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Cell yields and fermentation responses of a Salmonella Typhimurium poultry isolate at different dilution ratesin an anaerobic steady state continuous culture Abstract The objectives of these studies were todetermine cell yield and fermentation responses of aSalmonella enterica serovar Typhimurium poultry isolate using various dilution rates in steady state continuous culture incubations. S. enterica Typhimurium cells were propagated in continuous cultures with a total volume of 0.50 l of Luria Bertani medium containing 0.1% glucose. Dilution rates from 0.0125 to 1.44/h were used. Cell protein concentration generally increased linearly with increased dilution rate up to a rate of 0.54/h and declined at the higher dilution rates. Glucose consumption gave a similarpattern to cell protein concentration by declining at the three highest dilution rates. Short chain fatty acid production was inconsistently influenced by dilution rate. Acetate, the most predominant fatty acid produced, declined at the higher dilution rates, as did propionate. Ammonia production remained stable at the lowest dilution rates, but increased significantly at a dilution rates above 0.27/h. Keywords Salmonella Continuous culture Fermentation Glucose yield Introduction Foodborne salmonellosis is associated with poultry meat and by-products (Bryan and Doyle 1995;Humphrey 2000) and costs an estimated US $ 2.3 billion annually for medical care and loss of productivity(Frenzen et al. 1999). Salmonella can survive various environmental stresses including pH changes, nutrient deprivation, oxygen tension, osmotic shock and heat stress (Foster and Spector 1995). These stresses may also influence virulence potential(Archer 1996), especially in the gastrointestinal (GI) tract microenvironment of farm animals. Pathogenesis of Salmonella enterica serovar Typhimurium (ST) is dependent on the host infected; it causes gastroenteritis in humans but can cause lethal enteric fever in mice (Carter and Collins 1974). Age is also a factor ,as illustrated by the fact that young chicks exposed to ST may subsequently develop clinical gastroenteritis while adult chickens typically are asymptomatic (Barrow et al. 1987). Passage rate (flow rate) represents the amount of digesta that passes a particular point along the GI tract in a given time (Brant and Thacker 1958) and is considered essential in maintaining the overall microbial population and integrity of the GI tract of chickens. Passage rate varies in different segments of the GI tract and is dependent on the feed composition(Da¨nicke et al. 1997). Using markers to determine passage rates, Tukey et al. (1958) observed an initial appearance of excreta 2–2.5 h after intake, such that most of the marker was excreted within 12 h. Adequate feed retention time is essential, especially in the ceca, to encourage sufficient microbial degradation of feed substrate for extended time periods(McNab 1973). The continuous culture model has been proven to simulate bacterial interaction of microbial ecosystems such as the GI tract of chickens (Nisbet et al. 2000). Previously we have shown that pH shifts in continuous culture could alter genetic responses of ST (Dunkley et al. 2008b). The current study utilized continuous culture to grow ST pure cultures under various dilution rates (D). The underlying rationale was to mimic in vivo conditions in the GI tract during shifts in passage rate. The specific objectives of this study were to determine glucose yield, short chain fatty acid and ammonia production by ST using various dilution rates in a complex medium. |
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jinshubin
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2楼2012-11-18 10:14:00