The number of food poisoning cases caused by enteropathogens has increased in recent years. A significant part of the outbreaks associated with the consumption of raw vegetables has been attributed to Escherichia coli O157:H7 and Salmonella enterica subsp. enterica serovar Typhimurium. Bovine manure and slurry are the main environmental sources of these pathogens. Thus, reduction of the multiplication of E. coli O157:H7 and Salmonella serovar Typhimurium in cattle and their survival in manure and slurry are important tasks to minimize the risks of contamination of plant products and outbreaks of food-borne diseases. This thesis describes the influence of various environmental factors on survival of E. coli O157:H7 and Salmonella serovar Typhimurium in manure, slurry and soil amended with manure or slurry. Manure or slurry were inoculated with green fluorescent protein transformed strains of both enteropathogens at 106 - 107 cells g-1 dry weight, and their survival was studied in these substrates and in soil amended with inoculated manure or slurry. Population densities of the pathogens and autochthonous microbial communities were determined by dilution plating. The obtained survival data were fitted to non-linear models such as modified logistic or Weibull models, and estimated survival times in various substrates were compared. Analysis of the estimated parameter values showed that the pathogens survived longer at relatively low temperatures under anaerobic conditions especially at high concentrations of easily available substrate. Salmonella serovar Typhimurium was more resistant to environmental stresses than E. coli O157:H7. Survival of both pathogens significantly declined with increasing temperature amplitudes of daily temperature oscillations. Variations in fluctuations of E. coli O157:H7 populations around the decline curve were evaluated by the Approximate Entropy (ApEn) procedure. The instability of E. coli O157:H7 populations around the decline curve was greater in conventional than in organic and in loamy than in sandy soils, even though the mean survival periods did not differ. Multiple regression analysis of instability of E. coli O157:H7 survival on various soil characteristics showed a positive relation with the ratio of copiotrophic / oligotrophic bacteria, suggesting greater instability at higher available substrate concentrations. Percolation experiments with soil columns showed that surface application of solid manure decreased the risk of contamination of ground water and lettuce roots compared to injection of slurry, as more pathogen cells percolated to greater depths after slurry than after manure application. Detection of E. coli O157:H7 could be improved by incubation of Petri plates in anaerobic conditions, as this resulted in significantly higher numbers of recovered cells in comparison with the common aerobic plating procedure. Finally, a simulation model was developed based on our experimental data. The relative effects of temperature and substrate content were more important than that of oxygen concentration. The interaction with substrate resulted in oscillatory behavior of E. coli O157:H7 populations in manure and manure amended soil. Competition for substrate was the most important factor affecting the final survival time. The model was used to evaluate the effects of various manure and soil management scenarios on the survival of E. coli O157:H7. This simulation model provides a new approach to investigate dynamic changes of invasive microorganisms in natural substrates. The results presented in this thesis can be used for risk assessment of E. coli O157:H7 and Salmonella serovar Typhimurium in dairy farming systems and will help to identify and evaluate potential control strategies to minimize the chance of pathogen spread in the vegetable production chain.
|Qualification||Doctor of Philosophy|
|Award date||8 Dec 2008|
|Place of Publication||[S.l.]|
|Publication status||Published - 2008|
- physicochemical properties