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Bacterial attachment is a physicochemical interfacial process probably affected by the presence of humic acid (HA) in natural environments. However, the effect of HA coating on bacterial adhesion to soil minerals remains unknown, despite many studies focusing on cell attachment to pure or iron oxide-coated quartz in the presence of HA. The influence of HA on Bacillus subtilis adhesion to kaolinite, montmorillonite, and goethite under neutral pH and 1 mM ionic strength was examined using batch experiments coupled with Fourier-transform infrared spectroscopy (FTIR), isothermal titration calorimetry, and Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory. Results showed a reduction in cell adhesion on goethite and montmorillonite with increasing HA content from 0% to 2%, indicating an inhibitory effect of HA coating on the interaction. However, a promotive effect of HA was observed for kaolinite, which disagreed with previous findings based on cell–quartz systems. In addition, the presence of HA reduced the exothermic adhesion enthalpy for kaolinite and goethite, but increased that for montmorillonite. For goethite, the HA coating depressed its positive charge and increased its aggregation, thereby leading to a weaker electrostatic attraction and lesser available surface area, which probably contributed to the observed decreasing adhesion. For montmorillonite, the reduced attachment was likely caused by the increase in electrostatic repulsion that overcompensated for the appeared chemical interactions as suggested by FTIR analysis. The enhanced cell adhesion on kaolinite, which was unexpected from DLVO theory based on its surface charge change, is likely ascribed to the HA-induced decrease in its aggregation. Additionally, the effect of HA vanishes at HA contents higher than 2%, likely due to a saturation effect. Overall, the influence of HA on bacterial attachment is likely a combined result of multiple factors, including not only electrostatic forces and chemical interactions, but also mineral aggregation. [ABSTRACT FROM AUTHOR]

Substantial interactions of microbes with soil particles present fundamental influences on microbial activities relevant to a series of biogeochemical processes. However, how soil surface-active particles modulate microbial metabolism has received scant attention. The extent to which composition of soil colloids alter the metabolism is not well addressed. This work examined the impacts of soil colloids and minerals on the metabolic activity ofPseudomonas putidausing microcalorimetry and carbon utilization. The results showed that montmorillonite remarkably improved metabolic activity ofP. putida, whereas kaolinite, goethite and soil colloids significantly inhibited the activity. Humus may weaken the inhibition of soil colloids on bacterial metabolism via interfacial interaction rather than nutrient supplements. Soils bearing higher amount of kaolinite and iron oxide may have greater depression on bacterial activity. The thermodynamic method provides different and complementary information to that from other techniques in characterizing microbial activities. The quantity and affinity for the adhesion of bacteria onto soil components together with the detoxification of metabolites were assigned to the modifications of bacterial activities. [ABSTRACT FROM PUBLISHER]

Adhesion ofBacillus subtilison kaolinite, montmorillonite, and goethite was investigated over a wide range of ionic strength (IS) and pH using batch experiment. The related surface properties (size, zeta potential, and hydrophobicity) under varying conditions were systematically determined and the interaction energy between the cell and minerals were calculated using the extended Derjaguin, Landau, Verwey, and Overbeek (ExDLVO) theory. Adhesion on kaolinite and montmorillonite increased with IS at low level (< 0.01 mol L−1MgCl2) but declined at high IS level. An increase in IS generally depressed bacterial adhesion on goethite. Elevated pH resulted in decreasing the adhesions on all three minerals. The IS- and pH-effects on adhesion forphyllosilicatesystems followed the ExDLVO predictions. For goethite systems, this theory predicted the adhesion trend with IS and that under basic pH, but failed to explain the adhesion at low pH. Such deviation from the theory possibly resulted from chemical interactions between extracellular polymeric substances on cell surface and goethite. These results imply that bacterial adhesions onphyllosilicates are primarily governed by the ExDLVO interactions, and those on iron oxides are mediated by the combination of ExDLVO and non-ExDLVO interactions. [ABSTRACT FROM PUBLISHER]

