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

Dietary nutrition, intestinal microbiota dysbiosis and post-weaning diarrhea in piglets

Weaning is a critical transitional point in the life cycle of piglets. Early weaning can lead to post-weaning syndrome, destroy the intestinal barrier function and microbiota homeostasis, cause diarrhea and threaten the health of piglets. The nutritional components of milk and solid foods consumed by newborn animals can affect the diversity and structure of their intestinal microbiota, and regulate post-weaning diarrhea in piglets. Therefore, this paper reviews the effects and mechanisms of different nutrients, including protein, dietary fiber, dietary fatty acids and dietary electrolyte balance, on diarrhea and health of piglets by regulating intestinal function. Protein is an essential nutrient for the growth of piglets; however, excessive intake will cause many harmful effects, such as allergic reactions, intestinal barrier dysfunction and pathogenic growth, eventually aggravating piglet diarrhea. Dietary fiber is a nutrient that alleviates post-weaning diarrhea in piglets, which is related to its promotion of intestinal epithelial integrity, microbial homeostasis and the production of short-chain fatty acids. In addition, dietary fatty acids and dietary electrolyte balance can also facilitate the growth, function and health of piglets by regulating intestinal epithelial function, immune system and microbiota. Thus, a targeted control of dietary components to promote the establishment of a healthy bacterial community is a significant method for preventing nutritional diarrhea in weaned piglets.

Genetic-Phenotype Analysis of Bifidobacterium bifidum and Its Glycoside Hydrolase Gene Distribution at Different Age Groups

Human gut microbiota interfere with host development and aging. Bifidobacterium is a microbial genus found in the human digestive tract that has probiotic activities such as improving constipation and enhancing immunity. The species and numbers present change with age, but there has been limited research on probiotic gut microbiota at specific ages. This study analyzed the distribution of 610 bifidobacteria in subjects in several age groups (0-17, 18-65, and 66-108 y) using 486 fecal samples and determined the distribution of glycoside hydrolases based on genetic analysis of strains representing 85% of the Bifidobacterium species abundance in each age group. 6'-Sialyllactose is a major component of acidic breast milk oligosaccharides, which can promote human neurogenesis and bifidobacteria growth. Using genotypic and phenotypic association analysis, we investigated the utilization of 6'-sialyllactose by six B. bifidum strains isolated from subjects 0-17 and 18-65 y. A comparative genomic analysis of the six B. bifidum strains revealed differences in genomic features across age groups. Finally, the safety of these strains was evaluated by antibiotic gene and drug resistance phenotype analysis. Our results reveal that the distribution of glycoside hydrolase genes in B. bifidum varies with age, thus affecting the phenotypic results. This provides important insights for the design and application of probiotic products for different ages.

Pan-genome evolution and its association with divergence of metabolic functions in Bifidobacterium genus

Previous studies were mainly focused on genomic evolution and diversity of type species of Bifidobacterium genus due to their health-promoting effect on host. However, those studies were mainly based on species-level taxonomic resolution, adaptation, and characterization of carbohydrate metabolic features of the bifidobacterial species. Here, a comprehensive analysis of the type strain genome unveils the association of pan-genome evolution with the divergence of metabolic function of the Bifidobacterium genus. This study has also demonstrated that horizontal gene transfer, as well as genome expansion and reduction events, leads to the divergence of metabolic functions in Bifidobacterium genus. Furthermore, the genome-based search of probiotic traits among all the available bifidobacterial type strains gives hints on type species, that could confer health benefits to nutrient-deficient individuals. Altogether, the present study provides insight into the developments of genomic evolution, functional divergence, and potential probiotic type species of the Bifidobacterium genus.

Cross-talk between the infant/maternal gut microbiota and the endocrine system: a promising topic of research

The infant gut microbiota is the set of microorganisms colonizing the baby's intestine. This complex ecosystem appears to be related to various physiological conditions of the host and it has also been shown to act as one of the most crucial determinants of infant's health. Furthermore, the mother's endocrine system, through its hormones, can have an effect on the composition of the newborn's gut microbiota. In this perspective, we summarize the recent state of the art on the intricate relationships involving the intestinal microbiota and the endocrine system of mother/baby to underline the need to study the molecular mechanisms that appear to be involved.

Research progress on diarrhoea and its mechanism in weaned piglets fed a high-protein diet

In order to pursue faster growth and development of weaned piglets, increased dietary protein (CP) levels were favoured by the pig industry and the feed industry. The digestive organs of piglets were not fully developed at weaning, and the digestive absorption capacity of protein was limited. High-protein diets can cause allergic reactions in piglets, destroy intestinal structural integrity, reduce immunity, and cause intestinal flora imbalance. Undigested proteins were prone to produce toxic substances, such as ammonia and biogenic amines, after fermentation in the hindgut, which negatively affects the health of the intestine and eventually causes reduced growth performance and diarrhoea in piglets. This review revealed the mechanism of diarrhoea caused by high-protein diets in weaned piglets and provided ideas for preventing diarrhoea in weaned piglets.

Xylan utilisation promotes adaptation of Bifidobacterium pseudocatenulatum to the human gastrointestinal tract

Dietary carbohydrates impact the composition of the human gut microbiota. However, the relationship between carbohydrate availability for individual bacteria and their growth in the intestinal environment remains unclear. Here, we show that the availability of long-chain xylans (LCX), one of the most abundant dietary fibres in the human diet, promotes the growth of Bifidobacterium pseudocatenulatum in the adult human gut. Genomic and phenotypic analyses revealed that the availability of LCX-derived oligosaccharides is a fundamental feature of B. pseudocatenulatum, and that some but not all strains possessing the endo-1,4-β-xylanase (BpXyn10A) gene grow on LCX by cleaving the xylose backbone. The BpXyn10A gene, likely acquired by horizontal transfer, was incorporated into the gene cluster for LCX-derived oligosaccharide utilisation. Co-culturing with xylanolytic Bacteroides spp. demonstrated that LCX-utilising strains are more competitive than LCX non-utilising strains even when LCX-derived oligosaccharides were supplied. In LCX-rich dietary interventions in adult humans, levels of endogenous B. pseudocatenulatum increased only when BpXyn10A was detected, indicating that LCX availability is a fitness determinant in the human gut. Our findings highlight the enhanced intestinal adaptability of bifidobacteria via polysaccharide utilisation, and provide a cornerstone for systematic manipulation of the intestinal microbiota through dietary intervention using key enzymes that degrade polysaccharide as biomarkers.

Comparative Genomic Analysis of Bifidobacterium bifidum Strains Isolated from Different Niches

The potential probiotic benefits of Bifidobacterium bifidum have received increasing attention recently. We used comparative genomic analysis to explore the differences in the genome and the physiological characteristics of B. bifidum isolated from the fecal samples of Chinese adults and infants. The relationships between genotypes and phenotypes were analyzed to assess the effects of isolation sources on the genetic variation of B. bifidum. The phylogenetic tree results indicated that the phylogeny of B. bifidum may be related to the geographical features of its isolation source. B. bifidum was found to have an open pan-genome and a conserved core genome. The genetic diversity of B. bifidum is mainly reflected in carbohydrate metabolism- and immune/competition-related factors, such as the glycoside hydrolase gene family, bacteriocin operons, antibiotic resistance genes, and clustered regularly interspaced short palindromic repeats (CRISPR)-Cas. Additionally, the type III A CRISPR-Cas system was discovered in B. bifidum for the first time. B. bifidum strains exhibited niche-specific characteristics, and the results of this study provide an improved understanding of the genetics of this species.

