Considerable evidence points to the critical role of the gut microbiota in physiology and disease. The administration of live microbes as a therapeutic modality is increasingly being considered. However, key questions such as how to identify candidate microorganisms and which preclinical models are relevant to recapitulate human microbiota remain largely unanswered. The establishment of a humanized gnotobiotic mouse model through the fecal microbiota transplantation of human feces into germ-free mice provides an innovative and powerful tool to mimic the human microbial system. However, numerous considerations are required in designing such a model, as various elements, ranging from the factors pertaining to human donors to the mouse genetic background, affect how microbes colonize the gut. Thus, it is critical to match the murine context to that of human donors to provide a continuous and faithful progression of human flora in mice. This is of even greater importance when the need for accuracy and reproducibility across global research groups are taken into account. Here, we review the key factors that affect the formulation of a humanized mouse model representative of the human gut flora and propose several approaches as to how researchers can effectively design such models for clinical relevance.

Emerging research underscores the pivotal role of the gut–immune–brain axis, a dynamic bidirectional communication system involving intricate interactions between the gut microbiota, immune responses, and the central nervous system. Gut microbes and their metabolites have profound effects on immune and neurological homeostasis, influencing the development and function of multiple physiological systems. Disruption of the composition of the gut microbiota and barrier integrity has been implicated in various neurological and psychiatric disorders, including autism spectrum disorder, Alzheimer’s disease, Parkinson’s disease, depression, and anxiety. Most insights into these host–microbiota interactions come from preclinical models, revealing both the complexity and potential therapeutic opportunities of the gut–brain communication pathways. This review synthesizes the current understanding of these intricate interactions, exploring how microbiota-driven modulation of the gut and brain barriers, immune signaling, and neuronal pathways, such as those through the vagus nerve, contributes to health and disease. We further explore therapeutic implications, including personalized precision microbiota interventions, microbiome-derived biomarkers, and barrier-strengthening strategies. Advancing this field offers transformative potential for developing innovative, personalized therapies tailored to individual microbiomes and immune profiles, ultimately redefining clinical approaches to neurological and immune-mediated diseases.

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by alterations in social, repetitive, and anxiety-like behaviors. While emerging evidence suggest a gut-brain etiology in ASD, the underlying mechanisms remain unclear. To dissect this axis, we developed a germ-free BTBR mouse model for ASD. The absence of gut microbiota in male mice ameliorates ASD-associated behaviors and reduces populations of inflammatory brain-resident T cells. Additionally, CD4⁺ T cell depletion mitigates neuroinflammation and ASD behaviors, suggesting a gut-immune-brain axis. We identify several microbial and metabolic regulators of ASD, particularly those relevant to the glutamate/GABA ratio and 3-hydroxyglutaric acid. Using an in silico metabolite prediction model, we propose Limosilactobacillus reuteri IMB015 (IMB015) to be a probiotic candidate. Administration of IMB015 reduces the glutamate/GABA ratio and neuroinflammation, resulting in improved behaviors. Here we report a gut-immune-brain axis in which the gut microbiota and its metabolites can modulate brain-resident immune cells and ASD-associated behaviors.

The composition and diversity of gut microbiota significantly influence the immune system and are linked to various diseases, including inflammatory and allergy disorders. While considerable research has focused on exploring single bacterial species or consortia, the optimal strategies for microbiota-based therapeutics remain underexplored. Specifically, the comparative effectiveness of bacterial consortia versus individual species warrants further investigation. In our study, we assessed the impact of the bacterial consortium MPRO, comprising Lactiplantibacillus plantarum HY7712, Bifidobacterium animalis ssp. lactis HY8002, and Lacticaseibacillus casei HY2782, in comparison to its individual components. The administration of MPRO demonstrated enhanced therapeutic efficacy in experimental models of atopic dermatitis and inflammatory colitis when compared to single strains. MPRO exhibited the ability to dampen inflammatory responses and alter the gut microbial landscape significantly. Notably, MPRO administration led to an increase in intestinal CD103⁺CD11b⁺ dendritic cells, promoting the induction of regulatory T cells and the robust suppression of inflammation in experimental disease settings. Our findings advocate the preference for bacterial consortia over single strains in the treatment of inflammatory disorders, carrying potential clinical relevance.

Lee C, et al. Immune Netw. 2023 Feb;23(1):e7. https://doi.org/10.4110/in.2023.23.e7

The gut microbiota plays an essential role in mucosal immunity, with secretory immunoglobulin A (IgA) acting as a key effector in neutralizing pathogens an