Search

Deze natuurlijke stof fungeert als antioxidant, aroma en als conserveermiddel. Als conserveermiddel is Carvacrol niet toegestaan, wel als smaakstof

Bacillus cereus is a food spoilage microorganism and a pathogen. Growth of B. cereus can be prevented or delayed by adding growth limiting compounds to the food product or by altered storage conditions. Combinations of growth limiting factors can show synergy, or be multiplicative without synergy (gamma hypothesis). For food safety management, it is important to understand if combinations are synergistic or not, to avoid making faildangerous or overly fail-safe predictions. Therefore, the aim of this PhD project was to validate the gamma hypothesis for specific combinations of hurdles commonly used in food production. Since the relationship between growth and toxin production of B. cereus is little understood, a second aim was to investigate the production of the emetic toxin cereulide in more detail. Several new lines of research were set-up to deliver on these aims. For growth data collection to quantify hurdle effects and to study combinations of hurdles, the relative rate to detection method was found to be preferred over two other methods evaluated. The gamma hypothesis was validated for combinations of pH and undissociated acid. For combinations of pH and water activity lowering solutes, the gamma hypothesis could neither be validated nor rejected. The validity of the gamma hypothesis appeared to be dependent on the models chosen for the single hurdle effects, which are subsequently combined into the gamma model. A systematic way of model selection is therefore advocated. Investigating cells in the transition from lag phase (λ) to exponential growth phase, it was found that trends in physiological processes could be observed for different culture conditions, independent of the duration of λ. Esterase activity and electron transport chain activity were found to be useful quantitative markers for this transition phase. A new method to produce synthetic cereulide showing biological activity was developed, allowing accurate quantification of cereulide in samples. The use of valinomycin as a standard, the current procedure, underestimates the amount of cereulide by approximately 10 %. Considering the onset of emetic toxin production, LCMS analysis of B. cereus F4810/72 grown in BHI showed that cereulide production does not start before cells are in mid to late stationary phase, although significant variation was noted possibly related to variability in the growth parameters maximum specific growth rate (μmax) and λ. Addition of salt to the growth medium delayed the production of cereulide. This research has been able to deliver several new insights and tools that are useful for food safety management of the emetic toxin producer B. cereus.

Bacillus cereus is a food spoilage microorganism and a pathogen. Growth of B. cereus can be prevented or delayed by adding growth limiting compounds to the food product or by altered storage conditions. Combinations of growth limiting factors can show synergy, or be multiplicative without synergy (gamma hypothesis). For food safety management, it is important to understand if combinations are synergistic or not, to avoid making faildangerous or overly fail-safe predictions. Therefore, the aim of this PhD project was to validate the gamma hypothesis for specific combinations of hurdles commonly used in food production. Since the relationship between growth and toxin production of B. cereus is little understood, a second aim was to investigate the production of the emetic toxin cereulide in more detail. Several new lines of research were set-up to deliver on these aims. For growth data collection to quantify hurdle effects and to study combinations of hurdles, the relative rate to detection method was found to be preferred over two other methods evaluated. The gamma hypothesis was validated for combinations of pH and undissociated acid. For combinations of pH and water activity lowering solutes, the gamma hypothesis could neither be validated nor rejected. The validity of the gamma hypothesis appeared to be dependent on the models chosen for the single hurdle effects, which are subsequently combined into the gamma model. A systematic way of model selection is therefore advocated. Investigating cells in the transition from lag phase (λ) to exponential growth phase, it was found that trends in physiological processes could be observed for different culture conditions, independent of the duration of λ. Esterase activity and electron transport chain activity were found to be useful quantitative markers for this transition phase. A new method to produce synthetic cereulide showing biological activity was developed, allowing accurate quantification of cereulide in samples. The use of valinomycin as a standard, the current procedure, underestimates the amount of cereulide by approximately 10 %. Considering the onset of emetic toxin production, LCMS analysis of B. cereus F4810/72 grown in BHI showed that cereulide production does not start before cells are in mid to late stationary phase, although significant variation was noted possibly related to variability in the growth parameters maximum specific growth rate (μmax) and λ. Addition of salt to the growth medium delayed the production of cereulide. This research has been able to deliver several new insights and tools that are useful for food safety management of the emetic toxin producer B. cereus.