Comparative Genomic Analysis of Novel Bifidobacterium longum subsp. longum Strains Reveals Functional Divergence in the Human Gut Microbiota

Bifidobacterium longum subsp. longum is a prevalent group in the human gut microbiome. Its persistence in the intestinal microbial community suggests a close host-microbe relationship according to age. The subspecies adaptations are related to metabolic capabilities and genomic and functional diversity. In this study, 154 genomes from public databases and four new Chilean isolates were genomically compared through an in silico approach to identify genomic divergence in genes associated with carbohydrate consumption and their possible adaptations to different human intestinal niches. The pangenome of the subspecies was open, which correlates with its remarkable ability to colonize several niches. The new genomes homogenously clustered within subspecies longum, as observed in phylogenetic analysis. B. longum SC664 was different at the sequence level but not in its functions. COG analysis revealed that carbohydrate use is variable among longum subspecies. Glycosyl hydrolases participating in human milk oligosaccharide use were found in certain infant and adult genomes. Predictive genomic analysis revealed that B. longum M12 contained an HMO cluster associated with the use of fucosylated HMOs but only endowed with a GH95, being able to grow in 2-fucosyllactose as the sole carbon source. This study identifies novel genomes with distinct adaptations to HMOs and highlights the plasticity of B. longum subsp. longum to colonize the human gut microbiota.

Malnutrition and Gut Microbiota in Children

Malnutrition continues to threaten the lives of millions across the world, with children being hardest hit. Although inadequate access to food and infectious disease are the primary causes of childhood malnutrition, the gut microbiota may also contribute. This review considers the evidence on the role of diet in modifying the gut microbiota, and how the microbiota impacts childhood malnutrition. It is widely understood that the gut microbiota of children is influenced by diet, which, in turn, can impact child nutritional status. Additionally, diarrhoea, a major contributor to malnutrition, is induced by pathogenic elements of the gut microbiota. Diarrhoea leads to malabsorption of essential nutrients and reduced energy availability resulting in weight loss, which can lead to malnutrition. Alterations in gut microbiota of severe acute malnourished (SAM) children include increased Proteobacteria and decreased Bacteroides levels. Additionally, the gut microbiota of SAM children exhibits lower relative diversity compared with healthy children. Thus, the data indicate a link between gut microbiota and malnutrition in children, suggesting that treatment of childhood malnutrition should include measures that support a healthy gut microbiota. This could be of particular relevance in sub-Saharan Africa and Asia where prevalence of malnutrition remains a major threat to the lives of millions.

Phylogenomic disentangling of the Bifidobacterium longum subsp. infantis taxon

Members of the Bifidobacterium longum species have been shown to possess adaptive abilities to allow colonization of different mammalian hosts, including humans, primates and domesticated mammalian species, such as dogs, horses, cattle and pigs. To date, three subspecies have formally been recognized to belong to this bifidobacterial taxon, i.e. B. longum subsp. longum, B. longum subsp. infantis and B. longum subsp. suis. Although B. longum subsp. longum is widely distributed in the human gut irrespective of host age, B. longum subsp. infantis appears to play a significant role as a prominent member of the gut microbiota of breast-fed infants. Nevertheless, despite the considerable scientific relevance of these taxa and the vast body of genomic data now available, an accurate dissection of the genetic features that comprehensively characterize the B. longum species and its subspecies is still missing. In the current study, we employed 261 publicly available B. longum genome sequences, combined with those of 11 new isolates, to investigate genomic diversity of this taxon through comparative genomic and phylogenomic approaches. These analyses allowed us to highlight a remarkable intra-species genetic and physiological diversity. Notably, characterization of the genome content of members of B. longum subsp. infantis subspecies suggested that this taxon may have acquired genetic features for increased competitiveness in the gut environment of suckling hosts. Furthermore, specific B. longum subsp. infantis genomic features appear to be responsible for enhanced horizontal gene transfer (HGT) occurrences, underpinning an intriguing dedication toward acquisition of foreign DNA by HGT events.

Host-microbiome intestinal interactions during early life: considerations for atopy and asthma development

PURPOSE OF REVIEW

The body's largest microbial community, the gut microbiome, is in contact with mucosal surfaces populated with epithelial, immune, endocrine and nerve cells, all of which sense and respond to microbial signals. These mutual interactions have led to a functional coevolution between the microbes and human physiology. Examples of coadaptation are anaerobes Bifidobacteria and Bacteroides, which have adjusted their metabolism to dietary components of human milk, and infant immune development, which has evolved to become reliant on the presence of beneficial microbes. Current research suggests that specific composition of the early-life gut microbiome aligns with the maturation of host immunity. Disruptions of natural microbial succession patterns during gut colonization are a consistent feature of immune-mediated diseases, including atopy and asthma.

RECENT FINDINGS

Here, we catalog recent birth cohorts documenting associations between immune dysregulation and microbial alterations, and summarize the evidence supporting the role of the gut microbiome as an etiological determinant of immune-mediated allergic diseases.

SUMMARY

Ecological concepts that describe microbial dynamics in the context of the host environment, and a portray of immune and neuroendocrine signaling induced by host-microbiome interactions, have become indispensable in describing the molecular role of early-life microbiome in atopy and asthma susceptibility.

Amoxicillin-Clavulanic Acid Resistance in the Genus Bifidobacterium

Amoxicillin-clavulanic acid (AMC) is one of the most frequently prescribed antibiotic formulations in the Western world. Extensive oral use of this antimicrobial combination influences the gut microbiota. One of the most abundant early colonizers of the human gut microbiota is represented by different taxa of the Bifidobacterium genus, which include many members that are considered to bestow beneficial effects upon their host. In the current study, we investigated the impact of AMC administration on the gut microbiota composition, comparing the gut microbiota of 23 children that had undergone AMC antibiotic therapy to that of 19 children that had not been treated with antibiotics during the preceding 6 months. Moreover, we evaluated AMC sensitivity by MIC test of 261 bifidobacterial strains, including reference strains for the currently recognized 64 bifidobacterial (sub)species, as well as 197 bifidobacterial isolates of human origin. These assessments allowed the identification of four bifidobacterial strains that exhibit a high level of AMC insensitivity, which were subjected to genomic and transcriptomic analyses to identify the putative genetic determinants responsible for this AMC insensitivity. Furthermore, we investigated the ecological role of AMC-resistant bifidobacterial strains by in vitro batch cultures.IMPORTANCE Based on our results, we observed a drastic reduction in gut microbiota diversity of children treated with antibiotics, which also affected the abundance of Bifidobacterium, a bacterial genus commonly found in the infant gut. MIC experiments revealed that more than 98% of bifidobacterial strains tested were shown to be inhibited by the AMC antibiotic. Isolation of four insensitive strains and sequencing of their genomes revealed the identity of possible genes involved in AMC resistance mechanisms. Moreover, gut-simulating in vitro experiments revealed that one strain, i.e., Bifidobacterium breve PRL2020, is able to persist in the presence of a complex microbiota combined with AMC antibiotic.

Comparative genomics of the gut commensal Bifidobacterium bifidum reveals adaptation to carbohydrate utilization

Bifidobacterium bifidum is one of the most abundant members of the gut microbiota at the early stage of life. The established association of the bacterium with the human gut confers health benefits. Such successful persistence of B. bifidum necessitates metabolic adaptation to the host-derived carbohydrates, a process which is poorly understood. The current study focuses on revealing the genomic-based phylogeny (phylogenomics) of B. bifidum and utilizing comparative genomics to decipher the glycolytic abilities of bifidobacterial strains isolated from different human body niches (feces, human gut, vagina, and breast milk). When the phylogenomic analysis was performed on 95 B. bifidum strains, currently available on the RefSeq database, the bacterium was clearly distinguished from other members of the Bifidobacterium genus. Furthermore, a pairwise genomic comparison indicated that a large proportion of orthologous gene families were shared among the B. bifidum strains. These findings highlight the notion that the B. bifidum species is genetically similar and may perform similar functions in their host. When 15 B. bifidum genomes representing strains from different human body niches were annotated, the resulting functional profile showed the presence of enriched proteins involved in carbohydrate utilization. Moreover, mining the 15 B. bifidum genomes for the presence of Carbohydrate-Active Enzyme (CAZY) systems, the analysis found the existence of diverse protein families which include glycosyl hydrolases, glycosyl transferases, carbohydrate-binding modules, and carbohydrate esterases. Collectively, these CAZY systems enables B. bifidum to utilize host-derived glycans (e.g., mucin) and diet-derived carbohydrates (e.g., starch). In contrast, a correlation analysis revealed that B. bifidum strains isolated from the different body niches were indistinguishable in the context of presence-absence of CAZY systems. These findings emphasize the valuable use of comparative genomics in deciphering the glycolytic abilities of B. bifidum and consequently its adaptation to carbohydrate utilization in the human gut environment.