Abstract: The functions of extracellular polymeric substances (EPS) during the adhesion of Bacillus subtilis to kaolinite, montmorillonite, and goethite were examined by a direct comparison of the adhesion behaviors of native and EPS-free cells via cation exchange resin (CER) treatment using batch experiments, attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy, and potentiometric titration. The EPS removal had no apparent influence on bacterial adhesion when the wet bacteria/mineral mass ratio was low (<0.4 for the clay mineral systems and <1.8 for the goethite system). With higher mass ratios, the absence of EPS reduced adhesion to clay minerals but enhanced adhesion to goethite. The ATR-FTIR spectra suggested that protein conformational changes were involved in the adhesion of bacteria to clay minerals, whereas additional chemical interactions such as P OFe bonds were important for adhesion to goethite. In addition to electrostatic forces (repulsion for clays and attraction for goethite), absence of chemical interactions may also cause the relatively much weaker bacterial adhesion to clay minerals than to goethite. The absence of EPS did not change the interaction mode of the adhesion to clay minerals but enhanced the chemical interactions via carboxyl groups for bacteria–goethite adhesion. The potentiometric titration results coupled with the ATR-FTIR spectra showed a significant increase in site concentrations of the CER-treated bacteria as compared to the native cells. Changes in surface site concentrations and chemical interactions that were accompanied by the EPS removal may reasonably explain the influences of EPS on bacterial adhesion to different minerals. [Copyright &y& Elsevier]

Abstract: Equilibrium adsorption along with scanning electron microscopy (SEM), Fourier transform infrared (FTIR) and isothermal titration calorimetry (ITC) techniques were employed to investigate the adhesion of Pseudomonas putida on goethite. The adhesion isotherm revealed the high affinity of P. putida for goethite. The SEM analysis also showed a tight association between bacteria and mineral particles. Larger amounts of adhesion of bacteria on goethite were observed at pH lower than the isoelectric point (IEP) of goethite. The bacterial adhesion increased with increasing concentration of K+. The calorimetric results demonstrated that the P. putida–goethite adhesion was an exothermic process. The adhesion enthalpy increased with increasing pH and concentrations of electrolyte. The increase of the negative enthalpy with increment of temperature indicated that the bacteria–goethite adhesion was an enthalpy-driven process. Electrostatic interactions and hydrogen bonding were considered to contribute mainly to the adhesion of bacterial adhesion on goethite. The data obtained in this study would provide valuable information for a better understanding of the mechanisms of mineral–microorganism interactions in soil and associated environments. [Copyright &y& Elsevier]

Mineral-organic interfacial reactions strongly influence the adsorption, distribution and bioavailability of metal cations in soil systems. The molecular binding mechanisms and distribution of Cd onto goethite, humic acid, Pseudomonas putida cells, and their composites at different mass ratios were studied through the combination of bulk adsorption coupled with EXAFS, ITC and SCM. In binary and ternary composites, the energetics of the overall adsorption of Cd was dominated by the entropy of Cd adsorption onto the organic fraction. The formation of a type-B HA bridging complex >FeOH−HACOOCdOH enhanced Cd adsorption by 10−30% at low Cd concentrations, and more than 93.5% of the adsorbed Cd was bound onto HA fraction. In ternary systems, the component additivity over-estimated Cd adsorption onto bacteria by ~21.8%, likely due to site blocking effects. Models involving the masking of phosphoryl sites and HA bridging reactions can simulate the distribution of Cd in the composites. Our modelling suggests that HA is the main scavenger of Cd under a range of environmental conditions, and that bacteria become important in affecting the distribution of Cd under lower pH settings. This study demonstrates the impact of iron oxide−HA−bacteria interactions on the fate and distribution of Cd in soils and associated environments. ga1 • Adsorption of Cd was driven by entropy increment on organic fraction in composites. • Composite SCMs for Cd adsorption were established based on EXAFS and ITC. • Formation of type-B HA bridging complex enhanced Cd adsorption. • Masking of phosphoryl and HA bridging reduced allocation of Cd onto bacteria. • HA is the main scavenger of Cd under a range of environmental conditions. [ABSTRACT FROM AUTHOR]