Shiga toxin-producing Escherichia coli (STEC) are significant pathogenic bacteria that can cause severe gastrointestinal diseases and also the hemolytic-uremic syndrome. Domestic ruminants appear to be the main reservoirs of these organisms. Although Bangladesh is an endemic zone for diarrhea caused by different enteric pathogens, no systematic study on STEC has yet been done there. We estimated the prevalence of STEC infections among diarrheal patients and the occurrences of STEC in the human food chain in Bangladesh. In addition, we evaluated methods for the isolation of STEC O157 from animal feces and foods. We found that the prevalence of STEC was low among diarrhoeal patients compared with other diarrheagenic pathogens. In contrast, there is a high prevalence of STEC including serogroup O157 in animal reservoirs and in the food chain. We concluded that the lack of STEC O157 infection among Bangladeshi population might be due to the protective immunity against these pathogens acquired by the frequent exposure to the antigens.

Shiga toxin-producing Escherichia coli (STEC) are significant pathogenic bacteria that can cause severe gastrointestinal diseases and also the hemolytic-uremic syndrome. Domestic ruminants appear to be the main reservoirs of these organisms. Although Bangladesh is an endemic zone for diarrhea caused by different enteric pathogens, no systematic study on STEC has yet been done there. We estimated the prevalence of STEC infections among diarrheal patients and the occurrences of STEC in the human food chain in Bangladesh. In addition, we evaluated methods for the isolation of STEC O157 from animal feces and foods. We found that the prevalence of STEC was low among diarrhoeal patients compared with other diarrheagenic pathogens. In contrast, there is a high prevalence of STEC including serogroup O157 in animal reservoirs and in the food chain. We concluded that the lack of STEC O157 infection among Bangladeshi population might be due to the protective immunity against these pathogens acquired by the frequent exposure to the antigens.

Infection with verocytotoxin producing Escherichia coli(VTEC) O157 in humans can lead to mild or bloody diarrhoea, with e.g. the haemolytic uraemic syndrome (HUS) as possible complication. Cattle appear to be important reservoirs of O157 VTEC. The main objectives of research described in this thesis are investigating prevalences, risk factors, and transmission of O157 VTEC to understand the dynamics of O157 VTEC in Dutch cattle. Data from a monitoring program in Dutch animal herds indicated that O157 VTEC is endemic in The Netherlands, with higher prevalences during summer and early fall. Risk factors for infection were identified. Within-herd prevalence, potential environmental reservoirs, intermediate hosts and DNA types of O157 VTEC isolates were determined in a longitudinal study of a positive dairy farm. DNA clusters indicated persistence on the farm during winter and spring. Quantification of transmission using data from this dairy herd and from an experiment with calves showed that transmission was higher in calves. Transmission of O157 VTEC during summer might differ from transmission during winter. Two other experiments indicated that both previously infected heifers and previously contaminated pastures did not function as reservoir of O157 VTEC between shedding seasons. A retrospective cohort study established that positive farms from the Dutch monitoring program were only slightly more likely to be found positive in a next shedding season than previously negative farms, so between-herd transmission seems to occur. Factors associated with a positive test at second sampling implied (re-)introduction rather than long-term persistence of infection. Results of this thesis suggest that several types of O157 VTEC might persist on a farm for some time, with possibly different strains as the most prevailing types in subsequent shedding seasons. Ultimately, the within-herd infection might become extinct due to limiting numbers of susceptible cattle. Between-herd transmission of O157 VTEC can lead to persistence in a larger region, e.g. The Netherlands, maintaining the endemic status in Dutch cattle populations. Because of this, exposure of humans to O157 VTEC cannot be ruled out in The Netherlands. Humans might become infected through food- and waterborne transmission and by transmission directly from animals to humans. When aiming at reducing risks for humans by interventions at farm-level, it is of importance to reduce the number of positive animals and farms. For this, more specific research for the effect of intervention measures on introduction, transmission and survival of O157 VTEC on farms, and economic (cost-benefit) analysis, should be performed.