Modelling the effect of birth and feeding modes on the development of human gut microbiota

The human gut microbiota is transmitted from mother to infant through vaginal birth and breastfeeding. Bifidobacterium, a genus that dominates the infants' gut, is adapted to breast milk in its ability to metabolize human milk oligosaccharides; it is regarded as a mutualist owing to its involvement in the development of the immune system. The composition of microbiota, including the abundance of Bifidobacteria, is highly variable between individuals and some microbial profiles are associated with diseases. However, whether and how birth and feeding practices contribute to such variation remains unclear. To understand how early events affect the establishment of microbiota, we develop a mathematical model of two types of Bifidobacteria and a generic compartment of commensal competitors. We show how early events affect competition between mutualists and commensals and microbe-host-immune interactions to cause long-term alterations in gut microbial profiles. Bifidobacteria associated with breast milk can trigger immune responses with lasting effects on the microbial community structure. Our model shows that, in response to a change in birth environment, competition alone can produce two distinct microbial profiles post-weaning. Adding immune regulation to our competition model allows for variations in microbial profiles in response to different feeding practices. This analysis highlights the importance of microbe-microbe and microbe-host interactions in shaping the gut populations following different birth and feeding modes.

Assembly, Annotation, and Comparative Analysis of Bifidobacterial Genomes

Genome assembly and annotation are two of the key actions that must be undertaken in order to explore the genomic repertoire of (bifido)bacteria. The gathered information can be employed to genomically characterize a given microorganism, and can also be used to perform comparative genome analysis by including other sequenced (bifido)bacterial strains. Here, we highlight various bioinformatic programs able to manage next generation sequencing data starting from the assembly of a genome to the comparative analyses between strains.

Exploring the Ecology of Bifidobacteria and Their Genetic Adaptation to the Mammalian Gut

The mammalian gut is densely inhabited by microorganisms that have coevolved with their host. Amongst these latter microorganisms, bifidobacteria represent a key model to study host–microbe interaction within the mammalian gut. Remarkably, bifidobacteria naturally occur in a range of ecological niches that are either directly or indirectly connected to the animal gastrointestinal tract. They constitute one of the dominant bacterial members of the intestinal microbiota and are among the first colonizers of the mammalian gut. Notably, the presence of bifidobacteria in the gut has been associated with several health-promoting activities. In this review, we aim to provide an overview of current knowledge on the genetic diversity and ecology of bifidobacteria. Furthermore, we will discuss how this important group of gut bacteria is able to colonize and survive in the mammalian gut, so as to facilitate host interactions.

Multiple Transporters and Glycoside Hydrolases Are Involved in Arabinoxylan-Derived Oligosaccharide Utilization in Bifidobacterium pseudocatenulatum

Bifidobacteria commonly reside in the human intestine and possess abundant genes involved in carbohydrate utilization. Arabinoxylan hydrolysates (AXH) are hydrolyzed products of arabinoxylan, one of the most abundant dietary fibers, and they include xylooligosaccharides and those decorated with arabinofuranosyl residues. The molecular mechanism by which B. pseudocatenulatum, a common bifidobacterial species found in adult feces, utilizes structurally and compositionally variable AXH has yet to be extensively investigated. In this study, we identified three gene clusters (encoding five GH43 enzymes and three solute-binding proteins of ABC transporters) that were upregulated in B. pseudocatenulatum YIT 4072^T during AXH utilization. By investigating their substrate specificities, we revealed how these proteins are involved in the uptake and degradation of AXH. These molecular insights may provide a better understanding of how resident bifidobacteria colonize the colon.

Arabinoxylan hydrolysates (AXH) are the hydrolyzed products of the major components of the dietary fiber arabinoxylan. AXH include diverse oligosaccharides varying in xylose polymerization and side residue modifications with arabinose at the O-2 and/or O-3 position of the xylose unit. Previous studies have reported that AXH exhibit prebiotic properties on gut bifidobacteria; moreover, several adult-associated bifidobacterial species (e.g., Bifidobacterium adolescentis and Bifidobacterium longum subsp. longum) are known to utilize AXH. In this study, we tried to elucidate the molecular mechanisms of AXH utilization by Bifidobacterium pseudocatenulatum, which is a common bifidobacterial species found in adult feces. We performed transcriptomic analysis of B. pseudocatenulatum YIT 4072^T, which identified three upregulated gene clusters during AXH utilization. The gene clusters encoded three sets of ATP-binding cassette (ABC) transporters and five enzymes belonging to glycoside hydrolase family 43 (GH43). By characterizing the recombinant proteins, we found that three solute-binding proteins of ABC transporters showed either broad or narrow specificity, two arabinofuranosidases hydrolyzed either single- or double-decorated arabinoxylooligosaccharides, and three xylosidases exhibited functionally identical activity. These data collectively suggest that the transporters and glycoside hydrolases, encoded in the three gene clusters, work together to utilize AXH of different sizes and with different side residue modifications. Thus, our study sheds light on the overall picture of how these proteins collaborate for the utilization of AXH in B. pseudocatenulatum and may explain the predominance of this symbiont species in the adult human gut.

IMPORTANCE Bifidobacteria commonly reside in the human intestine and possess abundant genes involved in carbohydrate utilization. Arabinoxylan hydrolysates (AXH) are hydrolyzed products of arabinoxylan, one of the most abundant dietary fibers, and they include xylooligosaccharides and those decorated with arabinofuranosyl residues. The molecular mechanism by which B. pseudocatenulatum, a common bifidobacterial species found in adult feces, utilizes structurally and compositionally variable AXH has yet to be extensively investigated. In this study, we identified three gene clusters (encoding five GH43 enzymes and three solute-binding proteins of ABC transporters) that were upregulated in B. pseudocatenulatum YIT 4072^T during AXH utilization. By investigating their substrate specificities, we revealed how these proteins are involved in the uptake and degradation of AXH. These molecular insights may provide a better understanding of how resident bifidobacteria colonize the colon.

Communal living: glycan utilization by the human gut microbiota

Our lower gastrointestinal tract plays host to a vast consortium of microbes, known as the human gut microbiota (HGM). The HGM thrives on a complex and diverse range of glycan structures from both dietary and host sources, the breakdown of which requires the concerted action of cohorts of carbohydrate-active enzymes (CAZymes), carbohydrate-binding proteins, and transporters. The glycan utilization profile of individual taxa, whether 'specialist' or 'generalist', is dictated by the number and functional diversity of these glycan utilization systems. Furthermore, taxa in the HGM may either compete or cooperate in glycan deconstruction, thereby creating a complex ecological web spanning diverse nutrient niches. As a result, our diet plays a central role in shaping the composition of the HGM. This review presents an overview of our current understanding of glycan utilization by the HGM on three levels: (i) molecular mechanisms of individual glycan deconstruction and uptake by key bacteria, (ii) glycan-mediated microbial interactions, and (iii) community-scale effects of dietary changes. Despite significant recent advancements, there remains much to be discovered regarding complex glycan metabolism in the HGM and its potential to affect positive health outcomes.

Decoding the Genomic Variability among Members of the Bifidobacterium dentium Species

Members of the Bifidobacterium dentium species are usually identified in the oral cavity of humans and associated with the development of plaque and dental caries. Nevertheless, they have also been detected from fecal samples, highlighting a widespread distribution among mammals. To explore the genetic variability of this species, we isolated and sequenced the genomes of 18 different B. dentium strains collected from fecal samples of several primate species and an Ursus arctos. Thus, we investigated the genomic variability and metabolic abilities of the new B. dentium isolates together with 20 public genome sequences. Comparative genomic analyses provided insights into the vast metabolic repertoire of the species, highlighting 19 glycosyl hydrolases families shared between each analyzed strain. Phylogenetic analysis of the B. dentium taxon, involving 1140 conserved genes, revealed a very close phylogenetic relatedness among members of this species. Furthermore, low genomic variability between strains was also confirmed by an average nucleotide identity analysis showing values higher than 98.2%. Investigating the genetic features of each strain, few putative functional mobile elements were identified. Besides, a consistent occurrence of defense mechanisms such as CRISPR-Cas and restriction-modification systems may be responsible for the high genome synteny identified among members of this taxon.