Bacterial adhesion onto mineral surfaces and subsequent biofilm formation play key roles in aggregate stability, mineral weathering, and the fate of contaminants in soils. However, the mechanisms of bacteria-mineral interactions are not fully understood. Atomic force microscopy (AFM) was used to determine the adhesion forces between bacteria and goethite in water and to gain insight into the nanoscale surface morphology of the bacteria-mineral aggregates and biofilms formed on clay-sized minerals. This study yields direct evidence of a range of different association mechanisms between bacteria and minerals. All strains studied adhered predominantly to the edge surfaces of kaolinite rather than to the basal surfaces. Bacteria rarely formed aggregates with montmorillonite, but were more tightly adsorbed onto goethite surfaces. This study reports the first measured interaction force between bacteria and a clay surface, and the approach curves exhibited jump-in events with attractive forces of 97 ± 34 pN between E. coli and goethite. Bond strengthening between them occurred within 4 s to the maximum adhesion forces and energies of −3.0 ± 0.4 nN and −330 ± 43 aJ (10−18 J), respectively. Under the conditions studied, bacteria tended to form more extensive biofilms on minerals under low rather than high nutrient conditions. [ABSTRACT FROM AUTHOR]

The accurate modeling of heavy metal adsorption in complex systems is fundamental for risk assessments in soils and associated environments. Bacteria-iron (hydr)oxide associations in soils and sediments play a critical role in heavy metal immobilization. The reduced adsorption of heavy metals on these composites have been widely reported using the component additivity (CA) method. However, there is a lack of a mechanism model to account for these deviations. In this study, we established models for Cd adsorption on goethite- Pseudomonas putida composites at 1:1 and 5:1 mass ratios. Cadmium adsorption on the 5:1 composite was consistent with the additivity method. However, the CA method over predicted Cd adsorption by approximately 8% on the 1:1 composite at high Cd concentration. The deviation was corrected by adding the site blockage reactions between P. putida and goethite. Both CA and “CA-site masking” models for Cd adsorption onto the composites were in line with the ITC data. These results indicate that CA method in simulating Cd adsorption on bacteria-iron oxides composites is limited to low bacterial and Cd concentrations. Therefore the interfacial complexation reactions that occur between iron (hydr)oxides and bacteria should be taken into account when high concentrations of bacteria and heavy metals are present. [ABSTRACT FROM AUTHOR]

Abstract: The metabolic activity of microorganisms in soil is influenced by various colloidal soil particles especially clay minerals or iron oxide. The knowledge of the impacts of soil minerals on microorganisms is essential in understanding microbially mediated processes in soil. However, to date, the available information on this regard is scarce. Based on microcalorimetric techniques, a powerful tool for monitoring the activity of microorganisms, the metabolic activity of Bacillus thuringiensis in the presence of goethite, kaolinite and montmorillonite was investigated. The clay minerals and iron oxide stimulated the exponential growth of B. thuringiensis, whereas the sporulation of B. thuringiensis was somewhat inhibited by kaolinite and montmorillonite. Goethite significantly depressed the sporulation and the total metabolic activity of the bacterium. The surface interaction between microbial cell and soil mineral probably account for the effect of these minerals on the metabolic activity of B. thuringiensis. [Copyright &y& Elsevier]

Abstract: Bioavailability of pesticides adsorbed on mineral surfaces is an important determinant of their environmental fate. Carbaryl (1-naphthyl-N-methylcarbamate) was used as a model compound to illustrate the impact of adsorption characteristics and microbial activity on the bioavailability of organic compound adsorbed on clay minerals and goethite. Batch experiments were applied to determine the sorption isotherms and biodegradation kinetics. Metabolic activity of Pseudomonas putida in the absence and presence of montmorillonite, kaolinite or goethite was monitored by microcalorimetry. Adsorption mechanisms of carbaryl were studied by Fourier Transform Infrared Spectroscopy (FTIR). Montmorillonite presented a higher adsorption capacity for carbaryl than goethite and kaolinite. Degradation of adsorbed carbaryl by P. putida followed the sequence montmorillonite>kaolinite>goethite, which is in accordance with the binding strength of carbaryl on the minerals. The presence of montmorillonite enhanced the activity of P. putida and ultimately stimulated the bioavailability of carbaryl. Goethite displayed an inhibitory effect on bacterial activity and reduced carbaryl degradation. The biodegradation of mineral-adsorbed carbaryl was mainly controlled by the activity of the degrading microorganisms. [Copyright &y& Elsevier]