Short-chain fatty acids produced by gut commensal bacteria play an important role in human physiology. Gut Veillonella and the metabolites acetate and propionate, produced by lactate fermentation, are associated with human health.

The human gut hosts numerous ecological niches for microbe–microbe and host–microbe interactions. Gut lactate homeostasis in humans is crucial and relies on various bacteria. Veillonella spp., gut lactate‐utilizing bacteria, and lactate‐producing bacteria were frequently co‐isolated. A recent clinical trial has revealed that lactate‐producing bacteria in humans cross‐feed lactate to Veillonella spp.; however, their interspecies interaction mechanisms remain unclear. Veillonella dispar , an obligate anaerobe commonly found in the human gut and oral cavity, ferments lactate into acetate and propionate. In our study, we investigated the interaction between V. dispar ATCC 17748 ^T and three representative phylogenetically distant strains of lactic acid bacteria, Lactobacillus acidophilus ATCC 4356 ^T , Lacticaseibacillus paracasei subsp. paracasei ATCC 27216 ^T , and Lactiplantibacillus plantarum ATCC 10241. Bacterial growth, viability, metabolism and gene level adaptations during bacterial interaction were examined. V. dispar exhibited the highest degree of mutualism with L. acidophilus . During co‐culture of V. dispar with L. acidophilus , both bacteria exhibited enhanced growth and increased viability. V. dispar demonstrated an upregulation of amino acid biosynthesis pathways and the aspartate catabolic pathway. L. acidophilus also showed a considerable number of upregulated genes related to growth and lactate fermentation. Our results support that V. dispar is able to enhance the fermentative capability of L. acidophilus by presumably consuming the produced lactate, and that L. acidophilus cross‐feed not only lactate, but also glutamate, to V. dispar during co‐culture. The cross‐fed glutamate enters the central carbon metabolism in V. dispar . These findings highlight an intricate metabolic relationship characterized by cross‐feeding of lactate and glutamate in parallel with considerable gene regulation within both L. acidophilus (lactate‐producing) and V. dispar (lactate‐utilizing). The mechanisms of mutualistic interactions between a traditional probiotic bacterium and a potential next‐generation probiotic bacterium were elucidated in the production of short‐chain fatty acids.

Rheumatoid arthritis (RA), an autoimmune disease, is characterized by chronic joint inflammation and pain. We previously found that the deletion of T-cell death-associated gene 8 (TDAG8) significantly reduces disease severity and pain in RA mice. Whether it is by modulating gut microbiota remains unclear. In this study, 64 intestinal samples of feces, cecal content, and cecal mucus from the complete Freund’s adjuvant-induced arthritis mouse models were compared. The α- and β-diversity indices of the microbiome were significantly lower in RA mice. Cecal mucus showed a higher ratio of Firmicutes to Bacteroidetes in RA than healthy mice, suggesting the ratio could serve as an RA indicator. Four core genera, Eubacterium_Ventriosum, Alloprevotella, Rikenella, and Treponema, were reduced in content in both feces and mucus RA samples, and could serve microbial markers representing RA progression. TDAG8 deficiency decreased the abundance of proinflammation-related Eubacterium_Xylanophilum, Clostridia, Ruminococcus, Paraprevotella, and Rikenellaceae, which reduced local mucosal inflammation to relieve RA disease severity and pain. The pharmacological block of the TDAG8 function by a salicylanilide derivative partly restored the RA microbiome to a healthy composition. These findings provide a further understanding of specific bacteria interactions with host gut mucus in the RA model. The modulation by TDAG8 on particular bacteria can facilitate microbiota-based therapy.

Veillonella spp. are nitrate-reducing bacteria with anaerobic respiratory activity that reduce nitrate to nitrite. They are obligate anaerobic, Gram-negative cocci that ferment lactate as the main carbon source and produce short-chain fatty acids (SCFAs). Commensal Veillonella reside in the human body site where lactate level is, however, limited for Veillonella growth. In this study, nitrate was shown to promote the anaerobic growth of Veillonella in the lactate-deficient media. We aimed to investigate the underlying mechanisms and the metabolism involved in nitrate respiration. Nitrate (15 mM) was demonstrated to promote Veillonella dispar growth and viability in the tryptone-yeast extract medium containing 0.5 mM L-lactate. Metabolite and transcriptomic analyses revealed nitrate enabled V. dispar to actively utilize glutamate and aspartate from the medium and secrete tryptophan. Glutamate or aspartate was further supplemented to a medium to investigate individual catabolism during nitrate respiration. Notably, nitrate was demonstrated to elevate SCFA production in the glutamate-supplemented medium, and further increase tryptophan production in the aspartate-supplemented medium. We proposed that the increased consumption of glutamate provided reducing power for nitrate respiration and aspartate served as a substrate for fumarate formation. Both glutamate and aspartate were incorporated into the central metabolic pathways via reverse tricarboxylic acid cycle and were linked with the increased production of acetate, propionate, and tryptophan. This study provides further understanding of the promoted growth and metabolic mechanisms by commensal V. dispar utilizing nitrate and specific amino acids to adapt to the lactate-deficient environment. IMPORTANCE: Nitrate is a pivotal ecological factor influencing microbial community and metabolism. Dietary nitrate provides health benefits including anti-diabetic and anti-hypertensive effects via microbial-derived metabolites such as nitrite. Unraveling the impacts of nitrate on the growth and metabolism of human commensal bacteria is imperative to comprehend the intricate roles of nitrate in regulating microbial metabolism, community, and human health. Veillonella are lactate-utilizing, nitrate-reducing bacteria that are frequently found in the human body site where lactate levels are low and nitrate is at millimolar levels. Here, we comprehensively described the metabolic strategies employed by V. dispar to thrive in the lactate-deficient environment using nitrate respiration and catabolism of specific amino acids. The elevated production of SCFAs and tryptophan from amino acids during nitrate respiration of V. dispar further suggested the potential roles of nitrate and Veillonella in the promotion of human health.

The metabolic processes of Bifidobacterium longum subsp. infantis , an early coloniser of the human gut, are essential for gut health, mainly due to the production of indole derivatives from tryptophan. This study investigates the capacity of B. infantis ATCC 15697 to biosynthesise indole‐3‐lactate (ILA), indole‐3‐acetate (IAA), and indole‐3‐carboxaldehyde (I3CA) and the regulatory effects of substrate availability on these pathways. The tryptophan catabolic profile of B. infantis ATCC 15697 under a non‐growing but metabolically active state was investigated. Through HPLC‐PDA and LC–MS analyses, we confirmed for the first time the production of IAA and I3CA by B. infantis ATCC 15697. The results revealed a dose‐dependent relationship between tryptophan availability and the production of indole derivatives, highlighting the nutrient‐driven effect of these metabolic pathways. By integrating genomic analysis with metabolic profiles, we proposed potential pathways underlying the biosynthesis of IAA and I3CA from tryptophan. These findings enhance our understanding of the role of B. infantis ATCC 15697 in human health, with ILA, IAA, and I3CA contributing to immune modulation and gut health. We also provide a platform for using B. infantis ATCC 15697 as a biocatalyst for the biosynthesis of beneficial indole derivatives through whole‐cell bioconversion, which was further demonstrated in B. infantis ATCC 25962 and ATCC 15702. Future in vivo studies will help clarify the impact of these metabolites on the gut environment and inform dietary and probiotic strategies for enhancing indole derivatives production.