Analysis of the interfacial properties of fibrillated and nonfibrillated oral streptococcal strains from electrophoretic mobility and titration measurements : Evidence for the shortcomings of the classical soft-particle approach
Duval, J.F.L. ; Busscher, H.J. ; Belt-Gritter, B. van de; Mei, H.C. van der; Norde, W. - \ 2005
Langmuir 21 (2005)24. - ISSN 0743-7463 - p. 11268 - 11282.
atomic-force microscopy - colloidal particles - cell-wall - surface characteristics - protein antigens - network method - salivarius - electrokinetics - suspensions - recognition
Chemical and structural intricacies of bacterial cells complicate the quantitative evaluation of the physicochemical properties pertaining to the cell surface. The presence of various types of cell surface appendages has a large impact on those properties and therefore on various interfacial phenomena, such as aggregation and adhesion. In this paper, an advanced analysis of the electrophoretic mobilities of fibrillated and nonfibrillated strains (Streptococcus salivarius HB and Streptococcus salivarius HB-C12, respectively) is performed over a wide range of pH and ionic strength conditions on the basis of a recent electrokinetic theory for soft particles. The latter extends the approximate formalism originally developed by Ohshima by solving rigorously the fundamental electrokinetic equations without restrictions on the bacterial size, charge, and double layer thickness. It further allows (i) a straightforward implementation of the dissociation characteristics, as evaluated from titration experiments, of the ionogenic charged groups distributed throughout the bacterial cell wall and/or the surrounding exopolymer layer and (ii) the inclusion of possible specific interactions between the charged groups and ions from the background electrolyte other than charge-determining ions. The theory also enables an estimation of possible swelling/shrinking processes operating on the outer polymeric layer of the bacterium. Application of the electrokinetic model to HB and HB-C12 clearly shows a significant discrepancy between the amount of surface charges probed by electrophoresis and by protolytic titration. This is ascribed to the specific adsorption of cations onto pristine charged sites in the cell wall. Physicochemical parameters pertaining to the hydrodynamics (softness degree) and electrostatics of the bacterial cell wall (HB-C12) and soft polymeric layer (HB) are quantitatively derived
Characterization of spatial soil variability and its effect on Millet yield on Sudano-Sahelian coversands in SW Niger
Voortman, R.L. ; Brouwer, J. ; Albersen, P.J. - \ 2004
Geoderma 121 (2004)39479. - ISSN 0016-7061 - p. 65 - 82.
west-africa - surface characteristics - fertility constraints - precision agriculture - sandy soils - growth - systems - water - tests - heteroscedasticity
Very local spatial soil variability on Sudano-Sahelian coversands hampers the interpretation of agronomic research and is an obstacle for the dissemination of research findings. In an earlier paper, we specifically accounted for this spatial soil variability: Using novel tools for data exploration, such as non-parametric kernel density regression and spatial econometrics, on spatially explicit data for topsoil N, P, K and millet yield, we could explain 81% of the yield variation. However, the macronutrients explained a modest portion of millet yield only, while the good explanatory power was largely derived from spatial dependence/autocorrelation. This implies that variables other than N, P and K determine spatial variability of soils and millet growth. In the present paper we identify these variables and show that the proportions of the cations (Ca, Mg, K and Na) at the exchange complex, in combination with the Al saturation profile are the main source of spatial variability at this scale. Equations including these soil properties explain 82% of the millet yield variation, without spatial autocorrelation being present in the residuals. High proportions of Mg and Na in the topsoil coincide with low Al saturation levels in the subsoil and under such conditions yields are low (and vice versa). The likely operating mechanism is the destabilizing effect that Mg and Na have on the clay fraction, which, in turn, causes surface sealing. The latter affects seedling emergence and water infiltration and, consequently, subsoil Al saturation levels. The evidence provided further suggests a parent material connection to local soil variability: coversands of different source materials and age occurring as shallow layers.
Wind effects on spatial variability in pearl millet yields in the Sahel
Sterk, G. ; Stein, A. ; Stroosnijder, L. - \ 2004
Soil & Tillage Research 76 (2004)1. - ISSN 0167-1987 - p. 25 - 37.
crop growth variability - west-africa - erosion control - semiarid niger - surface characteristics - farmers knowledge - crusted soil - transport - residue - sediment
Growth and yield of pearl millet (Pennisetum glaucum) in the West African Sahel are characterized by significant spatial variability at short distances (5-20 m). Several studies have suggested that this variability is caused by aeolian redistribution of relatively fertile topsoil material. The objectives of this study were (i) to quantify storm-based erosion/deposition patterns within a Sahelian millet field and (ii) to determine the effects of topography, erosion and deposition on millet growth and yield. An experiment was conducted at a research station in southwest Niger, on a sandy, siliceous, isohyperthermic Psammentic Paleustalf. Twenty-one sediment catchers were installed in a 40 m x 60 m plot within a pearl millet field. Four wind erosion events occurred during the 1993 growing season. Maps of wind-blown mass transport were created for each storm by applying a geostatistical space-time procedure. Maps showing the spatial distribution in erosion and deposition were derived by differentiation of mass transport in the direction of the mean wind. For 21 sub-plots of 4 m x 5 m the elevation, wind-blown mass balance, millet growth and yield were quantified. A correlation analysis showed neither a significant relationship between millet yield and topography, nor between millet yield and erosion/deposition. It is concluded that only wind-blown sediment transport occurring within one growing season cannot explain the spatial variability in millet growth and yield. A new hypothesis is defined that suggests aeolian erosion/deposition patterns as observed in previous studies were not the cause but more likely the result of spatial variability in millet growth. (C) 2003 Elsevier B.V. All rights reserved.