Bifidobacterium bifidum PRL2010 modulates the host innate immune response. Appl Environ Microbiol. 2014;80:730-740. [PMID: 24242237 DOI: 10.1128/aem.03313-13]

Prebiotics, probiotics, synbiotics, and the immune system: experimental data and clinical evidence

PURPOSE OF REVIEW

The intestinal immune system is constantly exposed to foreign antigens, which for the most part should be tolerated. Certain probiotics, prebiotics, and synbiotics are able to influence immune responses. In this review, we highlight the recent publications (within the last 2 years) that have substantially progressed this field.

RECENT FINDINGS

The immunological mechanisms underpinning probiotics, prebiotics, and synbiotics effects continue to be better defined with novel mechanisms being described for dendritic cells, epithelial cells, T regulatory cells, effector lymphocytes, natural killer T cells, and B cells. Many of the mechanisms being described are bacterial strain or metabolite specific, and should not be extrapolated to other probiotics or prebiotics. In addition, the timing of intervention seems to be important, with potentially the greatest effects being observed early in life.

SUMMARY

In this review, we discuss the recent findings relating to probiotics, prebiotics, and synbiotics, specifically their effects on immunological functions.

Insights from genomes of representatives of the human gut commensal Bifidobacterium bifidum

Bifidobacteria are bacterial gut commensals of mammals, birds and social insects that are perceived to influence the metabolism/physiology of their host. In this context, members of the Bifidobacterium bifidum species are believed to significantly contribute to the overall microbiota of the human gut at infant stage. However, the molecular reasons for their adaptation to this environment are poorly understood. In this study, we analysed the pan-genome of B. bifidum species by decoding genomes of 15 B. bifidum strains, which highlighted the existence of a conserved gene uniquely present in this bifidobacterial taxon, underscoring a nutrient acquisition strategy that targets host-derived glycans, such as those present in mucin. Growth experiments and corresponding transcriptomic analyses confirmed the in silico data and supported these intriguing and unique host glycan-specific saccharolytic features. The ubiquity of the genetic features of B. bifidum for the breakdown of host glycans was confirmed by interrogating metagenomic datasets, thereby supporting the notion that metabolic access to host-derived glycans is a potent evolutionary force that has shaped B. bifidum genomes and consequently the ecology of the infant intestinal microbiota.

Impact of the gut microbiota on rodent models of human disease

Traditionally bacteria have been considered as either pathogens, commensals or symbionts. The mammal gut harbors 10¹⁴ organisms dispersed on approximately 1000 different species. Today, diagnostics, in contrast to previous cultivation techniques, allow the identification of close to 100% of bacterial species. This has revealed that a range of animal models within different research areas, such as diabetes, obesity, cancer, allergy, behavior and colitis, are affected by their gut microbiota. Correlation studies may for some diseases show correlation between gut microbiota composition and disease parameters higher than 70%. Some disease phenotypes may be transferred when recolonizing germ free mice. The mechanistic aspects are not clear, but some examples on how gut bacteria stimulate receptors, metabolism, and immune responses are discussed. A more deeper understanding of the impact of microbiota has its origin in the overall composition of the microbiota and in some newly recognized species, such as Akkermansia muciniphila, Segmented filamentous bacteria and Faecalibacterium prausnitzii, which seem to have an impact on more or less severe disease in specific models. Thus, the impact of the microbiota on animal models is of a magnitude that cannot be ignored in future research. Therefore, either models with specific microbiota must be developed, or the microbiota must be characterized in individual studies and incorporated into data evaluation.

Bifidobacterium bifidum as an example of a specialized human gut commensal

Bifidobacteria are considered dominant and for this reason key members of the human gut microbiota, particularly during the first one to two years following birth. A substantial proportion of the bifidobacterial population in the intestine of infants belong to the Bifidobacterium bifidum taxon, whose members have been shown to display remarkable physiological and genetic features involving adhesion to epithelia, as well as utilization of host-derived glycans. Here, we reviewed the current knowledge on the genetic features and associated adaptations of B. bifidum to the human gut.

Impact of genomics on the field of probiotic research: historical perspectives to modern paradigms

For thousands of years, humans have safely consumed microorganisms through fermented foods. Many of these bacteria are considered probiotics, which act through diverse mechanisms to confer a health benefit to the host. However, it was not until the availability of whole-genome sequencing and the era of genomics that mechanisms of probiotic efficacy could be discovered. In this review, we explore the history of the probiotic concept and the current standard of integrated genomic techniques to discern the complex, beneficial relationships between probiotic microbes and their hosts.

Murein lytic enzyme TgaA of Bifidobacterium bifidum MIMBb75 modulates dendritic cell maturation through its cysteine- and histidine-dependent amidohydrolase/peptidase (CHAP) amidase domain

Bifidobacteria are Gram-positive inhabitants of the human gastrointestinal tract that have evolved close interaction with their host and especially with the host's immune system. The molecular mechanisms underlying such interactions, however, are largely unidentified. In this study, we investigated the immunomodulatory potential of Bifidobacterium bifidum MIMBb75, a bacterium of human intestinal origin commercially used as a probiotic. Particularly, we focused our attention on TgaA, a protein expressed on the outer surface of MIMBb75's cells and homologous to other known bacterial immunoactive proteins. TgaA is a peptidoglycan lytic enzyme containing two active domains: lytic murein transglycosylase (LT) and cysteine- and histidine-dependent amidohydrolase/peptidase (CHAP). We ran immunological experiments stimulating dendritic cells (DCs) with the B. bifidum MIMBb75 and TgaA, with the result that both the bacterium and the protein activated DCs and triggered interleukin-2 (IL-2) production. In addition, we observed that the heterologous expression of TgaA in Bifidobacterium longum transferred to the bacterium the ability to induce IL-2. Subsequently, immunological experiments performed using two purified recombinant proteins corresponding to the single domains LT and CHAP demonstrated that the CHAP domain is the immune-reactive region of TgaA. Finally, we also showed that TgaA-dependent activation of DCs requires the protein CD14, marginally involves TRIF, and is independent of Toll-like receptor 4 (TLR4) and MyD88. In conclusion, our study suggests that the bacterial CHAP domain is a novel microbe-associated molecular pattern actively participating in the cross talk mechanisms between bifidobacteria and the host's immune system.