Gut Biofactory-Neurocompetent Metabolites within the Gastrointestinal Tract. A Scoping Review

The gut microbiota have gained much scientific attention recently. Apart from unravelling the taxonomic data, we should understand how the altered microbiota structure corresponds to functions of this complex ecosystem. The metabolites of intestinal microorganisms, especially bacteria, exert pleiotropic effects on the human organism and contribute to the host systemic balance. These molecules play key roles in regulating immune and metabolic processes. A subset of them affect the gut brain axis signaling and balance the mental wellbeing. Neurotransmitters, short chain fatty acids, tryptophan catabolites, bile acids and phosphatidylcholine, choline, serotonin, and L-carnitine metabolites possess high neuroactive potential. A scoping literature search in PubMed/Embase was conducted up until 20 June 2020, using three major search terms "microbiota metabolites" AND "gut brain axis" AND "mental health". This review aimed to enhance our knowledge regarding the gut microbiota functional capacity, and support current and future attempts to create new compounds for future clinical interventions.

Strain-Level Analysis of Bifidobacterium spp. from Gut Microbiomes of Adults with Differing Lactase Persistence Genotypes

When humans domesticated animals, some adapted genetically to digest milk into adulthood (lactase persistence). The gut microbiomes of people with lactase-persistent genotypes (AA or AG) differ from those with lactase-nonpersistent genotypes (GG) by containing fewer bacteria belonging to the bifidobacteria, a group which contains beneficial species. Here, we asked if the gut microbiomes of adults with GG and AA/AG genotypes differ in the species of bifidobacteria present. In particular, we used a novel technique which allowed us to compare bifidobacteria in adults at the strain level, without the traditional need for culturing. Our results show that the GG genotype enhances the abundance of bifidobacteria regardless of species. We also noted that a person’s specific strains are recoverable several years later, and twins can share the same ones. Given that bifidobacteria are inherited from mother to child, strain stability over time in adulthood suggests long-term, multigenerational inheritance.

One of the strongest associations between human genetics and the gut microbiome is a greater relative abundance of Bifidobacterium in adults with lactase gene (LCT) single nucleotide polymorphisms (SNPs) associated with lactase nonpersistence (GG genotypes), versus lactase persistence (AA/AG genotypes). To gain a finer-grained phylogenetic resolution of this association, we interrogated 1,680 16S rRNA libraries and 245 metagenomes from gut microbiomes of adults with various lactase persistence genotypes. We further employed a novel genome-capture-based enrichment of Bifidobacterium DNA from a subset of these metagenomes, including monozygotic (MZ) twin pairs, each sampled 2 or 3 times. B. adolescentis and B. longum were the most abundant Bifidobacterium species regardless of host LCT genotype. LCT genotypes could not be discriminated based on relative abundances of Bifidobacterium species or Bifidobacterium community structure. Three distinct metagenomic analysis methods of Bifidobacterium-enriched DNA revealed intraindividual temporal stability of B. longum, B. adolescentis, and B. bifidum strains against the background of a changeable microbiome. Two of our three methods also observed greater strain sharing within MZ twin pairs than within unrelated individuals for B. adolescentis, while no method revealed an effect of host LCT genotype on Bifidobacterium strain composition. Our results support a “rising tide lifts all boats” model for the dominant bifidobacteria in the adult gut: their higher abundance in lactase-nonpersistent than in lactase-persistent individuals results from an expansion at the genus level. Bifidobacterium species are known to be transmitted from mother to child and stable within individuals in infancy and childhood: our results extend this stability into adulthood.

IMPORTANCE When humans domesticated animals, some adapted genetically to digest milk into adulthood (lactase persistence). The gut microbiomes of people with lactase-persistent genotypes (AA or AG) differ from those with lactase-nonpersistent genotypes (GG) by containing fewer bacteria belonging to the bifidobacteria, a group which contains beneficial species. Here, we asked if the gut microbiomes of adults with GG and AA/AG genotypes differ in the species of bifidobacteria present. In particular, we used a novel technique which allowed us to compare bifidobacteria in adults at the strain level, without the traditional need for culturing. Our results show that the GG genotype enhances the abundance of bifidobacteria regardless of species. We also noted that a person’s specific strains are recoverable several years later, and twins can share the same ones. Given that bifidobacteria are inherited from mother to child, strain stability over time in adulthood suggests long-term, multigenerational inheritance.

The Infant-Derived Bifidobacterium bifidum Strain CNCM I-4319 Strengthens Gut Functionality

Bifidobacteria are among the first colonisers of the gastrointestinal tract of breast-fed newborns due to, among other things, their ability to metabolise oligosaccharides naturally occurring in human milk. The presence of bifidobacteria in the infant gut has been shown to promote intestinal health and homeostasis as well as to preserve a functional gut barrier, thus positively influencing host health and well-being. Among human-associated gut commensals, Bifidobacterium bifidum has been described as the only species capable of the extracellular degradation of both mucin-type glycans and HMOs, thereby giving this species a special role as a commensal gut forager of both host and diet-derived glycans. In the present study, we assess the possible beneficial properties and probiotic potential of B. bifidum strain CNCM I-4319. In silico genome analysis and growth experiments confirmed the expected ability of this strain to consume HMOs and mucin. By employing various animal models, we were also able to assess the ability of B. bifidum CNCM I-4319 to preserve gut integrity and functionality from stress-induced and inflammatory damage, thereby enforcing its potential as an effective probiotic strain.

Characterization of the phylogenetic diversity of two novel species belonging to the genus Bifidobacterium: Bifidobacterium cebidarum sp. nov. and Bifidobacterium leontopitheci sp. nov

Two Bifidobacterium strains, i.e., 2176B^T and 2177B^T, were isolated from Golden-Headed Lion Tamarin (Leontopithecus chrysomelas) and Goeldi's monkey (Callimico goeldii). Isolates were shown to be Gram-positive, non-motile, non-sporulating, facultative anaerobic and d-fructose 6-phosphate phosphoketolase-positive. Phylogenetic analyses based on 16S rRNA sequences, multilocus sequences (including hsp60, rpoB, dnaJ, dnaG and clpC genes) and the core genome revealed that bifidobacterial strains 2176B^T and 2177B^T exhibit close phylogenetic relatedness to Bifidobacterium felsineum DSM 103139^T and Bifidobacterium bifidum LMG 11041^T, respectively. Further genotyping based on the genome sequence of the isolated strains combined with phenotypic analyses, clearly show that these strains are distinct from each of the type strains of the so far recognized Bifidobacterium species. Thus, Bifidobacterium cebidarum sp. nov. (2176B^T=LMG 31469^T=CCUG 73785^T) and Bifidobacterium leontopitheci sp. nov. (2177B^T=LMG 31471^T=CCUG 73786^T are proposed as novel Bifidobacterium species.

Bifidobacteria cell wall-derived exo-polysaccharides, lipoteichoic acids, peptidoglycans, polar lipids and proteins - their chemical structure and biological attributes

A variety of health benefits has been documented to be associated with the consumption of probiotic bacteria, namely bifidobacteria and lactobacilli. Thanks to the scientific advances in recent years we are beginning to understand the molecular mechanisms by which bacteria in general and probiotic bacteria in particular act as host physiology and immune system modulators. More recently, the focus has shifted from live bacteria towards bacteria-derived defined molecules, so called postbiotics. These molecules may represent safer alternative compared to the live bacteria while retaining the desired effects on the host. The excellent source of effector macromolecules is the bacterial envelope. It contains compounds that are pivotal in the adhesion phenomenon, provide direct bacteria-to-host signaling capacity and the associated physiological impact and immunomodulatory properties of bacteria. Here we comprehensively review the structure and biological role of Bifidobacterium surface and cell wall molecules: exopolysaccharides, cell wall polysaccharides, lipoteichoic acids, polar lipids, peptidoglycans and proteins. We discuss their involvement in direct signaling to the host cells and their described immunomodulatory effects.

