Dissolved Carbon Dioxide Selects for Lactic Acid Bacteria Able to Grow In and Spoil Packaged Beer

Lactic acid bacteria (LAB) are frequently found as beer-spoilage organisms (BSOs). Correctly identifying a LAB as a BSO is problematic, given that there are few known genetic markers that distinguish beer-spoiling from non-beer-spoiling LAB. Currently, genes purported to participate in hop-tolerance mechanisms are heavily relied upon to indicate LAB isolates with the potential to spoil beer, even though these genes do not consistently correlate with beer-spoilage. Though the presence of hops certainly is a significant physiological stress for bacteria in beer, we demonstrate here that the presence of CO2 dissolved in beer is a strong selective pressure for true LAB beer-spoilage ability (i.e., the ability to grow in and spoil a finished and packaged beer). We screened 20 LAB for their capability to survive and grow in gassed beer at 22 and at 30 degrees C, and discuss the results in relation to ethanol and hop tolerance. Functional gene comparisons of nine dissolved CO2-tolerant and nontolerant genome-sequenced isolates reveal potential metabolic pathways of interest for further study, specifically those that deal with cell dormancy and stress responses. These results further our understanding of LAB BSOs and have implications for how best to analyze these bacteria in laboratory settings and to test for these bacteria in the brewery.