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 <i>Lactobacillus</i> and <i>Pediococcus</i>. It is currently believed that hop-resistance is the key factor(s) permitting <i>Lactobacillus</i> and <i>Pediococcus</i> 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 <i>Lactobacillus</i> and <i>Pediococcus</i> isolates. The ability of <i>Lactobacillus</i> and <i>Pediococcus</i> 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 <i>Lactobacillus</i> and <i>Pediococcus</i> 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 <i>Lactobacillus</i> and <i>Pediococcus</i> 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 <i>Lactobacillus</i> and <i>Pediococcus</i> 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 <i>Lactobacillus</i> and <i>Pediococcus</i> has been clarified.

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 <i>Lactobacillus</i> and <i>Pediococcus</i>. It is currently believed that hop-resistance is the key factor(s) permitting <i>Lactobacillus</i> and <i>Pediococcus</i> 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 <i>Lactobacillus</i> and <i>Pediococcus</i> isolates. The ability of <i>Lactobacillus</i> and <i>Pediococcus</i> 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 <i>Lactobacillus</i> and <i>Pediococcus</i> 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 <i>Lactobacillus</i> and <i>Pediococcus</i> 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 <i>Lactobacillus</i> and <i>Pediococcus</i> 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 <i>Lactobacillus</i> and <i>Pediococcus</i> has been clarified.