Bifidobacterium bifidum: A Key Member of the Early Human Gut Microbiota

Bifidobacteria typically represent the most abundant bacteria of the human gut microbiota in healthy breast-fed infants. Members of the Bifidobacterium bifidum species constitute one of the dominant taxa amongst these bifidobacterial communities and have been shown to display notable physiological and genetic features encompassing adhesion to epithelia as well as metabolism of host-derived glycans. In the current review, we discuss current knowledge concerning particular biological characteristics of the B. bifidum species that support its specific adaptation to the human gut and their implications in terms of supporting host health.

Milk Glycans and Their Interaction with the Infant-Gut Microbiota

Human milk is a unique and complex fluid that provides infant nutrition and delivers an array of bioactive molecules that serve various functions. Glycans, abundant in milk, can be found as free oligosaccharides or as glycoconjugates. Milk glycans are increasingly linked to beneficial outcomes in neonates through protection from pathogens and modulation of the immune system. Indeed, these glycans influence the development of the infant and the infant-gut microbiota. Bifidobacterium species commonly are enriched in breastfed infants and are among a limited group of bacteria that readily consume human milk oligosaccharides (HMOs) and milk glycoconjugates. Given the importance of bifidobacteria in infant health, numerous studies have examined the molecular mechanisms they employ to consume HMOs and milk glycans, thus providing insight into this unique enrichment and shedding light on a range of translational opportunities to benefit at-risk infants.

Production of Indole-3-Lactic Acid by Bifidobacterium Strains Isolated fromHuman Infants

Recent studies have shown that metabolites produced by microbes can be considered as mediators of host-microbial interactions. In this study, we examined the production of tryptophan metabolites by Bifidobacterium strains found in the gastrointestinal tracts of humans and other animals. Indole-3-lactic acid (ILA) was the only tryptophan metabolite produced in bifidobacteria culture supernatants. No others, including indole-3-propionic acid, indole-3-acetic acid, and indole-3-aldehyde, were produced. Strains of bifidobacterial species commonly isolated from the intestines of human infants, such as Bifidobacterium longum subsp. longum, Bifidobacterium longum subsp. infantis, Bifidobacterium breve, and Bifidobacterium bifidum, produced higher levels of ILA than did strains of other species. These results imply that infant-type bifidobacteria might play a specific role in host-microbial cross-talk by producing ILA in human infants.

Bifidobacterial Transfer from Mother to Child as Examined by an Animal Model

Bifidobacteria commonly constitute the most abundant group of microorganisms in the healthy infant gut. Their intestinal establishment is believed to be maternally driven, and their acquisition has even been postulated to occur during pregnancy. In the current study, we evaluated bifidobacterial mother-to infant transmission events in a rat model by means of quantitative PCR (qPCR), as well as by Internally Transcribed Spacer (ITS) bifidobacterial profiling. The occurrence of strains supplied by mothers during pregnancy to their corresponding newborns was observed and identified by analysis immediately following C-section delivery. These findings provide intriguing support for the existence of an unknown route to facilitate bifidobacterial transfer during the very early stages of life.

Genetic marker-based multi-locus sequence analysis for classification, genotyping, and phylogenetics of the family Bifidobacteriaceae as an alternative approach to phylogenomics

Bifidobacteria are widely known for their probiotic potential; however, little is known regarding the ecological significance and potential probiotic effects of the phylogenetically related 'scardovial' genera (Aeriscardovia, Alloscardovia, Bombiscardovia, Galliscardovia, Neoscardovia, Parascardovia, Pseudoscardovia and Scardovia) and Gardnerella classified with bifidobacteria within the Bifidobacteriaceae family. Accurate classification and genotyping of bacteria using certain housekeeping genes is possible, whilst current phylogenomic analyses allow for extremely precise classification. Studies of applicable genetic markers may provide results comparable to those obtained from phylogenomic analyses of the family Bifidobacteriaceae. Segments of the glyS (624 nucleotides), pheS (555 nucleotides), rpsA (630 nucleotides), and rpsB (432 nucleotides) genes and their concatenated sequence were explored. The mean glyS, pheS, rpsB and rpsA gene sequence similarities calculated for Bifidobacterium taxa were 84.8, 85.2, 90.2 and 86.8%, respectively. Interestingly, the average value of the Average Nucleotide Identity among 67 type strains of the family Bifidobacteriaceae (84.70%) calculated based on values published recently was in agreement with the average pairwise similarity (84.6%) among 75 type strains of Bifidobacteriaceae family computed in this study using the concatenated sequences of four gene fragments. Similar to phylogenomic analyses, several gene sequence and phylogenetic analyses revealed that concatenated gene regions allow for classification of Bifidobacteriaceae strains into particular phylogenetic clusters and groups. Phylogeny reconstructed from the concatenated sequences assisted in defining two novel phylogenetic groups, the Bifidobacterium psychraerophilum group consisting of B. psychraerophilum, Bifidobacterium crudilactis and Bifidobacterium aquikefiri species and the Bifidobacterium bombi group consisting of B. bombi, Bifidobacterium bohemicum and Bifidobacterium commune.

In silico Approach for Unveiling the Glycoside Hydrolase Activities in Faecalibacterium prausnitzii Through a Systematic and Integrative Large-Scale Analysis

This work presents a novel in silico approach to the prediction and characterization of the glycolytic capacities of the beneficial intestinal bacterium Faecalibacterium prausnitzii. Available F. prausnitzii genomes were explored taking the glycolytic capacities of F. prausnitzii SL3/3 and F. prausnitzii L2-6 as reference. The comparison of the generated glycolytic profiles offered insights into the particular capabilities of F. prausnitzii SL3/3 and F. prausnitzii L2-6 as well as the potential of the rest of strains. Glycoside hydrolases were mostly detected in the pathways responsible for the starch and sucrose metabolism and the biosynthesis of secondary metabolites, but this analysis also identified some other potentially interesting, but still uncharacterized activities, such as several hexosyltransferases and some hydrolases. Gene neighborhood maps offered additional understanding of the genes coding for relevant glycoside hydrolases. Although information about the carbohydrate preferences of F. prausnitzii is scarce, the in silico metabolic predictions were consistent with previous knowledge about the impact of fermentable sugars on the growth promotion and metabolism of F. prausnitzii. So, while the predictions still need to be validated using culturing methods, the approach holds the potential to be reproduced and scaled to accommodate the analysis of other strains (or even families and genus) as well as other metabolic activities. This will allow the exploration of novel methodologies to design or obtain targeted probiotics for F. prausnitzii and other strains of interest.

Unveiling Genomic Diversity among Members of the Species Bifidobacterium pseudolongum, a Widely Distributed Gut Commensal of the Animal Kingdom

Bifidobacteria are commensals of the animal gut and are commonly found in mammals, birds, and social insects. Specifically, strains of Bifidobacterium adolescentis, Bifidobacterium bifidum, Bifidobacterium longum, and Bifidobacterium pseudolongum are widely distributed in the mammalian gut. In this context, we investigated the genetic variability and metabolic abilities of the B. pseudolongum taxon, whose genomic characterization has so far not received much attention. Phylogenomic analysis of the genome sequences of 60 B. pseudolongum strains revealed that B. pseudolongum subsp. globosum and B. pseudolongum subsp. pseudolongum may actually represent two distinct bifidobacterial species. Furthermore, our analysis highlighted metabolic differences between members of these two subspecies. Moreover, comparative analyses of genetic strategies to prevent invasion of foreign DNA revealed that the B. pseudolongum subsp. globosum group exhibits greater genome plasticity. In fact, the obtained findings indicate that B. pseudolongum subsp. globosum is more adaptable to different ecological niches such as the mammalian and avian gut than is B. pseudolongum subsp. pseudolongum IMPORTANCE Currently, little information exists on the genetics of the B. pseudolongum taxon due to the limited number of sequenced genomes belonging to this species. In order to survey genome variability within this species and explore how members of this taxon evolved as commensals of the animal gut, we isolated and decoded the genomes of 51 newly isolated strains. Comparative genomics coupled with growth profiles on different carbohydrates has further provided insights concerning the genotype and phenotype of members of the B. pseudolongum taxon.

