Research

Current research established the role of crop plants as secondary hosts for human pathogens. It is yet unknown, how these interactions and technologies applied in food processing impact on the virulence of the pathogens. The focus of our research is on the interaction of the human pathogens Listeria monocytogenes with lettuce:

• Pathogens apply similar mechanisms to adhere on/internalize in plant and human hosts. We are studying gene expression patterns of selected pathogens in the presence of plants. The localization of the bacteria on and in the plant will be correlated with the expression of selected marker genes. Genes encoding relevant structures for bacterial adhesion and internalization will be deleted and their function thus confirmed. This will be done by microscopical methods.
• Pathogens react to their host by a change in their transcriptional patterns, which might result in changed virulence. We aim at studying this adaptation of adhered and/or internalized cells in parallel to the reaction of the plant (DualRNASeq), and how that impacts on bacterial virulence. As plants harbor an autochthonous microbiota and as microorganisms interact directly and/or indirectly, this results in a network of interactions rather than in a direct effect.

Suitable strains for targeted food fermentation will be selected by applying “omics” technologies

This project is based on a finished FEI-project (AiF 19688 N). Lactococcus lactis is industrially applied as a starter organism for dairy fermentations. The strains of this species are genetically diverse because of their plasmidome. Genes encoding key enzymes enabling growth in dairy environments are also plasmid-encoded. It is current knowledge from industrial practice that some strains induce bitterness in fermented dairy products, while others do not. We investigate which genetic determinants in combination with which environmental factors, especially enhanced calcium contents such as usually found in dairy fermentations, induce enhanced bitterness. Based on own results we hypothesize that calcium has an impact on bitter peptide formation which is not due to modified transcription levels of genes encoding the enzymes/proteins of the proteolytic system.

In a project funded by the Research Association of the German Food Industry (FEI) we investigate how plant-based alternatives for raw sausage and raw milk cheese can be produced by fermentation. This work is carried out at the University of Hohenheim. In a first step, suitable strains need to be identified, which reproducibly lower the pH-value in the selected raw materials and under the given fermentation conditions. The strains also need to inhibit the growth of pathogens and spoilage microorganisms by the production of antimicrobial compounds. The selected strains should also be capable of metabolizing antinutritive plant compounds, which would result in a reduced off-flavor.

This research is part of the Marie Skłodowska Curie Innovative Training Network project ITN-ETN-956257 HiStabJuice “Establishing a strong and lasting international training network for innovation in food and juice industries: A 4D-research approach for fruit juice processing”. In total 17 academic and industrial partners evaluate classical and non-thermal inactivation processes concerning their impact on microorganisms, enzymes and valuable compounds in food. The hypotheses of our project part are:

• Non-thermal technologies can yield comparable results to thermal processes by the selection of suitable parameters. To identify these parameters, the D- and z-values of spoilage organisms and pathogens are determined in diverse processes and food matrices.
• A selection of suitable surrogate microorganisms for enterohemorrhagic E. coli (in cooperation with the Department of Food Microbiology and Hygiene, University of Hohenheim) and Salmonella spp. Is carried out. The inactivation curves of potential surrogates is investigated in buffer systems and in food matrices. Selected strains are transferred to pilot-plant scale (cooperation with industrial partners).
• Conventional and non-thermal technologies have different impacts on the gene expression patterns of microorganisms. This influence is investigated for selected strains by RNASeq, with a special focus on the transcription of virulence genes.