Stress tolerance to low nutrient availability

Yeasts are used to consume and ferment nutrients in wort following the breakdown of grain starches during the malting and mashing processes, in order to produce ethanol. Yeasts can make use of the majority of sugars present in wort, in addition to using available amino acids as a source of nitrogen, in a sequence usually dependent on both the strain of yeast and conditions used (Lodolo et al., 2008; Perpete et al., 2005). Organic acids (acetic, citric, lactic, malic, pyruvic and succinic acid) are left behind by yeasts as metabolic by-products, in addition to unused compounds such as dextrins, arabinoxylans and в-glucans (Gupta et al., 2010). Remaining nutrients in beer following fermentation are typically in low abundance and are often ‘alternative' sources of carbon that can vary from brew to brew within and between breweries.

LAB naturally have an array of possible mechanisms to transfer nutrients into the cell, thus allowing them to inhabit various nutrient-rich or -poor environmental niches. In nutrient-depleted beer, primary nutrient transport via the use of ATP-binding-cassette (ABC) transporters is proposed to allow for advantageous growth (Kon- ings, 1997). These transporters typically have high affinity for a given solute and use ATP-hydrolysis for high-rate transport. Further, secondary transport mechanisms, not requiring ATP but relying on the electrochemical ion gradient to transport molecules across the membrane, involves uniporters, antiporters and symporters for effective uptake of molecules (White et al., 2012b). In some cases, this uptake can even contribute to the production of energy through contribution to the PMF gradient (White et al., 2012b). Lastly, group translocation, a mechanism that chemically modifies a solute that has been internalized can also facilitate the uptake of a range of carbohydrates (White et al., 2012b).

There is considerable evidence for the importance of each type of transport uptake mechanism for BSR LAB. First, there is a great number of ABC transporters among LAB in general, and the importance of these proteins for BSR LAB is probably not yet fully appreciated beyond hop tolerance mechanisms (Konings et al., 1997). Second, recent transcriptomic work on P claussenii ATCC- BAA344T (Pc344) when grown in beer revealed the importance of both secondary transport systems (e.g. the arginine or agmatine deiminase pathways, citrate fermentation) and group translocation such as the phosphotransferase system (PTS) (Pittet et al., 2013). Components of all these systems were actually found to be among the top 20 most significantly expressed transcripts in beer, suggesting the critical role of nutrient-acquisition pathways for survival in beer (Pittet et al., 2013). Interestingly, the significantly expressed agmatine deiminase pathway (AgDI operon) in Pc344 is very similar to the arginine deiminase (ADI) pathway, which is not specific to nutrient-stress, but has been shown to be up-regulated in response to low pH and acid stress, low oxygen concentration, low arginine supply (6 mM) and cell adaptation to arginine in L. sanfransciscensis (de Angelis and Gobetti, 2004). Though the ADI operon is not found in Pc344, the similar AgDI operon was shown to be critically important for survival in the beer environment, and is a major example of the cross-specificity of LAB stress responses. Finally, another example of cross-resistance is the induction of stationary phase in LAB when faced with nutritional starvation, at which point the cells become more resistant to stresses such as heat and acid (Angelis and Gobetti, 2011; Gouesbet et al., 2001).

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