Bifidobacteria are relatively abundant inhabitants of the gastrointestinal tract of humans and animals. Many bifidobacterial species, in conjunction with other members of the intestinal microbiota are believed to contribute to host nutrition, while also impacting on intestinal pH, cell proliferation and differentiation, development and activity of the immune system, and innate and acquired responses to pathogens. These perceived beneficial health effects have driven commercial exploitation of bifidobacteria as live components of many functional foods and therapeutic adjuncts. However, bifidobacteria have also been isolated from the human oral cavity, where their presence is linked to the progression of tooth decay: bifidobacteria have been detected in high numbers in infected dentine from carious lesions in children and have been associated with childhood dental caries. can be found as part of the microbiota implicated in human dental caries. In recent surveys of oral bifidobacteria associated with caries in adults and children and root caries in adults, Bifidobacterium dentium was the most frequently isolated Bifidobacterium species, representing approximately eight percent of the culturable bacteria isolated from active lesions. This species is capable of acidogenesis to produce a final pH in glucose-containing media below pH 4.2, sufficient to cause extensive demineralisation of tooth tissues. B. dentium may therefore significantly contribute to the pathogenesis of dental caries which is one of the most common chronic diseases, remaining untreated in many underdeveloped countries where dental pain is often alleviated only by the loss or extraction of the affected tooth.
Researchers have now uncovered the complete genetic make-up of the cavity-causing bacterium B. dentium Bd1, revealing the genetic adaptations that allow this microorganism to live and cause decay in the human oral cavity. Bifidobacteria, largely known as long-term beneficial gut bacteria, are often included as probiotic components of food to aid digestion and boost the immune system. However, not all species within the genus Bifidobacterium provide beneficial effects to the host’s health. In fact, B. dentium is an opportunistic pathogen since it has been linked to the development of tooth decay. The genome sequence of B. dentium Bd1 reveals how this microorganism has adapted to the oral environment through specialized nutrient acquisition features, acid tolerance, defences against antimicrobial substances and other gene products that increase fitness and competitiveness within the oral niche. This report identifies, through various genomic approaches, specific adaptations of a Bifidobacterium taxon to a lifestyle as a tooth decay-causing bacterium. The data in this study indicate that the genome of this opportunistic pathogen has evolved through only a small number of horizontal gene acquisition events, highlighting the narrow boundary that separates bacteria that are long-term residents on or in the human body from opportunistic pathogens.
The Bifidobacterium dentium Bd1 Genome Sequence Reflects Its Genetic Adaptation to the Human Oral Cavity. 2009 PLoS Genet 5(12): e1000785. doi:10.1371/journal.pgen.1000785
Bifidobacteria, one of the relatively dominant components of the human intestinal microbiota, are considered one of the key groups of beneficial intestinal bacteria (probiotic bacteria). However, in addition to health-promoting taxa, the genus Bifidobacterium also includes Bifidobacterium dentium, an opportunistic cariogenic pathogen. The genetic basis for the ability of B. dentium to survive in the oral cavity and contribute to caries development is not understood. The genome of B. dentium Bd1, a strain isolated from dental caries, was sequenced to completion to uncover a single circular 2,636,368 base pair chromosome with 2,143 predicted open reading frames. Annotation of the genome sequence revealed multiple ways in which B. dentium has adapted to the oral environment through specialized nutrient acquisition, defences against antimicrobials, and gene products that increase fitness and competitiveness within the oral niche. B. dentium Bd1 was shown to metabolize a wide variety of carbohydrates, consistent with genome-based predictions, while colonization and persistence factors implicated in tissue adhesion, acid tolerance, and the metabolism of human saliva-derived compounds were also identified. Global transcriptome analysis demonstrated that many of the genes encoding these predicted traits are highly expressed under relevant physiological conditions. This is the first report to identify, through various genomic approaches, specific genetic adaptations of a Bifidobacterium taxon, Bifidobacterium dentium Bd1, to a lifestyle as a cariogenic microorganism in the oral cavity. In silico analysis and comparative genomic hybridization experiments clearly reveal a high level of genome conservation among various B. dentium strains. The data indicate that the genome of this opportunistic cariogen has evolved through a very limited number of horizontal gene acquisition events.