Bacteroidetes

Bacteroidetes, a phylum of gram-negative bacteria, is a significant component of the human gut microbiota. 

Comprising about 30% of the gut microbiota, their abundance varies with factors such as population, geography, age, and diet.  This phylum includes several genera, notably Bacteroides, which are essential for degrading complex polysaccharides into short-chain fatty acids, providing energy to the host and regulating immune function. 

Bacteroidetes also play a vital role in maintaining gut barrier integrity, protecting against pathogens, and modulating host metabolism. Their presence and balance with other gut bacteria are critical for overall gut health, and alterations in their abundance are linked to various gastrointestinal and metabolic disorders.

Bacteroidetes: Microbiological Background

Bacteroidetes, a phylum of gram-negative bacteria, constitutes a significant portion of the human gut microbiota and is integral to maintaining gut homeostasis. 

Bacteroidetes make up about 30% of the human gut microbiota, with this proportion varying based on factors like population, geography, age, and diet.  This phylum includes several genera, notably Bacteroides, which are gram-negative, non-spore forming, anaerobic, rod-shaped bacteria. 

About 24 species of Bacteroides have been identified, with B. fragilis and B. thetaiotaomicron being the most studied.

Taxonomy and Characteristics of Bacteroidetes  [2., 9.]

Bacteroidetes encompasses a diverse group of Gram-negative, non-spore-forming bacteria with a wide range of morphological, metabolic, and physiological characteristics. 

Taxonomically, Bacteroidetes are classified into several families and genera, including Bacteroides, Prevotella, and Porphyromonas. 

These bacteria are typically anaerobic and inhabit the mucosal surfaces of the gastrointestinal tract, where they contribute to the degradation of complex polysaccharides from dietary fibers and mucins.

Role of Bacteroidetes in the Gut Microbiome  [2., 13.]

In the animal gut, Bacteroidetes play a mutualistic role, enhancing the host's fitness by degrading complex carbohydrates that the host cannot process on its own. This activity results in the production of short-chain fatty acids, which serve as an energy source for the host. 

Bacteroidetes ferment dietary fibers and produce short-chain fatty acids (SCFAs) such as acetate, propionate, and butyrate, which serve as energy sources for host epithelial cells and regulate immune function. 

Additionally, Bacteroidetes contribute to the maintenance of gut barrier integrity, modulation of host metabolism, and protection against pathogenic colonization by competing for nutrients and producing antimicrobial compounds. 

Bacteroidetes also interact with the immune system, promoting T-cell responses and limiting the colonization of pathogenic bacteria. Their ability to process various polysaccharides makes them essential for maintaining gut health and energy balance.

Alterations in Bacteroidetes abundance have been associated with various gastrointestinal disorders, metabolic diseases, and immune-related conditions, highlighting its importance in gut health and disease prevention.

Clinical and Health Implications of Bacteroidetes

Bacteroidetes play a crucial role in human health and have significant clinical implications related to their presence and abundance in the microbiome. 

Maintaining a balanced ratio of Bacteroidetes to other phyla like Firmicutes is essential for gut health, biodiversity, and homeostasis of metabolism, immune function, and colonization resistance.  [2.] 

Bacteroidetes are considered a "keystone taxon" in the gut microbiome, and their depletion can lead to detrimental shifts in the microbial community structure.  

Bacteroidetes species like Bacteroides fragilis and B. thetaiotaomicron play beneficial roles as commensals, fermenting polysaccharides to produce short-chain fatty acids that serve as an energy source for the host. They also contribute to bile acid metabolism and provide colonization resistance against pathogens like Clostridioides difficile. 

Reduced abundance of Bacteroidetes has been associated with obesity, though the evidence is not conclusive.  However, alterations in the Firmicutes/Bacteroidetes ratio and their metabolic capabilities may influence energy extraction from food and weight gain.  [6.]

Bacteroidetes are involved in the pathogenesis of inflammatory bowel diseases (IBD), with their dysbiosis contributing to disease development and progression through dysregulation of community composition, modulation of host immune response, and induction of chronic inflammation.  [4.]

Bacteroidetes are implicated in type 1 and type 2 diabetes, suggesting their potential role in these conditions.  [11.]

Modulating Bacteroidetes levels through approaches like fecal microbiota transplantation (FMT) and microbiota modulation are being explored as therapeutic strategies for conditions like C. difficile infection, diabetes, and IBD.  [4.]

