Mechanisms Of Bacterial Adhesion And Response Of Cell Activity To Soil Active Particles

Bacterial adhesion and response of cell activity to soil active particles play vital roles in the key biogeochemical processes including biofilm formation, mineral weathering, element cycle, the degradation of organic matter, aggregate stability, and the fates of contaminants. However, the mechanisms of bacteria-mineral interactions are not well understood. In this study, Gram-negative bacteria(Escherichia coli, Pseudomonas putida, Agrobacterium tumefaciens and Paracoccus sp.) and a Gram-positive bacterium (Bacillus subtilis) were selected. Biopolymers such as dextran, bovine serum albumin (BSA) and poly-L-lysine were used as analogues to represent bacterial cell surface biomacromolecules. The examined soil particles were Red and Brown soil colloids, kaolinite, montmorillonite, goethite, and muscovite. The traditionally physico-chemical methods and the advanced techniques including atomic force microscopy (AFM), isothermal microcalorimetry, attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, and fluorescence microscopy combined with cell staining were performed to study the following topics:1) bacterial adhesion onto soil colloids in a controlled flow system; 2) bacterial adhesion onto kaolinite at different growth phases; 3) the adhesion forces and association types of bacteria with clay-sized minerals; 4) the nanoscale surface morphology of biofilms formed on clay-sized minerals; 5) the adhesion forces of bacterial cell surface polymers with muscovite; 6) the response of bacterial activity to the presence of soil active particles. The main findings are listed as follows:(1) In situ ATR-FTIR spectroscopy can be used effectively to investigate the kinetics of bacterial adhesion to a soil colloid deposit. The adhesion followed a pseudo-first-order kinetics equation. Surface proteins may be involved in the bacterial adhesion to soil colloids. P. putida adsorption on the soil colloids was irreversible in a wide range of ionic strengths under controlled flow systems. The irreversible adhesion was likely ascribed to the Derjaguin-Landau-Verwey-Overbeek (DLVO) predicted deep secondary energy minima together with non-DLVO factors including polymer bridging, local charge heterogeneities, surface roughness, and Lewis acid-base interactions.(2) The larger adsorption density on kaolinite surfaces was shown for stationary-phase cells than mid-exponential-phase cells under a static deposition condition. In contrast to mid-exponential-phase cells, stationary-phase cells presented higher saturation coverage fitted by a pseudo-first-order kinetics equation in continuous flow systems. The greater adhesion amount of stationary-phase cells was possibly attributed to their smaller cell size and less negative surface charges resulting in deeper secondary energy minima and lower energy barriers.(3) AFM imaging yields direct evidence of a range of different association mechanisms between bacteria and various clay minerals. All strains studied adhered predominantly to the edge surfaces of kaolinite rather than to the basal surfaces. Bacteria were only loosely associated with montmorillonite, but bacteria were more tightly adsorbed onto goethite surfaces. We 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 (10-12 N) between E. coli and goethite. Bond strengthening between E. coli and goethite occurred within 4 s to the maximum adhesion forces and energies of -3.0 ± 0.4 nN (10-9 N) and -330 ± 43 aJ (10-18 J), respectively. The large adhesion energies are sufficient for cells to exhibit irreversible adhesion onto goethite in the primary energy minimum.(4) Under the conditions studied, bacteria tended to form more extensive biofilms on clay minerals at low rather than high nutrient levels. Of the 4 bacterial species used here, E. coli formed the most extensive biofilms on clay-sized minerals.(5) At a weakly acidic environment (pH 5.7), the adhesion forces of the positively charged biopolymers with muscovite significantly decreased with increasing ionic strength, which were attributed to the decline of electrostatic attraction and the changes of conformations from unfolded to compressed states. The adhesion forces of the tested biopolymers with muscovite notably fell with pH changing from 3.9 to 8.4 primarily due to the decrease of electrostatic attraction and hydrogen bonding. Poly-L-lysine showed the highest adhesion forces with muscovite among the used biopolymers at all tested conditions. BSA had larger adhesion forces with muscovite in acidic solutions, whereas the result was reversed in basic solutions.(6) Soil active particles stimulated cell activities of E. coli and B. subtilis, whereas inhibited the activities of P. putida and Paracoccus sp. The cell activity of the same bacteria showed the same response to all examined soil active particles which bear large surface negative charges. Native humus promoted bacterial activities probably via interfacial interactions between bacteria and soil particles. The electrostatic interactions may be responsible for the modulation of soil active particles on bacterial activities.