Identification of bacterial contaminants in beverages by MALDI-TOF MS

Carola C Kern, Technische Universität München, Freising, Germany
Co-author(s): Julia Usbeck, Rudi Vogel, and Jürgen Behr Technische Universität München, Freising, Germany

ABSTRACT: The growth of microbial contaminants in industrially produced beverages can cause turbidity, haze, and off-flavors resulting in quality loss and often rendering the product undrinkable. Therefore, rapid and reliable identification and differentiation of spoilage bacteria is crucial in the beverage industry to ensure efficient quality control. As traditional methods for bacterial identification are usually very laborious and time-consuming, there is a high demand for alternative methods. In this work we present matrix assisted laser desorption ionization–time-of-flight mass spectrometry (MALDI-TOF MS) based on the generation of peptide mass fingerprints, which form a distinctive protein peak pattern, as a rapid, reliable, and powerful tool for the identification of spoilage bacteria in beverages. Three strains belonging to the species Lactobacillus brevis, Pediococcus damnosus, and Leuconostoc mesenteroides were used to optimize sample preparation and MALDI-TOF MS settings to ensure resulting spectra were of the best achievable quality. Since MALDI-TOF MS requires a culturing step, and routine quality control procedures differ vastly in practice, growth conditions such as culturing time and availability of oxygen and nutrients were varied to assess their influence on the acquired protein peak pattern. Additionally, about 50 strains, belonging to the families Lactobacillaceae, Leuconostocaceae, and Acetobacteraceae, which are frequently encountered in spoilage incidents, were used for the establishment of a reference spectra database upon optimization of sample preparation. Data processing was performed using ClinProTools 2.2 and MALDI Biotyper 3.0. Routine identification of bacterial samples was successfully implemented. Among the tested parameters, neither culturing time nor availability of oxygen or nutrients impaired identification of the bacterial isolates on the species level, yielding only slight differences in spectra. Closer examination of these differences showed that more than 90% of all spectra could be correctly assigned to the medium used for culturing, whereas no such correlation could be established for culturing time and oxygen availability. Taken together, MALDI-TOF MS allowed differentiation on the species level regardless of the culture conditions used. The application of specific environmental conditions resulted in variations in spectra, which were sufficient to be detected and reliably assigned.

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.