Utility of BSR LAB hop tolerance genes
Questions remain concerning the utility of hop tolerance genes in predicting beer spoilage ability; however, there is no denying these genes are relevant to BSR LAB and the brewing environment.
The most notable feature of these genes is that they are not species-specific markers for hop-tolerance. For example, horA and horC (in addition to their flanking open reading frame (ORF) regions) are found to be well conserved in other BSR LAB isolates such as L. backii, L. lindneri, L. paracollinoides, P claussenii, and P damnosus, in addition to L. brevis (Iijima et al., 2007; Pittet et al., 2012; Suzuki et al., 2004a, 2005, 2011). It has even been reported that these two genes are found at rates as high as 94% and 96% of BSR LAB tested and that all strains have at least one of the genes (Suzuki, 2011). Caution is required; however, since the full gene length of hop tolerance genes is rarely sequenced in brewery settings; therefore, the sequence similarity, let alone the functional integrity of these genes, is rarely guaranteed.
Though targeting hop tolerance genes currently remain the strongest discriminatory method to detect intra-species differences in beer spoilage ability (Iijima et al., 2007; Sami et al., 1997b), these genes are still unable to predict the beer spoilage capacity of the full spectrum of BSR LAB that have been described. Unfortunately, to date, there are few data suggesting alternative hop tolerance genes or mechanisms in the absence of any of hitA, horA, or horC (Menz et al., 2010). This lack of compensatory theories is frustrating in light of the physiological variability (hop-tolerance, growth phenotype) between strains that have identical hop tolerance genes profiles; other uncharacterized hop tolerance mechanisms must exist (Haakensen et al., 2008, 2009c; Menz et al., 2010). Since the known hop tolerance functions are ABC transporters or efflux pumps, and since both types of transporter are common within LAB (Konings et al., 1997; White et al., 2012b), it is short-sighted to not conceive of other similar genes and proteins across the spectrum of BSR LAB that deal with hops directly or indirectly, or that deal with other stresses in beer. Indeed, given the multitude of stress in beer, the ability to grow in and mediate the damage of both beer and hops, is probably the result of a synergy of mechanisms and redundant genetic traits. Until more detailed and high-throughput analyses of these processes are conducted, we remain hindered in our capacity to screen for elements that describe true beer spoilage ability.