Carola C Kern, Technische Universität München, Freising, Germany
Co-author(s): Patrick Preissler, Rudi Vogel, and Jürgen Behr, Technische Universität München, Freising, Germany
ABSTRACT: Lactobacillus brevis is one of the most frequently encountered bacteria in beer-spoilage incidents. The species L. brevis comprises strains with the ability to grow in low hopped wheat beer, as well as strains that can grow in lager beer or even highly hopped pilsner beer. Accordingly, differentiation and classification of L. brevis based on their beer-spoiling ability improves quality control in breweries. Matrix assisted laser desorption ionization–time-of-flight mass spectrometry (MALDI-TOF MS) has been shown as a powerful and rapid tool for species and subspecies differentiation of bacterial isolates. In principle, MALDI-TOF MS of microbial samples is based on the generation of fingerprints of biomarker molecules, where low molecular weight proteins form a distinctive peak pattern, reflecting protein expression in the cell. The aim of this work was to elucidate whether differences in the proteome of two ecotypes of L. brevis are sufficient to differentiate L. brevis below-species ecotypes by MALDI-TOF MS and with special emphasis on their beer-spoiling ability. L. brevis strains were characterized according to their tolerance to iso-alpha-acids and their growth in wheat, lager, and pilsner beers. Two groups of L. brevis strains, each representing a distinct ecotype, were formed, among which one group showed the ability to grow in pilsner beer or lager beer and generally had a tolerance to iso-alpha-acids above 10 µM. The other group was only able to grow in low hopped wheat beer and exhibited an iso-alpha-acids tolerance below 10 µM. The two groups were analyzed using MALDI-TOF MS. Strain level identification and cluster analysis of the data were performed. Strain level identification was achieved in 85% of a total of more than 200 samples. Mis-identification occurred only among strains belonging to the same ecotype, referring to a strong or weak ability to spoil beer. Cluster analysis split strains into three subgroups: one closely related group of strains that were, with the exception of a single strain, able to grow in lager beer, and two groups of strains, which were only able to grow in wheat beer. Taken together, MALDI-TOF MS allowed strain level classification of L. brevis in about 85% of all samples analyzed and furthermore showed potential to discriminate strains and group them according to ecotypes based on their ability to grow in beer.
Carola C. Kern was born in 1984 in Austria. She obtained her master’s degree in nutritional sciences from the University of Vienna in 2010 and is currently a Ph.D. student at the Technical University of Munich, where she’s working on the identification and differentiation of microbial contaminants in beverages by MALDI-TOF mass spectrometry at the chair of Technische Mikrobiologie under the supervision of Rudi F. Vogel.
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