Adaptation of lactic acid bacteria for growth in beer
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Growth of bacteria in beer leads to turbidity and off-flavors, resulting in a spoiled and unpalatable product and thus economic loss. The most common beer-spoilage organisms (BSOs) are lactic acid bacteria (LAB), with Lactobacillus and Pediococcus species being the most problematic. Because of the harsh environment (low nutrients, antimicrobial compounds ethanol and hops, anaerobic), only select isolates are able to sustain growth in and spoil beer. To begin understanding the phenomenon of LAB adapting to overcome stresses in beer, ethanol tolerance, hop resistance, and nutrient acquisition mechanisms were investigated. First, ethanol tolerance was analyzed in the context of beer-spoilage ability, and it was found that it is intrinsically high in LAB, thus leading to the conclusion that LAB ability to spoil beer is not dependent on ethanol resistance levels. This was then followed by genome sequencing of the BSO Pediococcus claussenii ATCC BAA-344T (Pc344) to elucidate mechanisms being used to resist hops and acquire low abundance or alternative nutrients. Subsequent analysis of Pc344 and Lactobacillus brevis BSO 464 via reverse transcription quantitative PCR demonstrated the variability found among BSOs in the presence of beer-spoilage-related genes and their use during growth in beer. Further analysis of Pc344 was performed via RNA-sequencing to get a global view of gene expression during mid-logarithmic growth in beer. It was found that several alternative nutrients were being used by Pc344 to sustain growth, and that hop resistance was enabled by a variety of mechanisms including oxidative stress response and pH control. Finally, genomic comparison of BSOs determined that conservation is only present for closely related organisms and that no specific genes/proteins are indicative of an isolate’s beer-spoilage potential. It is more likely that horizontal gene transfer plays a major role in LAB adaption for growth in beer, and that plasmids are very important for this evolution, as was demonstrated by plasmid-variants of Pc344. The main conclusions of this thesis are therefore that hop resistance is the main factor determining ability to grow in beer, and that transfer of genetic elements is the driving force behind LAB evolving into BSOs.
DegreeDoctor of Philosophy (Ph.D.)
CommitteeDecoteau, John; Deneer, Harry; Xiao, Wei; Kusalik, Anthony; Qualtiere, Lou
Copyright DateAugust 2012