Genetic markers for beer-spoilage by lactobacilli and pediococci

Abstract

The brewing industry has considerable economic impact worldwide; therefore, demand exists for a better understanding of the organisms that cause beer-spoilage. Low nutrient levels, depleted oxygen levels, high alcohol levels, and the presence of antimicrobial hop compounds all play a role in making beer an inhospitable environment for most microorganisms. Nonetheless, there are bacteria that are resistant to all of these selective pressures. The most common beer-spoilage bacteria are the Gram-positive lactic acid bacteria Lactobacillus and Pediococcus. It is currently believed that hop-resistance is the key factor(s) permitting Lactobacillus and Pediococcus bacteria to grow in beer. However, it is likely that in addition, ethanol-tolerance and the ability to acquire nutrients also play roles in the beer-spoilage ability of Lactobacillus and Pediococcus isolates. The ability of Lactobacillus and Pediococcus to grow in beer was assessed and correlated to the presence of previously described beer-spoilage related genes, as well as with the presence of novel genes identified in this study. Molecular and culture-based techniques for detection and differentiation between Lactobacillus and Pediococcus isolates that can and cannot grow in beer were established and described in detail. Interestingly, beer-spoilage related proteins were often found to share homology with multi-drug transporters. As such, the presence of these beer-spoilage associated genes was also compared to the ability of isolates to grow in the presence of a variety of antibiotics and, unexpectedly, beer-spoiling bacteria were found to be more susceptible to antibiotics than were non beer-spoiling isolates of the same genus. Additionally, it was found that isolates of Lactobacillus and Pediococcus that can grow in beer do not group phylogenetically. In order to fully appreciate the relationship of speciation with beer-spoilage, phylogenetic and whole genome/proteome studies were conducted to clarify the taxonomy of the Lactobacillus and Pediococcus genera. Through the research in this thesis, a greater understanding of the mechanism(s) enabling bacteria to grow in beer has been gained and taxonomy of the genera Lactobacillus and Pediococcus has been clarified.