# _Lactobacillus rhamns GG_ as Biosensor for Oral and Systemic Health Conditions: A Pilot Study
## 3 Results and Discussion
Glucose-6-phosphate dehydrogenase (G6 PD) is a key enzyme in the pentose phosphate pathway, essential for maintaining cellular redox balance and facilitating biosynthesis. G6 PD helps maintain the levels of NADPH, which is crucial in combating oxidative stress [41]. Human saliva contains various components that can induce oxidative stress, and an increase in G6 PD expression can indicate a response to this stress [42]. Changes in G6 PD expression can reflect the overall health and viability of LGG cells under different conditions, making it  
a useful biomarker for studying the effects of human saliva on these bacteria [43]. In several studies, the G6 PD gene was noted as a potential prognostic biomarker for cancers [44] in cancer metabolism and may be a potential therapeutic target gene for tumor therapy [45]. Another study discusses the role of G6 PD in predicting glioma risk and its potential as a biomarker for prognosis and treatment response [46]. Figure 3: Expression levels of the selected protein in LGG are measured in relative expression units. Whiskers indicate means + 1 standard deviation. Significant group differences are represented as visually separated circles (Tukey’s test, \(p\) < 0.05). (**a**) Glucose-6-phosphate 1-dehydrogenase (G6 PD), (**b**) NADH-flavin reductase, (**c**) peptide methionine sulfoxide reductase (MsrA), (**d**) PTS System IIA Component, (**e**) Ribose-phosphate pyrophosphate kinase (RPPK), (**f**) Adhesin Domain-Containing Protein. In Figure 3b, the expression levels of NADH-flavin reductase are shown. BUFR has the lowest levels, indicating limited activity of flavin-dependent redox reactions in unstressed conditions. On the contrary, LAC shows very high upregulation. SU10 shows intermediate values. SLFU and SLPO exhibit moderate upregulation, indicating a greater reliance on flavin-mediated detoxification and redox balance. This trend suggests that the SLAN, SANT, and sucrose groups experience increased oxidative stress, engaging in metabolic pathways that require increased NADH-flavin reductase activity. The results underscore this enzyme's role in mitigating oxidative damage and maintaining cellular homeostasis under stressful conditions. NADH-flavin reductase is an enzyme that catalyzes the reduction of flavins, such as flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), using NADH as the electron donor [47]. This enzyme is crucial in maintaining redox balance and regulating energy metabolism. When LGG is exposed to different environments, such as lactic acid, buffer, or sucrose, the activity of NADH-flavin reductase can indicate how the bacteria adapt their metabolism to these conditions. It is specifically upregulated in a Lactic acid environment, as described by [PERSON] et al. in 2005, LGG is intrinsically resistant to lactic acid, and it needs glucose for survival in the gut environment. The survival of probiotic lactobacillus in acidic environments is enhanced in the presence of metabolizable sugars [48; 49]. Changes in the expression or activity of NADH-flavin reductase can reflect the overall health and viability of LGG cells under various environmental stresses, making it a useful biomarker. In Figure 3c, MsrA (peptide methionine sulfoxide reductase) expression was lowest in sucrose (SU10), moderately upregulated in SLPO, and highest in LAC and SLFU. The observed variance suggests that ROS levels are low and oxidative damage to methionine residues is minimal in BUF but escalates in SLAN, SANT, and SLPA. The pronounced upregulation in SLFU suggests that this situation entails higher oxidative stress, necessitating extensive repair of oxidized methionine residues. These results highlight a critical protective function against ROS-induced protein damage, especially under the stress conditions represented by SLFU and LAC. Peptide methionine sulfoxide reductase is an enzyme that reduces methionine sulfoxide to methionine, repairing the oxidatively damaged proteins. This enzyme is crucial for protecting cells from oxidative stress [50].