O.J. de Vos

Manure-amended soil characteristics affecting the survival of E. coli O157:H7 in 36 Dutch soils

The recent increase in foodborne disease associated with the consumption of fresh vegetables stresses the importance of the development of intervention strategies that minimize the risk of preharvest contamination. To identify risk factors for Escherichia coli O157:H7 persistence in soil, we studied the survival of a Shiga-toxin-deficient mutant in a set of 36 Dutch arable manure-amended soils (organic/conventional, sand/loam) and measured an array of biotic and abiotic manure-amended soil characteristics. The Weibull model, which is the cumulative form of the underlying distribution of individual inactivation kinetics, proved to be a suitable model for describing the decline of E. coli O157:H7. The survival curves generally showed a concave curvature, indicating changes in biological stress over time. The calculated time to reach the detection limit ttd ranged from 54 to 105 days, and the variability followed a logistic distribution. Due to large variation among soils of each management type, no differences were observed between organic and conventional soils. Although the initial decline was faster in sandy soils, no significant differences were observed in ttd between both sandy and loamy soils. With sandy, loamy and conventional soils, the variation in ttd was best explained by the level of dissolved organic carbon per unit biomass carbon DOC/biomC, with prolonged survival at increasing DOC/biomC. With organic soils, the variation in ttd was best explained by the level of dissolved organic nitrogen (positive relation) and the microbial species diversity as determined by denaturing gradient gel electrophoresis (negative relation). Survival increased with a field history of low-quality manure (artificial fertilizer and slurry) compared with high-quality manure application (farmyard manure and compost). We conclude that E. coli O157:H7 populations decline faster under more oligotrophic soil conditions, which can be achieved by the use of organic fertilizer with a relatively high C/N ratio and consequently a relatively low rate of nutrient release.

Formate releases carbon dioxide/bicarbonate from thylakoid membranes

Photosystem (PS) II acts as a waterplastoquinone oxidoreductase; it transfers four electrons from two molecules of water to plastoquinone producing molecular 02 and two molecules of doubly reduced plastoquinone. During this process, water protons are released into the lumen and additional protons are taken up into the thylakoid membrane from its stromal side. These protons are utilized to produce plastoquinol from the doubly reduced plastoquinone [1, 2]. Bicarbonate has been suggested to regulate PS II electron flow under a variety of conditions [3]. This bicarbonate effect is assumed to be through the binding of HCO3 to the reaction center II complex, particularly the D1 and D2 proteins [3-5] . In this model, addition of formate removes HCO3-/CO z from their binding sites, thus causing inhibition of electron flow. Addition of bicarbonate to formate-treated samples restores the electron flow by displacing the bound formate ions. Another view is that the anion binding sites can be empty in the native membranes; addition of formate ions causes inhibition of electron flow as these ions bind to empty sites. Further addition of bicarbonate ions restores electron flow because the latter displace the inhibitory formate ions. In support of the latter view, Stemler [6] reported that formate addition, which caused drastic inhibition of electron flow in maize thylakoids at pH 6, did

The effect of the native bacterial community structure on the predictability of E. coli O157:H7 survival in manure-amended soil

Aims:  The survival capability of pathogens like Escherichia coli O157:H7 in manure‐amended soil is considered to be an important factor for the likelihood of crop contamination. The aim of this study was to reveal the effects of the diversity and composition of soil bacterial community structure on the survival time (ttd) and stability (irregularity, defined as the intensity of irregular dynamic changes in a population over time) of an introduced E. coli O157:H7 gfp‐strain were investigated for 36 different soils by means of bacterial PCR‐DGGE fingerprints.

Comparison of photosynthetic activities in triazine-resistant and susceptible biotypes of Chenopodium album

Abstract Triazine-resistant and susceptible Chenopodium album plants were grown at low and at high light irradiances. At the lower light irradiance the dry matter production of the resistant and the susceptible plants were almost similar. At the higher irradiance the resistant biotype had a significantly lower production. Fluorescence studies showed that the photochemical yield and the photosystem II electron transport rate were lower in the resistant biotype. It could be demonstrated in intact leaves that the lower productivity of the resistant biotype is caused by a higher sensitivity to photoinhibition. However, when studying effects of photoinhibition on electron flow and photophosphorylation in isolated thylakoids of the two biotypes, no significant differences between resistant and susceptible plant materials were observed. It is suggest ed that the difference between resistant and susceptible biotypes connected with processes protective against photoinhibition in intact leaves, are lost during the isolation of thylakoids.

Biochemical Mechanisms of Resistance to Photosystem II Herbicides

Photosystem II herbicides bind to the D1 protein of the reaction centre of photosystem II. Resistance to these herbicides in plants is confined almost exclusively to the triazine group and involves an altered D1 protein: at site 264 the serine of the wild type is replaced by a glycine. The electron flow rate between the primary electron acceptor of photosystem II and the plastoquinone pool is about three times slower in triazine-resistant plants, but the overall electron transport rate is not different from that in sensitive plants. When plants are grown at a low light intensity, there is no difference in photosynthetic capacity and 1n fitness. However, when grown at a high light intensity, resistant plants have a lower photosynthetic capacity and are less fit. This difference is ascribed to a higher sensitivity of resistant plants to photoinhibition. Measurements of the reversible binding and release kinetics revealed that the binding kinetics are not much different in sensitive and resistant plants. Triazine resistance appears to originate from a significant increase of the release kinetics.

Molecular mechanisms of photoinhibition, an abiotic stress factor limiting primary plant production.

Molecular mechanisms of photoinhibition are clarified on the basis of experiments with leaves and isolated chloroplasts of peas and triazine-resistant and susceptible Chenopodium plants.