THE ROLE OF MICROORGANISMS IN THE EFFECT OF FOOD ON HUMAN HEALTH: TWO CASE STUDIES

This PhD project aims to investigate the interactions that microorganisms may have with some food components. These interactions can be exerted on different levels and can have positive or negative consequences. In the first study presented we tried to develop a protocol to identify and quantify the presence of enzymatic activity, in the gut, linked to the conversion of choline into trimethylamine (TMA), the precursor of a pro-atherosclerotic agent, trimethylamine oxide (TMAO). First we designed two pairs of primers, cut-Kp and cut-Dd, on the cutC gene sequence, identified as the gene capable of encoding for the enzyme responsible for converting choline to TMA. Then we proceeded to select 16 healthy volunteers who provided fecal and urine samples once for three weeks. A qPCR was performed on the fecal samples in order to quantify the cutC gene and a 16S rRNA gene profiling. The urine samples were used to quantify TMAO via UPLC-MS. The results obtained from this developed qPCR protocol allowed us to find an association between the levels of TMAO in urine, some taxonomic groups in the gut and a subgroup of the cutC gene present in Enterobacteriaceae. In the second work we used a dairy lactic acid bacteria inoculum for the fermentation of soy drink. Then we characterized and assessed the pro-estrogenic activity of these bacteria. We started inoculated kefir grains into soy drink. We isolated the four best adapted strains from fermented soy: Lactococcus lactis K03, Leuconostoc pseutomesenteroides K05, Leuconostoc mesenteroides K09 and Lentilactobacillus kefiri K10. We tested the viability of the four strains at the end of fermentation, their ability to produce a texture similar to a commercial product and their resistance to antibiotics, and we evaluated their ability to use the oligosaccharides naturally present in soybeans. Finally, we were able to evaluate, through UPLC-MS and the use of a hormonal biosensor in Saccharomyces cerevisiae, the ability of the strains to increase the bioavailability of phytoestrogens present in soybeans. All strains have been shown to ferment sugars and produce a creamy texture. In addition two Leuconostoc belonging genus strains, shows the complete fermentation of soy oligosaccharides and the increase of the estrogenic activity of soy drink. These two works have enabled us to investigate two bacterial enzymatic effects that are very different from each other, which can provide the basis for the development of new practices for human health intervention using bacteria.