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?5). Exposure of five species of bifidobacteria to acidic conditions adversely affected cell viability, which was particularly evident at pH? http://www.selleckchem.com/products/Bortezomib.html of an initial lag phase followed by a logarithmic reduction in viability. Overall, stationary phase cells were more resistant to the inhibitory effects of acid, because of both an increased lag phase and a reduced rate of viability loss. In common with previous studies, we observed differences in acid sensitivity among the different species (Matsumoto et?al. 2004; Vernazza et?al. 2006). However, our rates of viability loss were somewhat lower than previous http://www.selleckchem.com/products/sch772984.html reports. This may reflect the fact that our experiments were carried out under strict anaerobic conditions, thereby alleviating the additional oxygen stress resulting from the use of anaerobic jars (Shimura et?al. 1992; Imhof and Heinzer 1996). Provision of fermentable substrate improved the ability of all five species to resist the detrimental effects of acidic pH. Again, the observed improvements in viability were primarily the result of an increased lag phase, although in a few cases, there was also a reduction in the rate of cell viability loss compared to determinations carried out in absence of fermentable substrate. The effect of fermentable substrate on acid tolerance is consistent with previous observations in bifidobacteria (Sanch��z et?al. 2006) and other Gram-positive bacteria (Shabala et?al. 2002; Charalampopoulos et?al. 2003; Corcoran et?al. 2005; Sheng and Marquis 2006). For example, in Listeria monocytogenes, the provision of glucose resulted in a prolonged lag phase, which was attributed to the ability of cells to maintain pH homeostasis (Shabala et?al. 2002). Similarly, http://www.selleck.cn/products/gdc-0068.html fermentable substrate may prolong pH homeostasis in bifidobacteria, rather than improving the ability of these cells to tolerate acid. In many bacteria, pre-exposure of exponential cells to a sub-lethal pH increases the ability to survive a subsequent acid stress (Davis et?al. 1996). Pre-exposure of exponential phase Bif.?longum cells to pH 4��5 resulted in modest improvements in the ability of cells to resist the lethal effect of low pH exposure. Again, improvements in viability were due to an extension of the lag phase, with little difference in the actual rate of cell viability loss following the lag phase in acid pretreated compared to control cells. While a modest ATR appeared to occur following pre-exposure at pH 4��5, no similar effect was observed with Bif.?longum when the cells were preconditioned at pH 5��5. Furthermore, identical treatments of Bif.?breve had no impact on cell survival at low pH. In contrast, Park et?al. (1995) reported increased survival of Bif.