8:00 am - 9:00 am
A graduate exam seminar is a presentation of the student’s final research project for their degree.
This is an ALES PhD Final Exam Seminar by Qing Li. This seminar is open to the general public to attend.
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Conference ID: 873163402
Thesis Topic: Characterization of lifestyle-associated metabolic traits in host-adapted lactobacilli
PhD with Dr. Michael Gaenzle.
Lactobacilli are present in a majority of food fermentations, and the fermentation organisms of different fermented foods are recruited from free-living, host-adapted, and nomadic lactobacilli. The metabolic traits of host-adapted lactobacilli relate to their ecological success in their vertebrate or insect hosts and also facilitate the ecological fitness of host-adapted lactobacilli in fermented foods. The corresponding metabolites contribute to the flavor, structure, and quality of fermented food. This dissertation aimed to characterize glutaminase, and extracellular fructanase and arabinanases that are lifestyle-associated and exclusively presence in host-adapted lactobacilli.
Acid resistance is essential for the competitiveness and persistence of host-adapted lactobacilli in fermented food and in host habitats. Glutaminase contributes to acid resistance of Limosilactobacillus reuteri at pH 2.5 but not pH 3.5 but its specific contribution to glutamate and GABA accumulation was not elucidated. Experimentation with L. reuteri derivative with deletions of three gls genes demonstrated that glutaminases are not required for deamidation of glutamine in cereal substrates. Moreover, the presence of gls-gadB / C operon is not a requirement for acid resistance mediated by glutaminase.
The sourdough isolate Lactobacillus crispatus DSM29598 harbors the putative extracellular fructosidase FruA and the putative arabinanases AbnA and AbnB. Surface layer protein domains at the C-terminor of FruA and AbnA indicate their association with the cell wall. FruA degraded inulin and levan to produce fructose as end products, and the activity was not repressed by glucose. The application of conventional sourdough in bread making substantially reduced fructan in bread, improving the tolerance in many individuals; sourdough fermentation with FruA-expressing L. crispatus reduced fructans in bread by more than 90% to produce a low-FODMAP bread.
Extracellular arabinanases allowed the utilization of linear arabinan as the carbon source for the growth of L. crispatus. Two arabinanases, AbnA and AbnB, are located at the same arabinan utilization operon encoding multiple arabinan and arabinose metabolic enzymes. Both arabinanases and the operon are exceptional in L. crispatus as strains of this species are generally unable to use arabinose as carbon source. The characterization of AbnA and AbnB demonstrated that both enzymes hydrolyzed linear arabinan. AbnA also degraded branched arabinan.
This dissertation provides basic knowledge on lifestyle-associated metabolic traits of host-adapted lactobacilli related to acid resistance and extracellular polysaccharide hydrolysis, which is essential to guide the application of lactobacilli in food fermentation or as probiotics, and to explore their impact on host health.