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Hop tolerance genes and the brewing environment

Until a recent study by Bokulich et al. (2015), there was no previous analysis of hop tolerance gene dispersion in the brewery environment. This study made use of next-generation ddPCR to quantify the abundance of hitA, horA, horB and horC, in conjunction with associated microbial community profiles assessed by LAB-TRFLP on various brewery surfaces over time.

Firstly, this study determined that areas involved with sour beer production had the highest gene frequencies, specifically horC (Bokulich et al., 2015). This gene was the most abundant gene in general and was found in a nearly equal ratio with that of its putative transcriptional regulator horB, enforcing the notion that horC is an important and prevalent hop tolerance gene selected for in the brewing environment. The hitA gene had the lowest frequencies throughout the brewery, corroborating a previous report of low detected frequencies in BSR LAB (Haakensen et al., 2008). Indeed, hitA also had a lower correlation with the presence of the other hop tolerance genes, which shared amongst themselves high degrees of intercorrelation (Bokulich et

al., 2015). horA was the only gene correlated with Pediococcus in the brewery, supporting the previous observation that this gene is the primary known resistance gene for this genus (Haakensen et al., 2008). Most interesting was the fact that no genes correlated with L. brevis, which is not only considered the most common brewery contaminant species, but has also been shown to be among the LAB most commonly positive for hop-resistance genes (Haakensen et al., 2008). However, given that L. brevis was only a minor component of sour beer and processing surfaces, this finding is probably particular to the brewery under analysis (Bokulich et al., 2015).

These results reveal the importance of tracking spoilage genes within the brewery environment in order to understand contamination risks and patterns, especially where more than one beer style is produced. For examples, barrel surfaces, fermenters and packaging-line surfaces (that all come into contact with beer) exhibited fairly high levels of hop-resistance genes with the highest detection on surfaces that contacted sour beers and in unsanitary areas such as the packaging-line sink and below the packaging belt (Bokulich et al., 2015). These findings are highly illuminating for development of brewery best practices, in that equipment for the production of sour or specialty beer must be specifically dedicated and adequately separated from equipment used in conventional brewing. Further, contact with beer is strongly implicated in transmission of hop tolerance genes and BSR LAB between different areas of the brewery; thus protocol and human activities must limit this transfer. Tracking transmission of hop tolerance (and other important genes) within the brewing environment is an incredibly worthwhile undertaking given that it will add insight into the role of environmental versus raw material contamination, and into the propagation of hop tolerance genes and BSR LAB within the brewery (Bokulich et al., 2015).

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