Dissecting the Evolutionary Development of the Species Bifidobacterium animalis through Comparative Genomics Analyses

Bifidobacteria are members of the gut microbiota of animals, including mammals, birds, and social insects. In this study, we analyzed and determined the pangenome of Bifidobacterium animalis species, encompassing B. animalis subsp. animalis and the B. animalis subsp. lactis taxon, which is one of the most intensely exploited probiotic bifidobacterial species. In order to reveal differences within the B. animalis species, detailed comparative genomics and phylogenomics analyses were performed, indicating that these two subspecies recently arose through divergent evolutionary events. A subspecies-specific core genome was identified for both B. animalis subspecies, revealing the existence of subspecies-defining genes involved in carbohydrate metabolism. Notably, these in silico analyses coupled with carbohydrate profiling assays suggest genetic adaptations toward a distinct glycan milieu for each member of the B. animalis subspecies, resulting in a divergent evolutionary development of the two subspecies.IMPORTANCE The majority of characterized B. animalis strains have been isolated from human fecal samples. In order to explore genome variability within this species, we isolated 15 novel strains from the gastrointestinal tracts of different animals, including mammals and birds. The present study allowed us to reconstruct the pangenome of this taxon, including the genome contents of 56 B. animalis strains. Through careful assessment of subspecies-specific core genes of the B. animalis subsp. animalis/lactis taxon, we identified genes encoding enzymes involved in carbohydrate transport and metabolism, while unveiling specific gene acquisition and loss events that caused the evolutionary emergence of these two subspecies.

Characterization of the phylogenetic diversity of five novel species belonging to the genus Bifidobacterium: Bifidobacterium castoris sp. nov., Bifidobacterium callimiconis sp. nov., Bifidobacterium goeldii sp. nov., Bifidobacterium samirii sp. nov. and Bifidobacterium dolichotidis sp. nov

Five Bifidobacterium strains, i.e. 2020B^T, 2028B^T, 2033B^T, 2034B^T and 2036B^T, were isolated from European beaver (Castor fiber), Goeldi's marmoset (Callimicogoeldii), black-capped squirrel monkey (Saimiriboliviensissubsp. peruviensis) and Patagonian mara (Dolichotispatagonum). All of these isolates were shown to be Gram-positive, facultative anaerobic, d-fructose 6-phosphate phosphoketolase-positive, non-motile and non-sporulating. Phylogenetic analyses based on 16S rRNA gene sequences, multilocus sequences (including hsp60, rpoB, dnaJ, dnaG and clpC genes) and the core genome revealed that bifidobacterial strains 2020B^T, 2028B^T, 2033B^T, 2034B^T and 2036B^T exhibit close phylogenetic relatedness to Bifidobacterium biavatii DSM 23969^T, Bifidobacterium bifidum LMG 11041^T, Bifidobacterium choerinum LMG 10510^T, Bifidobacterium gallicum LMG 11596^T, Bifidobacterium imperatoris LMG 30297^T, Bifidobacterium italicum LMG 30187^T and Bifidobacterium vansinderenii LMG 30126^T, respectively. Further genotyping based on the genome sequence of the isolated strains combined with phenotypic analyses, clearly show that these strains are distinct from each of the type strains of the so far recognized Bifidobacterium species. Thus, Bifidobacterium castoris sp. nov. (2020B^T=LMG 30937^T=CCUG 72816^T), Bifidobacterium callimiconis sp. nov. (2028B^T=LMG 30938^T=CCUG 72814^T), Bifidobacterium samirii sp. nov. (2033B^T=LMG 30940^T=CCUG 72817^T), Bifidobacterium goeldii sp. nov. (2034B^T=LMG 30939^T=CCUG 72815^T) and Bifidobacterium dolichotidis sp. nov. (2036B^T=LMG 30941^T=CCUG 72818^T) are proposed as novel Bifidobacterium species.

Systematic Review of Gut Microbiota and Major Depression

Background: Recently discovered relationships between the gastrointestinal microbiome and the brain have implications for psychiatric disorders, including major depressive disorder (MDD). Bacterial transplantation from MDD patients to rodents produces depression-like behaviors. In humans, case-control studies have examined the gut microbiome in healthy and affected individuals. We systematically reviewed existing studies comparing gut microbial composition in MDD and healthy volunteers. Methods: A PubMed literature search combined the terms "depression," "depressive disorder," "stool," "fecal," "gut," and "microbiome" to identify human case-control studies that investigated relationships between MDD and microbiota quantified from stool. We evaluated the resulting studies, focusing on bacterial taxa that were different between MDD and healthy controls. Results: Six eligible studies were found in which 50 taxa exhibited differences (p < 0.05) between patients with MDD and controls. Patient characteristics and methodologies varied widely between studies. Five phyla-Bacteroidetes, Firmicutes, Actinobacteria, Fusobacteria, and Protobacteria-were represented; however, divergent results occurred across studies for all phyla. The largest number of differentiating taxa were within phylum Firmicutes, in which nine families and 12 genera differentiated the diagnostic groups. The majority of these families and genera were found to be statistically different between the two groups in two identified studies. Family Lachnospiraceae differentiated the diagnostic groups in four studies (with an even split in directionality). Across all five phyla, nine genera were higher in MDD (Anaerostipes, Blautia, Clostridium, Klebsiella, Lachnospiraceae incertae sedis, Parabacteroides, Parasutterella, Phascolarctobacterium, and Streptococcus), six were lower (Bifidobacterium, Dialister, Escherichia/Shigella, Faecalibacterium, and Ruminococcus), and six were divergent (Alistipes, Bacteroides, Megamonas, Oscillibacter, Prevotella, and Roseburia). We highlight mechanisms and products of bacterial metabolism as they may relate to the etiology of depression. Conclusions: No consensus has emerged from existing human studies of depression and gut microbiome concerning which bacterial taxa are most relevant to depression. This may in part be due to differences in study design. Given that bacterial functions are conserved across taxonomic groups, we propose that studying microbial functioning may be more productive than a purely taxonomic approach to understanding the gut microbiome in depression.

Exploration of Survival Traits, Probiotic Determinants, Host Interactions, and Functional Evolution of Bifidobacterial Genomes Using Comparative Genomics

Members of the genus Bifidobacterium are found in a wide-range of habitats and are used as important probiotics. Thus, exploration of their functional traits at the genus level is of utmost significance. Besides, this genus has been demonstrated to exhibit an open pan-genome based on the limited number of genomes used in earlier studies. However, the number of genomes is a crucial factor for pan-genome calculations. We have analyzed the pan-genome of a comparatively larger dataset of 215 members of the genus Bifidobacterium belonging to different habitats, which revealed an open nature. The pan-genome for the 56 probiotic and human-gut strains of this genus, was also found to be open. The accessory- and unique-components of this pan-genome were found to be under the operation of Darwinian selection pressure. Further, their genome-size variation was predicted to be attributed to the abundance of certain functions carried by genomic islands, which are facilitated by insertion elements and prophages. In silico functional and host-microbe interaction analyses of their core-genome revealed significant genomic factors for niche-specific adaptations and probiotic traits. The core survival traits include stress tolerance, biofilm formation, nutrient transport, and Sec-secretion system, whereas the core probiotic traits are imparted by the factors involved in carbohydrate- and protein-metabolism and host-immunomodulations.

