Stress tolerance and adaptation of BSR LAB
There is considerable literature that discusses general and specific stress responses of LAB in a variety of industries. Though stress tolerance can differ among isolates of the same species, LAB are highly adaptable to stressful environments and adaptation to one particular stress often affords LAB increased tolerance to the challenge of another stress, due to the cross-regulation and functions of stress- response pathways (de Angelis and Gobetti, 2011; Parente et al., 2010). BSR LAB isolates exemplify complex stress-response regulation given that isolates must simultaneously employ tolerance mechanisms to a variety of stresses.
Stress tolerance to ethanol and low pH
Ethanol levels and pH differ among styles of beer worldwide, typically within the ranges of 0.5-14% (v/v) ethanol and pH 3.8-4.7 (Suzuki et al., 2008a). As a consequence, LAB recovered from beer within or outside these ranges are typically well adapted to one or both of these stresses (Suzuki, 2011). Further, most BSR LAB produce either lactic or acetic acid due to their basic fermentation, which naturally lowers the pH of the surrounding environment. Indeed, it has been reported that decreased pH and increased ethanol in beer had little effect on the growth of LAB, and that there is no correlation between these two factors and contamination, though pH values near 4.0 or below had some inhibitory effect on LAB (Menz et al., 2010). Nonetheless, adaptation to the acidity found in beer is necessary, as low pH can interfere with enzymatic reactions, protein folding and other intracellular processes of non-pH-tolerant organisms. LAB, and other pH-tolerant organisms are capable of regulating their intracellular pH in face of acidic conditions through means of proton transport across the cellular membrane (often coupled to cation transport) or through proton-translocating ATP synthase (de Angelis and Gobetti, 2011).
Ethanol, like hops, is an antimicrobial component of beer, easily crossing the bacterial membrane and then modifying activity of cytoplasmic processes such as protein folding and inhibiting enzymatic actions. Ethanol also increases cell membrane permeability through alteration of the polarity of aqueous and hydrophobic regions of the phospholipid membrane, causing leakage of small molecules from the cell and cell death (Ingram, 1990). Various tolerance mechanisms may combat these effects, such as membrane fortification through an increase in long-chain fatty acids (> 20 carbons) (Uchida, 1974). Other general stress-response proteins such as the GroES chaperone, heat-shock proteins (HSP), and glutathione reductase (Fiocco et al., 2007; Silveira et al., 2004) confer increased survival during ethanol stress, as well as to other stresses (Angelis and Gobetti, 2011). For BSR LAB, it has been found that ethanol tolerance does not differ significantly between BSR and non-BSR LAB, and that overall LAB ethanol- tolerance levels were species-conserved, unlike beer spoilage capacity (Pittet et al., 2011). Though BSR LAB adaptation to low pH and ethanol are important, it does not appear that either is necessarily predictive of the ability to tolerate hops, nor ability to spoil beer (Bergsveinson et al., 2015a,b; Menz et al., 2010; Pittet et al., 2011).