Polymorphism and distribution of putative cell-surface adhesin-encoding ORFs among human fecal isolates of Bifidobacterium longum subsp. longum

Roles of the Cell Surface Architecture of Bacteroides and Bifidobacterium in the Gut Colonization

There are numerous bacteria reside within the mammalian gastrointestinal tract. Among the intestinal bacteria, Akkermansia, Bacteroides, Bifidobacterium, and Ruminococcus closely interact with the intestinal mucus layer and are, therefore, known as mucosal bacteria. Mucosal bacteria use host or dietary glycans for colonization via adhesion, allowing access to the carbon source that the host’s nutrients provide. Cell wall or membrane proteins, polysaccharides, and extracellular vesicles facilitate these mucosal bacteria-host interactions. Recent studies revealed that the physiological properties of Bacteroides and Bifidobacterium significantly change in the presence of co-existing symbiotic bacteria or markedly differ with the spatial distribution in the mucosal niche. These recently discovered strategic colonization processes are important for understanding the survival of bacteria in the gut. In this review, first, we introduce the experimental models used to study host-bacteria interactions, and then, we highlight the latest discoveries on the colonization properties of mucosal bacteria, focusing on the roles of the cell surface architecture regarding Bacteroides and Bifidobacterium.

FACTORS OF ADHESION OF BIFIDOBACTERIA

Data on fimbrial and afimbrial adhesion factors of bifidobacteria are presented. Pili-like structures, their composition and conditions of formation in various species of bifidobacteria are described. Several sugar-lytic enzymes serve as afimbrial adhesins in bifidobacteria. Transaldolase and enolase are detected in bifidobacteria on cells’ surface. Transaldolase ensures binding of bifidobacteria with mucin and their auto-aggregation. Surface enolase has an affinity to plasminogen, thus bifidobacteria obtain a surface-bound protein with proteolytic activity. Molecular structures giving bifidobacteria hydrophobic properties are described - surface lipoprotein Bop A and lipoteichoic acids.

Adhesion Properties of Lactic Acid Bacteria on Intestinal Mucin

Lactic acid bacteria (LAB) are Gram-positive bacteria that are natural inhabitants of the gastrointestinal (GI) tracts of mammals, including humans. Since Mechnikov first proposed that yogurt could prevent intestinal putrefaction and aging, the beneficial effects of LAB have been widely demonstrated. The region between the duodenum and the terminal of the ileum is the primary region colonized by LAB, particularly the Lactobacillus species, and this region is covered by a mucus layer composed mainly of mucin-type glycoproteins. The mucus layer plays a role in protecting the intestinal epithelial cells against damage, but is also considered to be critical for the adhesion of Lactobacillus in the GI tract. Consequently, the adhesion exhibited by lactobacilli on mucin has attracted attention as one of the critical factors contributing to the persistent beneficial effects of Lactobacillus in a constantly changing intestinal environment. Thus, understanding the interactions between Lactobacillus and mucin is crucial for elucidating the survival strategies of LAB in the GI tract. This review highlights the properties of the interactions between Lactobacillus and mucin, while concomitantly considering the structure of the GI tract from a histochemical perspective.

Bosom Buddies: The Symbiotic Relationship Between Infants and Bifidobacterium longum ssp. longum and ssp. infantis. Genetic and Probiotic Features

The intestinal microbiota is a complex community that plays an important role in human health from the initial steps of its establishment. Its microbial composition has been suggested to result from selective pressures imposed by the host and is modulated by competition among its members. Bifidobacterium longum is one of the most abundant species of the Bifidobacterium genus in the gut microbiota of healthy breast-fed infants and adults. The recent advancements of 'omics techniques have facilitated the genetic and functional studies of different gut microbiota members. They have revealed the complex genetic pathways used to metabolize different compounds that likely contribute to the competitiveness and persistence of B. longum in the colon. The discovery of a genomic island in B. longum ssp. infantis that encodes specific enzymes for the metabolism of human milk oligosaccharides suggests a specific ecological adaptation. Moreover, B. longum is widely used as probiotic, and beneficial effects in infant health have been reported in several studies.

Adhesion properties of a putative polymorphic fimbrial subunit protein from Bifidobacterium longum subsp. longum

In our previous study, we found that the open reading frame bl0675 in the genome of Bifidobacterium longum subsp. longum isolated from human feces encoded a novel putative fimbrial protein, was highly polymorphic, and had five variants (A, B, C, D, and E types). The aim of this study was to evaluate the affinity of these variants to porcine colonic mucins (PCMs). Protein-binding properties were examined using the recombinant BL0675 protein containing a C-terminal 6 × His tag (His-BL0675). Surface plasmon resonance analysis demonstrated that the His-BL0675 A type had strong affinity to PCMs (K_D = 9.82 × 10⁻⁸ M), whereas the B, C, D, and E types exhibited little or no binding. In a competitive enzyme-linked immunosorbent assay, His-BL0675 A type binding was reduced by addition of mucin oligosaccharides, suggesting that the binding occurs via carbohydrate chains of PCMs. The localization of BL0675 to the B. longum subsp. longum cell surface was confirmed by western blot analysis using A type polyclonal antibodies. Bacterial adhesion of B. longum subsp. longum to PCMs was also blocked by A type-specific antibodies; however, its adhesion properties were strain specific. Our results suggest that the BL0675 variants significantly contribute to the adhesion of B. longum subsp. longum strains. The expression and the adhesive properties of this protein are affected by genetic polymorphisms and are specific for B. longum subsp. longum strains. However, further studies are required on the properties of binding of these putative fimbrial proteins to the human gastrointestinal tract.

Technological advances in bifidobacterial molecular genetics: application to functional genomics and medical treatments

Bifidobacteria are well known as beneficial intestinal bacteria that exert health-promoting effects in humans. In addition to physiological and immunological investigations, molecular genetic technologies have been developed and have recently started to be applied to clarify the molecular bases of host-Bifidobacterium interactions. These technologies include transformation technologies and Escherichia coli-Bifidobacterium shuttle vectors that enable heterologous gene expression. In this context, a plasmid artificial modification method that protects the introduced plasmid from the restriction system in host bifidobacteria has recently been developed to increase transformation efficiency. On the other hand, targeted gene inactivation systems, which are vital for functional genomics, seemed far from being practically applicable in bifidobacteria. However, remarkable progress in this technology has recently been achieved, enabling functional genomics in bifidobacteria. Integrated use of these molecular genetic technologies with omics-based analyses will surely boost characterization of the molecular basis underlying beneficial effects of bifidobacteria. Applications of recombinant bifidobacteria to medical treatments have also progressed.