Effects of Different Doses of Fructooligosaccharides (FOS) on the Composition of Mice Fecal Microbiota, Especially the Bifidobacterium Composition

Fructooligosaccharides (FOS) are a well-known class of prebiotic and are considered to selectively stimulate the growth of bifidobacteria in the gut. Previous studies focused on the growth stimulation of Bifidobacterium, but they did not further investigate the bifidobacterial composition and the specific species that were stimulated. In this study, mice were fed with FOS in different doses for four weeks and the composition of fecal microbiota, in particular Bifidobacterium, was analyzed by sequencing the V3–V4 region and the groEL gene on the MiSeq platform, respectively. In the high-dose group, the relative abundance of Actinobacteria was significantly increased, which was mainly contributed by Bifidobacterium. At the genus level, the relative abundances of Blautia and Coprococcus were also significantly increased. Through the groEL sequencing, 14 species of Bifidobacterium were identified, among which B. pseudolongum was most abundant. After FOS treatment, B. pseudolongum became almost the sole bifidobacterial species (>95%). B. pseudolongum strains were isolated and demonstrated their ability to metabolize FOS by high performance liquid chromatography (HPLC). Therefore, we inferred that FOS significantly stimulated the growth of B. pseudolongum in mice. Further investigations are needed to reveal the mechanism of selectiveness between FOS and B. pseudolongum, which would aid our understanding of the basic principles between dietary carbohydrates and host health.

Complementary Mechanisms for Degradation of Inulin-Type Fructans and Arabinoxylan Oligosaccharides among Bifidobacterial Strains Suggest Bacterial Cooperation

Inulin-type fructans (ITF) and arabinoxylan oligosaccharides (AXOS) are broken down to different extents by various bifidobacterial strains present in the human colon. To date, phenotypic heterogeneity in the consumption of these complex oligosaccharides at the strain level remains poorly studied. To examine mechanistic variations in ITF and AXOS constituent preferences present in one individual, ITF and AXOS consumption by bifidobacterial strains isolated from the simulator of the human intestinal microbial ecosystem (SHIME) after inoculation with feces from one healthy individual was investigated. Among the 18 strains identified, four species-independent clusters displaying different ITF and AXOS degradation mechanisms and preferences were found. Bifidobacterium bifidum B46 showed limited growth on all substrates, whereas B. longum B24 and B. longum B18 could grow better on short-chain-length fractions of fructooligosaccharides (FOS) than on fructose. B. longum B24 could cleave arabinose substituents of AXOS extracellularly, without using the AXOS-derived xylose backbones, whereas B. longum B18 was able to consume oligosaccharides (up to xylotetraose) preferentially and consumed AXOS to a limited extent. B. adolescentis B72 degraded all fractions of FOS simultaneously, partially degraded inulin, and could use xylose backbones longer than xylotetraose extracellularly. The strain-specific degradation mechanisms were suggested to be complementary and indicated resource partitioning. Specialization in the degradation of complex carbohydrates by bifidobacteria present on the individual level could have in vivo implications for the successful implementation of ITF and AXOS, aiming at bifidogenic and/or butyrogenic effects. Finally, this work shows the importance of taking microbial strain-level differences into account in gut microbiota research.IMPORTANCE It is well known that bifidobacteria degrade undigestible complex polysaccharides, such as ITF and AXOS, in the human colon. However, this process has never been studied for strains coexisting in the same individual. To examine strain-dependent mechanistic variations in ITF and AXOS constituent preferences present in one individual, ITF and AXOS consumption by bifidobacterial strains isolated from the SHIME after inoculation with feces from one healthy individual was investigated. Among the 18 bifidobacterial strains identified, four species-independent clusters displaying different ITF and AXOS degradation mechanisms and preferences were found, indicating that such strains can coexist in the human colon. Such specialization in the degradation of complex carbohydrates by bifidobacteria present on the individual level could have in vivo implications for the successful implementation of ITF and AXOS, aiming at bifidogenic and/or butyrogenic effects.

Tracking the Taxonomy of the Genus Bifidobacterium Based on a Phylogenomic Approach

For decades, bacterial taxonomy has been based on in vitro molecular biology techniques and comparison of molecular marker sequences to measure the degree of genetic similarity and deduce phylogenetic relatedness of novel bacterial species to reference microbial taxa. Due to the advent of the genomic era, access to complete bacterial genome contents has become easier, thereby presenting the opportunity to precisely investigate the overall genetic diversity of microorganisms. Here, we describe a high-accuracy phylogenomic approach to assess the taxonomy of members of the genus Bifidobacterium and identify apparent misclassifications in current bifidobacterial taxonomy. The developed method was validated by the classification of seven novel taxa belonging to the genus Bifidobacterium by employing their overall genetic content. The results of this study demonstrate the potential of this whole-genome approach to become the gold standard for phylogenomics-based taxonomic classification of bacteria.IMPORTANCE Nowadays, next-generation sequencing has given access to genome sequences of the currently known bacterial taxa. The public databases constructed by means of these new technologies allowed comparison of genome sequences between microorganisms, providing information to perform genomic, phylogenomic, and evolutionary analyses. In order to avoid misclassifications in the taxonomy of novel bacterial isolates, new (bifido)bacterial taxons should be validated with a phylogenomic assessment like the approach presented here.

Gene-trait matching across the Bifidobacterium longum pan-genome reveals considerable diversity in carbohydrate catabolism among human infant strains

Background

Bifidobacterium longum is a common member of the human gut microbiota and is frequently present at high numbers in the gut microbiota of humans throughout life, thus indicative of a close symbiotic host-microbe relationship. Different mechanisms may be responsible for the high competitiveness of this taxon in its human host to allow stable establishment in the complex and dynamic intestinal microbiota environment. The objective of this study was to assess the genetic and metabolic diversity in a set of 20 B. longum strains, most of which had previously been isolated from infants, by performing whole genome sequencing and comparative analysis, and to analyse their carbohydrate utilization abilities using a gene-trait matching approach.

Results

We analysed their pan-genome and their phylogenetic relatedness. All strains clustered in the B. longum ssp. longum phylogenetic subgroup, except for one individual strain which was found to cluster in the B. longum ssp. suis phylogenetic group. The examined strains exhibit genomic diversity, while they also varied in their sugar utilization profiles. This allowed us to perform a gene-trait matching exercise enabling the identification of five gene clusters involved in the utilization of xylo-oligosaccharides, arabinan, arabinoxylan, galactan and fucosyllactose, the latter of which is an abundant human milk oligosaccharide (HMO).

Conclusions

The results showed high diversity in terms of genes and predicted glycosyl-hydrolases, as well as the ability to metabolize a large range of sugars. Moreover, we corroborate the capability of B. longum ssp. longum to metabolise HMOs. Ultimately, their intraspecific genomic diversity and the ability to consume a wide assortment of carbohydrates, ranging from plant-derived carbohydrates to HMOs, may provide an explanation for the competitive advantage and persistence of B. longum in the human gut microbiome.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4388-9) contains supplementary material, which is available to authorized users.

Mucin Cross-Feeding of Infant Bifidobacteria and Eubacterium hallii

Mucus production is initiated before birth and provides mucin glycans to the infant gut microbiota. Bifidobacteria are the major bacterial group in the feces of vaginally delivered and breast milk-fed infants. Among the bifidobacteria, only Bifidobacterium bifidum is able to degrade mucin and to release monosaccharides which can be used by other gut microbes colonizing the infant gut. Eubacterium hallii is an early occurring commensal that produces butyrate and propionate from fermentation metabolites but that cannot degrade complex oligo- and polysaccharides. We aimed to demonstrate that mucin cross-feeding initiated by B. bifidum enables growth and metabolite formation of E. hallii leading to short-chain fatty acid (SCFA) formation. Growth and metabolite formation of co-cultures of B. bifidum, of Bifidobacterium breve or Bifidobacterium infantis, which use mucin-derived hexoses and fucose, and of E. hallii were determined. Growth of E. hallii in the presence of lactose and mucin monosaccharides was tested. In co-culture fermentations, the presence of B. bifidum enabled growth of the other strains. B. bifidum/B. infantis co-cultures yielded acetate, formate, and lactate while co-cultures of B. bifidum and E. hallii formed acetate, formate, and butyrate. In three-strain co-cultures, B. bifidum, E. hallii, and B. breve or B. infantis produced up to 16 mM acetate, 5 mM formate, and 4 mM butyrate. The formation of propionate (approximately 1 mM) indicated cross-feeding on fucose. Lactose, galactose, and GlcNAc were identified as substrates of E. hallii. This study shows that trophic interactions of bifidobacteria and E. hallii lead to the formation of acetate, butyrate, propionate, and formate, potentially contributing to intestinal SCFA formation with potential benefits for the host and for microbial colonization of the infant gut. The ratios of SCFA formed differed depending on the microbial species involved in mucin cross-feeding.

