Many studies have shown that the chemical composition and physiological activity of tea can change upon microbial fermentation [16]. In fact, the predominant microorganisms within dark tea exert positive effects on tea quality. Fermented autumn green tea is produced using Eurotium cristatum, with significant differences reported in the contents of amino acids, catechins, alkaloids, polyphenols and flavonoids following fermentation [17]. Meanwhile, no significant differences were observed in the total polyphenols of Tibetan tea following B. circulans fermentation. However, UHPLC-Q-TOF analysis revealed a significant increase in tea catechin derivatives, flavonoids, phenolic acids, and other active substances compared to unfermented (p < 0.05). These chemical components have certain regulatory effects on the intestinal microbiota, as well as anti-inflammatory and anti-oxidation properties [18–20].
In this study, fermented Tibetan tea was found to promote the intestinal microbiota diversity and growth of probiotics in mice, in particular, the Lactobacillus spp., a probiotic, occupies a large portion of the intestinal microbiota and plays a central role in normal intestinal regulation and homeostasis. Lactobacillus spp. can regulate intestinal flora and inhibit the growth of harmful bacteria [21], while also altering the acid–base properties of the intestinal flora by secreting acidic substances (such as lactic acid) to facilitate multiplication of the beneficial intestinal flora. Moreover, fermented Tibetan tea reduces the abundance of Proteobacteria,but increases the abundance of Muribaculaceae and Lachnospiraceae_NK4A136_group, which may be due to an increased abundance of Lactobacillus spp.. Notably, Lactobacillus spp., Muribaculaceae, and the Lachnospiraceae_NK4A136_group were also identified as the dominant short-chain fatty acid (SCFA) producers. SCFAs are crucial mediators that regulate the intestinal immune response and maintain intestinal epithelial integrity [22].
Lactobacillus spp. plays a role in epithelial growth and survival, as well as innate and adaptive immune development and regulation [23]. In this way, they can promote intestinal cell growth and regulate immune functions[24]. Moreover, these bacteria support the growth of intestinal mucosal epithelial cells, thereby promoting the regeneration of the intestinal mucosal epithelium. Lactobacillus spp. also regulate the homeostasis of the intestinal mucosal layer via intestinal immune cells, such as RORγt (+) IL-22(+) ILC3 cells, which can influence the proliferation of mucosal cells and mucin production [25]. Indeed, increasing the number of Lactobacillus spp. cells has been shown to increase the number of goblet cells and enhance their mucin secretion [26], forming a dense protective layer through which the intestinal epithelium and microbiota interact [27]. Additionally, Lactobacillus species reduce the expression inflammatory markers and improve the immunity of mice [28, 29]. More specifically, fermentation products secreted by Lactobacillus spp. contain a large number of active factors that promote the repair of intestinal immune functions and regulate the number of immune cells. In particular, the SCFAs secreted by Lactobacillus spp. in the intestine serve as an important energy source for the intestinal flora and intestinal epithelial cells, which maintain the intestinal acid–base balance, repair intestinal mucosal damage, relieve intestinal inflammation, regulate host intestinal immunity, and inhibit the growth of harmful pathogens [30]. Hence, given that we observed an upregulation of mucin-1, mucin-2, and ZO-1 mRNA expression, and downregulation of TNF-α, IL-1β, and P53 expression in the BC group compared to the other groups, the fermented Tibetan tea may regulate Lactobacillus spp. to promote these specific physical and chemical barriers of the intestinal mucosa.
The liver is an important antioxidant and immune organ that is closely related to intestinal metabolism. In fact, gut microbiota affects the hepatic metabolic capacity [31]. More specifically, Lactobacillus spp., and their metabolites, can improve the antioxidant and anti-inflammatory capabilities of the liver and protect mouse livers [32]. The T-AOC is a measure of various antioxidant substances and enzymes, representing the total antioxidant level; GSH and CAT, the major oxidases, can scavenge free radicals to maintain a low concentration and avoid accumulation of excessive free radicals in damaged tissues and organs [33]. Moreover, redox imbalances can activate pro-inflammatory signaling pathways, with excess free radicals inducing persistent (chronic) upregulation of pro-inflammatory mediators, such as IL-1β, COX-2, and iNOS [34]. In this study, mice administered the fermented tea exhibited increased T-AOC, GSH, and CAT activities, as well as reduced MDA levels in the liver. Fermented tea also significantly increased the expression of IκBα, Cu/Zn-SOD, SOD, and GSH, while decreasing the expression of COX-2 and iNOS. The inhibitory protein IκBα sequesters the transcription factor NF-κB as an inactive complex in the cytoplasm. Interestingly, Lactobacillus spp., via inhibition of IκBα degradation, reportedly reduces NF-κB levels induced by Shigella Flexneri [35]. Hence, collectively, the findings of the current study indicate that fermented Tibetan tea could improve the antioxidant capacity of the liver, potentially through the enhancement of intestinal probiotics such as Lactobacillus spp..