The increasing scientific understanding of the health and clinical implications of Bacteroidetes abundance has emerged as an important biomarker for assessing gut health, metabolic status, and disease risk. 

Specific Role of Bacteroidetes as a Biomarker

Bacteroidetes abundance has been associated with various health outcomes and disease states, making it a promising biomarker in medical diagnostics. 

Bacteroidetes abundance reflects the composition and function of the gut microbiota, offering insights into the dynamic interplay between microbial communities and host physiology.

Studies have demonstrated correlations between Bacteroidetes levels and metabolic health parameters including obesity, insulin resistance, and dyslipidemia.  [7., 11.]

Additionally, alterations in Bacteroidetes abundance have been linked to gastrointestinal disorders such as inflammatory bowel disease (IBD), irritable bowel syndrome (IBS), and colorectal cancer, suggesting its utility as a diagnostic marker for gut dysbiosis and gastrointestinal health.  [8., 10.]

Implications of High and Low Bacteroidetes Abundance

Both high and low Bacteroidetes abundance have clinical implications for human health.  High Bacteroidetes abundance is generally associated with a healthy gut microbiome and favorable metabolic outcomes, while low Bacteroidetes abundance may indicate dysbiosis, inflammation, and increased disease risk.  [5., 8., 10.] 

Understanding the implications of Bacteroidetes abundance as a biomarker can aid in early disease detection, risk stratification, and personalized treatment strategies tailored to individual patients' gut microbiota composition.

Laboratory Testing for Bacteriodetes Levels

Test Type, Sample Collection and Preparation

Bacteriodetes levels are assessed in stool samples.  Stool samples may be collected from the comfort of home.  

Testing may require avoidance of certain medications and/or supplements including probiotics prior to sample collection.  It is important to consult with the ordering provider for full test preparation instructions.  

Interpretation of Test Results

Optimal Levels of Bacteriodetes

It is important to consult with the laboratory company used for test interpretation.  

One lab company provides the following reference range for Bacteriodetes levels: 8.6e11 - 3.3e12  [12.]

Clinical Implications of High Bacteroidetes 

High levels of Bacteroidetes in the gut microbiome are generally associated with a healthy state and favorable metabolic outcomes. 

A high relative abundance of Bacteroidetes compared to other phyla like Firmicutes is considered essential for maintaining gut health, biodiversity, and homeostasis of metabolism, immune function, and colonization resistance.  Bacteroidetes are considered a "keystone taxon" in the gut microbiome, and their depletion can lead to detrimental shifts in the microbial community structure.  [8., 14.]

Several studies have found an association between high Bacteroidetes levels and a lean phenotype or healthy weight.  A higher Bacteroidetes/Firmicutes ratio was associated with a lower body mass index (BMI).  [5., 7.] 

Conversely, a lower abundance of Bacteroidetes has been observed in obese individuals compared to lean individuals.  [5.] 

High levels of Bacteroidetes species like Bacteroides fragilis and B. thetaiotaomicron are considered beneficial as commensals, fermenting polysaccharides to produce short-chain fatty acids that serve as an energy source for the host. They also contribute to bile acid metabolism and provide colonization resistance against pathogens like Clostridioides difficile.  [14.]

Low Bacteroidetes Abundance  [2., 7., 12.] 

Lower levels of Bacteroidetes have been associated with inflammatory bowel diseases (IBD) like ulcerative colitis, where specific Bacteroidetes species exhibit significantly lower abundance compared to healthy controls.

The loss of these species is suggested to result from disease exacerbation and may serve as potential biomarkers for disease activity.

Low Bacteroidetes levels may disrupt microbial community dynamics, favoring the overgrowth of potentially harmful bacteria and diminishing the beneficial effects of Bacteroidetes-mediated functions, such as the fermentation of dietary fibers and production of short-chain fatty acids (SCFAs).  [3.] 

Therefore, maintaining a high abundance of Bacteroidetes in the gut microbiome is generally considered a favorable state, associated with better metabolic health, a lean phenotype, and a lower risk of inflammatory conditions like IBD. 

Monitoring Bacteroidetes levels may have clinical significance in assessing gut health, disease risk, and potential therapeutic interventions aimed at restoring a balanced microbiome.

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