Ethanol Tolerance of Lactic Acid Bacteria, Including Relevance of the Exopolysaccharide Gene Gtf

Abstract

Lactic acid bacteria (LAB) are generally ethanol-tolerant organisms that have a higher resistance to ethanol than most bacteria. However, little is known with regard to the role ethanol tolerance plays in beer spoilage. Various stresses found in beer need to be overcome for an organism to be able to grow and cause spoilage. Because of this, a broad range of beer-spoilage abilities is found in LAB, and no conservation of this phenotype exists within species. As such, it is very difficult to accurately predict when a contaminating LAB would be able to spoil beer. Analysis of LAB ethanol tolerance was performed to determine whether a predictive factor could be found for the ability to grow in beer. Minimum inhibitory and bactericidal concentrations were determined for 61 LAB that were also analyzed for their ability to spoil beer. No significant correlation was found between ethanol tolerance and ability to spoil beer because ethanol tolerance was essentially conserved within species. In addition, 153 LAB isolates were screened for the glucosyltransferase gene gtf, which is responsible for exopolysaccharide (EPS) production, to determine whether the presence of the gene was correlated with the ability to spoil beer or to tolerate high ethanol concentrations. The gtf gene was found in only six isolates, and no difference in beer-spoilage ability was found between ropy and nonropy isolates. Further, ethanol tolerance of EPS-producing variants was comparable with their nonropy counterparts. The results of this study show that ethanol tolerance does not play a discriminating role in LAB beer spoilage and that the presence of the gtf gene does not provide a selective advantage for ethanol tolerance or beer spoilage.