Prokaryotic biofilms that predominate in a diverse range of ecosystems are often composed of highly structured multispecies communities. Within these communities metabolic activities are integrated, and developmental sequences, not unlike those of multicellular organisms, can be detected. These structural adaptations and interrelationships are made possible by the expression of sets of genes that result in phenotypes that differ profoundly from those of planktonically grown cells of the same species. Molecular and microscopic evidence suggest the existence of a succession of de facto biofilm phenotypes. We submit that complex cell-cell interactions within prokaryotic communities are an ancient characteristic, the development of which was facilitated by the localization of cells at surfaces. In addition to spatial localization, surfaces may have provided the protective niche in which attached cells could create a localized homeostatic environment. In a holistic sense both biofilm and planktonic phenotypes may be viewed as integrated components of prokaryote life.

Biofilms as complex differentiated communities.

Extracellular polymeric substances (EPS) of microbial aggregates in biological wastewater treatment systems: a review.

An increasing body of evidence suggests that microorganisms are far more sensitive to heavy metal stress than soil animals or plants growing on the same soils. Not surprisingly, most studies of heavy metal toxicity to soil microorganisms have concentrated on effects where loss of microbial function can be observed and yet such studies may mask underlying effects on biodiversity within microbial populations and communities. The types of evidence which are available for determining critical metal concentrations or loadings for microbial processes and populations in agricultural soil are assessed, particularly in relation to the agricultural use of sewage sludge. Much of the confusion in deriving critical toxic concentrations of heavy metals in soils arises from comparison of experimental results based on short-term laboratory ecotoxicological studies with results from monitoring of long-term exposures of microbial populations to heavy metals in field experiments. The laboratory studies in effect measure responses to immediate, acute toxicity (disturbance) whereas the monitoring of field experiments measures responses to long-term chronic toxicity (stress) which accumulates gradually. Laboratory ecotoxicological studies are the most easily conducted and by far the most numerous, but are difficult to extrapolate meaningfully to toxic effects likely to occur in the field. Using evidence primarily derived from long-term field experiments, a hypothesis is formulated to explain how microorganisms may become affected by gradually increasing soil metal concentrations and this is discussed in relation to defining “safe” or “critical” soil metal loadings for soil protection.

Toxicity of heavy metals to microorganisms and microbial processes in agricultural soils: a review.

Recent advances in membrane bioreactors (MBRs): membrane fouling and membrane material.

This article reviews the mechanisms of bacterial adhesion to biomaterial surfaces, the factors affecting the adhesion, the techniques used in estimating bacteria-material interactions and the models that have been developed in order to predict adhesion. The process of bacterial adhesion includes an initial physicochemical interaction phase and a late molecular and cellular one. It is a complicated process influenced by many factors, including the bacterial properties, the material surface characteristics, the environmental factors, such as the presence of serum proteins and the associated flow conditions. Two categories of techniques used in estimating bacteria-material interactions are described: those that utilize fluid flowing against the adhered bacteria and counting the percentage of bacteria that detach, and those that manipulate single bacteria in various configurations which lend themselves to more specific force application and provide the basis for theoretical analysis of the receptor-ligand interactions. The theories that are reviewed are the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, the thermodynamic approach and the extended DLVO theory. Over the years, significant work has been done to investigate the process of bacterial adhesion to biomaterial surfaces, however a lot of questions still remain unanswered.

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Concise review of mechanisms of bacterial adhesion to biomaterials and of techniques used in estimating bacteria-material interactions.

Biosorption isotherms, kinetics and thermodynamics

The essential role of hydrodynamic shear force in the formation of biofilm and granular sludge

State of the art of biogranulation technology for wastewater treatment.

The effect of shear force on aerobic granulation was studied in four column-type, sequential aerobic sludge blanket reactors. Hydrodynamic turbulence caused by upflow aeration served as the main shear force in the systems. Results showed that aerobic granulation was closely associated with the strength of shear force. Compact and regular aerobic granules were formed in the reactors with a superficial upflow air velocity higher than 1.2 cm s(-1). However, only typical bioflocs were observed in the reactor with a superficial upflow air velocity of 0.3 cm s(-1) during the whole experimental period. The characteristics of the aerobic granules in terms of settling ability, specific gravity, hydrophobicity, polysaccharide and protein content and specific oxygen utilization rate (SOUR) were examined. It was found that the shear force has a positive effect on the production of polysaccharide, SOUR, hydrophobicity of cell surface and specific gravity of granules. The hydrophobicity of granular sludge is much higher than that of bioflocs. Therefore, it appears that hydrophobicity could induce and further strengthen cell-cell interaction and might be the main force for the initiation of granulation. The shear-stimulated production of polysaccharides favors the formation of a stable granular structure. This research provides experimental evidence to show that shear force plays a crucial role in aerobic granulation and further influences the structure and metabolism of granules.

The effects of shear force on the formation, structure and metabolism of aerobic granules

Thermodynamic methods based on conductor-like screening models (COSMO) originated from the use of solvation thermodynamics and computational quantum mechanics. These methods rely on sigma profiles specific to each molecule. A sigma profile is the probability distribution of a molecular surface segment having a specific charge density. Two COSMO-based thermodynamic models are COSMO-RS (realistic solvation) developed by Klamt and his colleagues, and COSMO-SAC (segment activity coefficient) published by Lin and Sandler. Quantum mechanical calculations for generating the sigma profiles represent the most time-consuming and computationally expensive aspect of using COSMO-based methods. A growing number of scientists and engineers are interested in the COSMO-based thermodynamic models but are intimidated by the complexity of performing quantum mechanical calculations. This paper presents the first free, web-based sigma profile database of 1432 compounds. We describe the procedure for sigma profile generation, and we have validated our database by comparing COSMO-based predictions of activity coefficients, normal boiling point and solubility with experimental data and thermodynamic property database. We discuss improvements which include using supplemental geometry optimization software packages to provide good initial guesses for molecular conformations as a precursor to the COSMO calculation. Finally, this paper provides a FORTRAN program and a procedure to generate additional sigma profiles, as well as a FORTRAN program to generate binary phase-equilibrium predictions using the COSMO-SAC model. Our sigma profile database will facilitate predictions of thermodynamic properties and phase behaviors from COSMO-based thermodynamic models.

Yu Liu

Papers

Biosorption isotherms, kinetics and thermodynamics

The essential role of hydrodynamic shear force in the formation of biofilm and granular sludge

State of the art of biogranulation technology for wastewater treatment.

The effects of shear force on the formation, structure and metabolism of aerobic granules

Sigma-Profile Database for Using COSMO-Based Thermodynamic Methods