Infection with verocytotoxin producing Escherichia coli(VTEC) O157 in humans can lead to mild or bloody diarrhoea, with e.g. the haemolytic uraemic syndrome (HUS) as possible complication. Cattle appear to be important reservoirs of O157 VTEC. The main objectives of research described in this thesis are investigating prevalences, risk factors, and transmission of O157 VTEC to understand the dynamics of O157 VTEC in Dutch cattle. Data from a monitoring program in Dutch animal herds indicated that O157 VTEC is endemic in The Netherlands, with higher prevalences during summer and early fall. Risk factors for infection were identified. Within-herd prevalence, potential environmental reservoirs, intermediate hosts and DNA types of O157 VTEC isolates were determined in a longitudinal study of a positive dairy farm. DNA clusters indicated persistence on the farm during winter and spring. Quantification of transmission using data from this dairy herd and from an experiment with calves showed that transmission was higher in calves. Transmission of O157 VTEC during summer might differ from transmission during winter. Two other experiments indicated that both previously infected heifers and previously contaminated pastures did not function as reservoir of O157 VTEC between shedding seasons. A retrospective cohort study established that positive farms from the Dutch monitoring program were only slightly more likely to be found positive in a next shedding season than previously negative farms, so between-herd transmission seems to occur. Factors associated with a positive test at second sampling implied (re-)introduction rather than long-term persistence of infection. Results of this thesis suggest that several types of O157 VTEC might persist on a farm for some time, with possibly different strains as the most prevailing types in subsequent shedding seasons. Ultimately, the within-herd infection might become extinct due to limiting numbers of susceptible cattle. Between-herd transmission of O157 VTEC can lead to persistence in a larger region, e.g. The Netherlands, maintaining the endemic status in Dutch cattle populations. Because of this, exposure of humans to O157 VTEC cannot be ruled out in The Netherlands. Humans might become infected through food- and waterborne transmission and by transmission directly from animals to humans. When aiming at reducing risks for humans by interventions at farm-level, it is of importance to reduce the number of positive animals and farms. For this, more specific research for the effect of intervention measures on introduction, transmission and survival of O157 VTEC on farms, and economic (cost-benefit) analysis, should be performed.

The cocoa tree ( Theobroma cacao L.) produces the beans that are the source of cacao, the basis for chocolate production, and an important commodity crop in South America, West Africa, and Southeast Asia.Cocoa Pod Borer (CPB,( Conopomorpha cramerella)has been the single most important limiting factor for cacao production in Southeast Asia.So far, there has been no single cost effective and environmentally safe way to control this pest.This thesis describes the first steps in a biotechnological approach to the control of CPB through the production of transgenic cocoa trees, which produce an insecticidal toxin from Bacillus thuringiensis (Bt) in their pod wall, making them resistant to CPB. Selection of a CPB-active toxin, optimization of its expression, making the expression tissue-specific and optimization of cocoa transformation and regeneration are described. Moreover, the isolation of two transcription factor genes involved in flower timing and architecture is described.

The cocoa tree ( Theobroma cacao L.) produces the beans that are the source of cacao, the basis for chocolate production, and an important commodity crop in South America, West Africa, and Southeast Asia.Cocoa Pod Borer (CPB,( Conopomorpha cramerella)has been the single most important limiting factor for cacao production in Southeast Asia.So far, there has been no single cost effective and environmentally safe way to control this pest.This thesis describes the first steps in a biotechnological approach to the control of CPB through the production of transgenic cocoa trees, which produce an insecticidal toxin from Bacillus thuringiensis (Bt) in their pod wall, making them resistant to CPB. Selection of a CPB-active toxin, optimization of its expression, making the expression tissue-specific and optimization of cocoa transformation and regeneration are described. Moreover, the isolation of two transcription factor genes involved in flower timing and architecture is described.

The oral toxicity of excretion products of several Photorhabdus and Xenorhabdus strains was tested on two thrips species: Frankliniella occidentalis and Thrips tabaci. Out of 46 Photorhabdus isolates and 6 Xenorhabdus isolates only 6 North American P. temperata isolates were toxic to the thrips species. After 7 days of drinking from P. temperata supernatant a mortality of 90% could be reached. Thrips were also killed after sucking from leaves covered with the toxins. Possibilities of using P. temperata in the control of thrips will be discussed

The oral toxicity of excretion products of several Photorhabdus and Xenorhabdus strains was tested on two thrips species: Frankliniella occidentalis and Thrips tabaci. Out of 46 Photorhabdus isolates and 6 Xenorhabdus isolates only 6 North American P. temperata isolates were toxic to the thrips species. After 7 days of drinking from P. temperata supernatant a mortality of 90% could be reached. Thrips were also killed after sucking from leaves covered with the toxins. Possibilities of using P. temperata in the control of thrips will be discussed

Deze natuurlijke stof fungeert als antioxidant, aroma en als conserveermiddel. Als conserveermiddel is Carvacrol niet toegestaan, wel als smaakstof