Bifidobacteria and the infant gut: an example of co-evolution and natural selection

Throughout the human life, the gut microbiota interacts with us in a number of different ways, thereby influencing our health status. The acquisition of such an interactive gut microbiota commences at birth. Medical and environmental factors including diet, antibiotic exposure and mode of delivery are major factors that shape the composition of the microbial communities in the infant gut. Among the most abundant members of the infant microbiota are species belonging to the Bifidobacterium genus, which are believed to confer beneficial effects upon their host. Bifidobacteria may be acquired directly from the mother by vertical transmission and their persistence in the infant gut is associated with their saccharolytic activity toward glycans that are abundant in the infant gut. Here, we discuss the establishment of the infant gut microbiota and the contribution of bifidobacteria to this early life microbial consortium.

Omics of bifidobacteria: research and insights into their health-promoting activities

Members of the genus Bifidobacterium include gut commensals that are particularly abundant among the microbial communities residing in the gut of healthy breast-fed infants, where their presence has been linked to many beneficial host effects. Next-generation DNA sequencing and comparative and functional genome methodologies have been shown to be particularly useful in exploring the diversity of this genus. These combined approaches have allowed the identification of genetic features related to bifidobacterial establishment in the gut, involving host-microbe as well as microbe-microbe interactions. Among these, proteinaceous structures, which protrude from the bacterial surface, i.e. pili or fimbriae, and exopolysaccharidic cell surface layers or capsules represent crucial features that assist in their colonization and persistence in the gut. As bifidobacteria are colonizers of the large intestine, they have to be able to cope with various sources of osmotic, oxidative, bile and acid stress during their transit across the gastric barrier and the small intestine. Bifidobacterial genomes thus encode various survival mechanisms, such as molecular chaperones and efflux pumps, to overcome such challenges. Bifidobacteria represent part of an anaerobic gut community, and feed on nondigestible carbohydrates through a specialized fermentative metabolic pathway, which in turn produces growth substrates for other members of the gut community. Conversely, bifidobacteria may also be dependent on other (bifido)bacteria to access host- and diet-derived glycans, and these complex co-operative interactions, based on resource sharing and cross-feeding strategies, represent powerful driving forces that shape gut microbiota composition.

The First Microbial Colonizers of the Human Gut: Composition, Activities, and Health Implications of the Infant Gut Microbiota

The human gut microbiota is engaged in multiple interactions affecting host health during the host's entire life span. Microbes colonize the neonatal gut immediately following birth. The establishment and interactive development of this early gut microbiota are believed to be (at least partially) driven and modulated by specific compounds present in human milk. It has been shown that certain genomes of infant gut commensals, in particular those of bifidobacterial species, are genetically adapted to utilize specific glycans of this human secretory fluid, thus representing a very intriguing example of host-microbe coevolution, where both partners are believed to benefit. In recent years, various metagenomic studies have tried to dissect the composition and functionality of the infant gut microbiome and to explore the distribution across the different ecological niches of the infant gut biogeography of the corresponding microbial consortia, including those corresponding to bacteria and viruses, in healthy and ill subjects. Such analyses have linked certain features of the microbiota/microbiome, such as reduced diversity or aberrant composition, to intestinal illnesses in infants or disease states that are manifested at later stages of life, including asthma, inflammatory bowel disease, and metabolic disorders. Thus, a growing number of studies have reported on how the early human gut microbiota composition/development may affect risk factors related to adult health conditions. This concept has fueled the development of strategies to shape the infant microbiota composition based on various functional food products. In this review, we describe the infant microbiota, the mechanisms that drive its establishment and composition, and how microbial consortia may be molded by natural or artificial interventions. Finally, we discuss the relevance of key microbial players of the infant gut microbiota, in particular bifidobacteria, with respect to their role in health and disease.

The Sortase-Dependent Fimbriome of the Genus Bifidobacterium: Extracellular Structures with Potential To Modulate Microbe-Host Dialogue

Bifidobacteria are important gut commensals of mammals, including humans, of any age. However, the molecular mechanisms by which these microorganisms establish themselves in the mammalian gut and persist in this environment are largely unknown. Here, we analyzed the genetic diversity of the predicted arsenal of sortase-dependent pili of known and sequenced members of the Bifidobacterium genus and constructed a bifidobacterial sortase-dependent fimbriome database. Our analyses revealed considerable genetic variability of the sortase-dependent fimbriome among bifidobacterial (sub)species, which appears to have been due to horizontal gene transfer events and for which we were able to perform evolutionary mapping. Functional assessment by transcriptome analysis and binding assays involving different substrates demonstrates how bifidobacterial pili are pivotal in promoting various abilities for adhesion to glycans and extracellular matrix proteins, thereby supporting the ecological success of bifidobacteria in the mammalian gut.IMPORTANCE Adhesion of bifidobacterial cells to the mucosa of the large intestine is considered a hallmark for the persistence and colonization of these bacteria in the human gut. In this context, we analyzed the genetic diversity of the predicted arsenal of sortase-dependent pili of known and sequenced members of the Bifidobacterium genus, and constructed a bifidobacterial sortase-dependent fimbriome database. Our analyses revealed considerable genetic variability of the sortase-dependent fimbriome among bifidobacterial (sub)species, which appears to have been due to horizontal gene transfer events. In addition, functional assessment by transcriptome analysis and binding assays involving different substrates demonstrates how bifidobacterial pili are crucial in promoting various abilities for adhesion to glycans and extracellular matrix proteins, thereby supporting the ecological success of bifidobacteria in the mammalian gut. This study represents a complete genomic study regarding the presence of fimbriae in the genus Bifidobacterium.

Comparative genomic and phylogenomic analyses of the Bifidobacteriaceae family

Background

Members of the Bifidobacteriaceae family represent both dominant microbial groups that colonize the gut of various animals, especially during the suckling stage of their life, while they also occur as pathogenic bacteria of the urogenital tract. The pan-genome of the genus Bifidobacterium has been explored in detail in recent years, though genomics of the Bifidobacteriaceae family has not yet received much attention. Here, a comparative genomic analyses of 67 Bifidobacteriaceae (sub) species including all currently recognized genera of this family, i.e., Aeriscardovia, Alloscardovia, Bifidobacterium, Bombiscardovia, Gardnerella, Neoscardovia, Parascardovia, Pseudoscardovia and Scardovia, was performed. Furthermore, in order to include a representative of each of the 67 (currently recognized) (sub) species belonging to the Bifidobacteriaceae family, we sequenced the genomes of an additional 11 species from this family, accomplishing the most extensive comparative genomic analysis performed within this family so far.

Results

Phylogenomics-based analyses revealed the deduced evolutionary pathway followed by each member of the Bifidobacteriaceae family, highlighting Aeriscardovia aeriphila LMG 21773 as the deepest branch in the evolutionary tree of this family. Furthermore, functional analyses based on genome content unveil connections between a given member of the family, its carbohydrate utilization abilities and its corresponding host. In this context, bifidobacterial (sub) species isolated from humans and monkeys possess the highest relative number of acquired glycosyl hydrolase-encoding genes, probably in order to enhance their metabolic ability to utilize different carbon sources consumed by the host.

Conclusions

Within the Bifidobacteriaceae family, genomics of the genus Bifidobacterium has been extensively investigated. In contrast, very little is known about the genomics of members of the other eight genera of this family. In this study, we decoded the genome sequences of each member of the Bifidobacteriaceae family. Thanks to subsequent comparative genomic and phylogenetic analyses, the deduced pan-genome of this family, as well as the predicted evolutionary development of each taxon belonging to this family was assessed.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3955-4) contains supplementary material, which is available to authorized users.