responsible for the reduction in biofilm formation when NalP is expressed. Both NhbA and the a-peptide of IgA protease were shown to bind DNA. We conclude that NhbA and the a-peptide of IgA protease are implicated in biofilm formation by binding eDNA and that NalP is an important regulator of this process through the proteolysis of these surface-exposed proteins.

The need to understand important factors affecting the spread of bacteria in groundwater aquifers is evident for fields as diverse as drinking water safety or environmental engineering concerned with bioremediation of polluted sites. For example, increasing concentrations of dissolved minerals tend to increase the deposition efficiency of bacteria in porous media. As bacteria and mineral surfaces are mostly negatively charged, this is generally assumed to be a consequence of the higher ionic strength, which leads to stronger shielding of the surface charges by the counterion-cloud in solution. We found Mg 2+ to enhance deposition of Pseudomonas sp. strain B13 in sand columns with respect to a solution of identical ionic strength containing Na + . Hence bivalent cations are likely to affect microbial deposition specifically, for example due to specific binding to the cell surface. Moreover, low concentrations of Pb 2+ or Cu 2+ reverted the surface potential of strain B13, thus providing additional evidence for this hypothesis. Recently, we showed strain B13 to split up in a well-adhering and in a nonadhering subpopulation. In experiments conducted with Mg 2+ and Na + at various ionic strength, bivalent cations seemed to increase the welladhering subpopulation as well as its adhesion efficiency.

All three isomers of trichlorobenzene were reductively dechlorinated to monochlorobenzene via dichlorobenzenes in anaerobic sediment columns. The dechlorination was specific: 1,2,3‐ and 1,3,5‐trichlorobenzene were solely transformed to 1,3‐dichlorobenzene, while 1,4‐dichlorobenzene was the only product of 1,2,4‐trichlorobenzene transformation. Microorganisms were responsible for the observed transformations. Since monochlorobenzene and dichlorobenzene are mineralized by bacteria in the presence of oxygen, the process of reductive dechlorination may be an important initial step to obtain complete mineralization of otherwise recalcitrant trichlorobenzenes. This is especially true for the 1,3,5‐isomer, which seems to resist biodegradation in oxic environments.

The dissolved hydrogen concentrations under various redox processes were investigated based on batch experiments. Chloroethenes including tetrachloroethene (PCE), cis-dichloroethene (cis-DCE) and vinylchloride (VC) were respectively used as culture substrates. For each chloroethene, a series of bottles were prepared with the additions of different electron acceptors or donors such as nitrate, manganese oxide, ferrous iron, sulfate, carbondioxide and volatile fatty acids. Hydrogen concentrations as well as redox species were measured over time to ensure the achievements of characteristic hydrogen levels in various enrichment batches. The results showed that redox processes with nitrate, manganese oxide and ferric iron as the electron acceptors exhibited hydrogen threshold values close to PCE/TCE dechlorination, whereas cis-DCE and VC dechlorinations exhibited hydrogen threshold values in the range of sulfate reduction and methanogenesis, respectively. Characteristic hydrogen concentrations for various redox processes were as follows (nM): denitrification, 0.1-0.4; manganese reduction, 0.1-2.0; iron reduction, 0.1-0.4; sulfate reduction, 1.5-4.5; methanogenesis, 2.5-24; PCE/TCE dechlorination, 0.6-0.9; eis-DCE dechlorination, 0.1-2.5; and VC dechlorination, 2-24.

No abstract