Background

Hundreds of trillions of microorganisms, including bacteria, virus, fungi and archaea, reside in our distal intestines and mutually interact with co-evolved host immune cells in a beneficial reciprocal relationship that is influenced by host genetics and environmental factors, including the diet [1,2,3]. The microbiota evolved to colonize specialized ecological niches of the human gastrointestinal tract and to utilize variable diets, while the human mucosal immune system evolved to protect the host from harmful microbial pathogen exposures, yet prevent chronic intestinal inflammation [3, 4]. Enteric resident microbiota exists as a consortium that contains both putative proinflammatory and protective strains [5, 6]. A delicate balance between those functionally distinct populations is maintained in healthy individuals, while patients with inflammatory bowel diseases (IBD), including Crohn’s disease (CD) and ulcerative colitis (UC), harbor an altered gut microbial composition (dysbiosis) defined as increased potentially aggressive species in parallel with decreased anti-inflammatory groups [5,6,7,8]. Gut microbial diversity decreases and metabolic functions are altered in IBD patients, suggesting a loss of protective bacteria and their functions in IBD [9,10,11]. Prolonged dysbiotic conditions lead to dysfunction of the host immune system, which is considered the key mediator of the chronic inflammation of IBD [6, 11] (Fig. 1).

Fig. 1
figure 1

Dysbiosis-associated mucosal immune-dysfunction in IBD. Enteric infection, medications including antibiotics, NSAIDs and immunosuppressive drugs, diet, smoking and alcohol, psychological stress in susceptible genetic individuals cause microbial dysbiosis and metabolic changes. Prolonged dysbiotic conditions characterized by increased aggressive bacterial strains and decreased regulatory species lead to dysfunction of mucosal immune response. Aggressive microbial groups activate inflammatory response by inducing Th1/Th17-effector cells, while decreased regulatory species impair the induction and function of regulatory cells that include regulatory T cells (Treg), B cells (Breg), macrophages (MΦ), dendritic cells (DC) and innate lymphoid cells (ILCs). This imbalance of mucosal cytokine profiles in combination with defective barrier function sustains mucosal inflammation and can potentially lead to IBD in susceptible individuals

The activation, migration, proliferation, differentiation and maintenance of a variety of mucosal immune cells are directly regulated by resident microbiota. These activated immune cells cooperate to maintain intestinal homeostasis in normal hosts [4]. Inflammatory immune cells help eliminate invading pathogens by highly effective redundant innate and adaptive immune mechanisms. Microbiota boosts the innate immune response against pathogens by stimulating secretion of antimicrobial peptides and cytokines such as TNFα, IL-22 and IL-17, and activating the inflammasome for anti-pathogen defense [2]. On the other hand, regulatory immune cells including regulatory T cells [12,13,14,15], B cells [16,17,18,19], dendritic cells [20, 21], macrophages [22] and innate lymphoid cells (ILCs) [143].

Fully understanding the interactions between microbiota and the host immune system, in concert with environmental and genetic factors unique to each individual, is necessary to target the most effective therapies for each patient. Personalized diagnostic profiles will require identifying an individual’s metabolic functions and dominant microbial antigens by shotgun metagenomic and metabolomic profiling, in concert with host microbial transcriptomic and genetic profiling. Microbiota reciprocally interacts with each other and the diet to provide immunological signals to host and the same microbe sometimes behaves differently in different individuals [1, 11, 144]. Host genetic and nutritional factors will need to be considered in an integrated and personalized manner to increase the effectiveness and efficacy of microbiota-based therapies [111, 145]. Selection of optimal approaches and therapeutic targets based on analysis of an individual’s microbiota pattern will be important to replace missing or dysfunctional bacterial components. We believe that a combined strategy to promote homeostatic immune responses, improve mucosal barrier function and restore eubiosis by targeting dominant pathobionts and replacing missing protective species or their functions by manipulating the bacterial microbiota and diet may be best. This integrated approach should provide a more physiologic, safer and more cost-effective means to sustained remission of IBD than the current lifelong treatments with immunosuppressives. It is our belief that this approach will be more effective as maintenance therapies once induction of disease remission has been accomplished by traditional therapies, but then toxic induction regimens can be withdrawn to decrease toxicity.

Conclusions and a path to improve personalized treatment

Human IBD includes genetically and clinically heterozygous patient subpopulations with very unique intestinal bacterial compositions and functions that help determine immune responses and disease outcomes. Therefore, we believe that it will be feasible to evaluate the microbe/immune profiles by rapid diagnostic tests of microbiota functional and mucosal immune profiles to direct highly effective and safe treatments in a personalized manner (Fig. 2). Restoring impaired regulatory immune cell activity by correcting dysbiosis and defective microbial metabolic functions is a novel and highly promising therapeutic approach to managing IBD in a more physiologic, safer and sustained manner. Unveiling the mechanisms underlying specific defective bacteria–host interactions in each IBD patient will enable precision editing of microbiota and their function with maximum effectiveness and efficiency.

Fig. 2
figure 2

Current and proposed treatment strategies in microbe-based treatment for IBD. Currently, we diagnose and treat IBD patients based on clinical parameters including fecal calprotectin, serum CRP level, disease activity index (DAI) and endoscopic findings. These clinical observations do not provide insight into the degree of mucosal dysbiosis or impaired regulatory immune response in IBD patients. Therefore, empiric microbe-based therapies are used, such as existing probiotics, prebiotics, antibiotics, fecal microbial transplantation in addition to standard of care anti-TNF agents or immunomodulators (IM). Since these empiric treatments have a limited efficacy in current clinical practice, we propose a more rational and scientific approach based on the fecal microbial and mucosal immune profiles in each IBD patient determined by rapid diagnosis tests. These fecal metabolic profiles and mucosal immune cytokine expression levels allow us to provide more effective and lower toxic microbe-based treatments based on various combinations of protective bacterial strains (LBP live biotherapeutic products) that are then applied in a customized way to restore microbial homeostasis based on dysbiosis in an individual patient. This approach can potentially provide cost-effective, nontoxic treatment and higher quality of life for IBD patients. HC healthy control, Pt IBD patient