Interaction of the probiotic bacterium Lacticaseibacillus rhamnosusGG with the phenolic compound carvacrol and its analysis by proteomic methods

In this thesis study, the effects of carvacrol, a phenolic compound commonly found in many food sources, on the widely known probiotic bacterium Lacticaseibacillus rhamnosus GG (LGG) were evaluated. The central hypothesis posits that carvacrol may contribute to the enhancement of LGG's probiotic properties and potentially influence its proteomic profile. Based on the coexistence of probiotic bacteria and phenolic compounds in the same ecosystem (e.g., the gut microbiota), this study aimed to investigate the interaction between these two health-promoting food components and to elucidate how the synbiotic relationship they may form affects the physiological properties and proteome of the probiotic bacteria. In the experimental design, LGG was treated with carvacrol concentrations ranging from 0 to 800 µg/mL. The minimum inhibitory concentration (MIC) was determined to be ≥800 µg/mL, and therefore, 25, 50, 100, 200, and 300 µg/mL concentrations were selected for further analysis. The results showed that carvacrol did not exhibit any inhibitory effects on LGG at the tested concentrations and, in some cases, even supported bacterial growth. In auto-aggregation assays, a 22% increase in LGG's auto-aggregation ability was observed at 300 µg/mL carvacrol. Co-aggregation analyses revealed time- and concentration-dependent changes in the interaction between LGG and E. coli or S. aureus. Notably, co-aggregation with S. aureus increased by approximately 22% at 200 µg/mL carvacrol. Antioxidant activity tests showed that carvacrol alone had strong antioxidant capacity in both DPPH and ABTS assays; however, growing LGG in the presence of carvacrol did not significantly enhance the antioxidant activity. Similarly, the adhesion ability of LGG was increased 11% by 200 µg/mL carvacrol. In bile salt tolerance tests, the LGG control group demonstrated 94% resistance, while the treated groups ranged between 75% and 93%, suggesting that carvacrol generally did not enhance bile salt tolerance. Regarding pepsin resistance, a decrease was observed at all tested carvacrol concentrations. In the BSH enzyme analysis, an increase in enzyme activity was observed at all concentrations of carvacrol. In the proteomic analysis, carvacrol altered the abundancies of "CHAP domain-containing protein" and "UPF0291 protein AWJ15_03705" among the detected spots. In conclusion, carvacrol was found to support certain probiotic characteristics of LGG while exhibiting neutral or mildly suppressive effects on others. These findings suggest that the use of phenolic compounds in combination with probiotics should be re-evaluated in the context of synbiotic interactions. Furthermore, this study provides a foundational perspective on the potential impact of carvacrol on the proteome of LGG and serves as a preliminary framework for future comprehensive proteomic analyses.