A review of hop resistance in beer spoilage lactic acid bacteria

Hop bitter acids play a major role in enhancing the microbiological stability of beer. However, beer spoilage lactic acid bacteria (LAB) are able to grow in beer by exhibiting strong hop resistance. Recently two hop resistance genes, horA and horC, have been identified in beer spoilage Lactobacillus brevis ABBC45. The horA gene was shown to encode an xrP dependent multidrug transporter that extrudes hop bitter acids out of bacterial cells. In contrast, the product of the horC gene confers hop resistance by presumably acting as a proton motive force (PMF)-dependent multidrug transporter. Strikingly, the homologs of horA and horC genes were found to be widely and almost exclusively distributed in various species of beer spoilage LAB strains. indicating these two hop resistance genes are excellent species-independent genetic markers for differentiating the beer spoilage ability of LAB. Furthermore the nucleotide sequence analysis of horA and horC homologs revealed that both genes are essentially identical among distinct beer spoilage species, indicating horA and horC have been acquired by beer spoilage LAB through horizontal gene transfer. Taken collectively, these insights provide a basis for applying horA and horC to the species-independent determination of beer spoilage LAB, including yet uncharacterized species. In addition to the hop resistance mechanisms mediated by multidrug transporters, proton translocating ATPase and the ATP production system were shown to contribute to the hop resistance mechanisms in beer spoilage LAB by generating PMF and ATP that are necessary or survival in beer.