Gut Microbiome: From Microbial Composition to Functional Activity

The human gut harbors an exceptionally complex ecosystem of microorganisms that perform important biological functions, from digestion and nutrient metabolism to the regulation of immune responses. This ecosystem is the gut microbiome, one of the fastest-evolving areas of modern biomedical research.

The term gut microbial community refers to the microorganisms residing in the intestine, primarily bacteria, but also archaea, fungi, and viruses. The term microbiome is used more broadly to describe these microorganisms, their genetic material, and the environment in which they interact.

For many years, laboratory investigations of the microbiome focused primarily on identifying microorganisms and estimating their relative abundance. Advances in DNA sequencing technologies have enabled more detailed mapping of microbial composition, including bacteria that are not readily detected using conventional culture methods.

In recent years, research interest has increasingly shifted from identifying “which microorganisms are present” to understanding “what they are capable of doing and what they actually do”. This development offers new opportunities to understand intestinal health, while also presenting significant challenges for the clinical interpretation of laboratory findings.

From Composition to Function: What Is Changing in Gut Microbiome Analysis?

Modern technologies enable the investigation of different characteristics of the microbiome, ranging from microbial composition and biodiversity to genetic potential and the products of metabolic activity.

1. PCR-Based Molecular Analysis: Which Specific Microorganisms Are Present in the Gut?
Polymerase chain reaction (PCR) is a targeted molecular method that detects specific microorganisms through their genetic material. Quantitative PCR (qPCR) also quantifies selected microbial targets. Unlike sequencing-based methods, PCR targets predefined microorganisms and does not provide a comprehensive profile of microbial diversity.

2. 16S rRNA Sequencing: Which Bacteria Are Present in the Gut?
Sequencing of the 16S rRNA gene is a widely used method for profiling bacterial composition. It enables the identification of microbial groups, estimation of their relative abundance and assessment of microbial diversity.

3. Metagenomics: What Are Microorganisms Capable of Doing?
Whole microbial DNA metagenomic analysis, known as shotgun metagenomics, is an advanced method used primarily in scientific research and specialized applications. It enables the investigation of genes associated with metabolic functions, such as short-chain fatty acid production and bile acid metabolism. However, it mainly provides information about the microbial community's functional potential rather than its actual metabolic activity.

4. Metabolomics: What Does the Microbiome Actually Produce?
Metabolomics investigates small molecules, known as metabolites, that are produced or modified through biochemical processes. Analyzing them provides information about the intestine's metabolic environment.

Combining microbial composition and functional markers can provide a more comprehensive picture, although it does not replace clinical diagnosis.

How Microbiome Activity Influences Gut Health

The significance of the gut microbiome extends beyond the microorganisms that constitute it to the biological functions they perform. By breaking down and metabolizing dietary components, intestinal microorganisms produce or modify substances that may influence digestion, metabolism, immune responses, and intestinal barrier function.

1. Microbial Metabolites and Metabolic Activity
A key example is short-chain fatty acids (SCFAs), such as acetate, propionate, and butyrate, produced primarily through microbial fermentation of dietary fiber. Butyrate is an important energy source for colon cells, while SCFAs help regulate inflammatory responses and maintain normal intestinal function.
The microbiome also participates in the metabolism of bile acids, which contribute to fat digestion, as well as tryptophan, an essential amino acid obtained through the diet. These microbial processes produce or modify substances that may influence metabolism, immune function, and communication between the intestine and the rest of the body.

2. Intestinal Barrier and Inflammatory Response
The intestinal barrier acts as a protective system, allowing controlled absorption while limiting the passage of potentially harmful microbial factors. Its normal function is influenced, among other factors, by interactions between the intestinal epithelium, the microbiome, and its metabolites.
When intestinal barrier integrity is compromised, the immune system may be exposed to higher levels of microbial products, such as lipopolysaccharides (LPS), potentially activating inflammatory mechanisms. Persistent inflammation may, in turn, further impair intestinal barrier function.

These interactions highlight why investigating microbial composition is more informative when combined with an understanding of metabolic activity and the intestinal environment.

Laboratory Investigation of the Gut Microbiome

Assessing the gut microbiome involves specialized molecular analyses that examine microbial composition, along with selected biochemical and functional markers that provide complementary information about the intestinal environment. The selection of appropriate tests depends on the individual's medical history, symptoms, and specific clinical question, while findings are interpreted in conjunction with the overall clinical picture.

Specialized Functional Medicine Tests
In personalized and functional medicine, specialized tests assess the gut microbiome and select aspects of intestinal function using advanced molecular and biochemical analysis technologies.

• EnteroScan® NGS - Gut Microbiome Analysis Using Next-Generation Sequencing: EnteroScan® NGS uses next-generation sequencing (NGS) technology, specifically 16S rRNA gene sequencing, to provide comprehensive profiling of the bacterial composition of the gut microbiome. It provides information on the relative abundance of detected bacteria, microbial diversity, and the overall structure of the intestinal microbial ecosystem.
• EnteroScan® with PCR - Molecular Analysis of the Gut Microbiome: EnteroScan® PCR is based on polymerase chain reaction (PCR) technology, which enables the targeted detection and quantification of selected microorganisms through their genetic material. Unlike the broader microbial profiling offered by NGS technology, PCR analysis focuses on predefined microbial groups and species.
   

Complementary Biochemical and Functional Markers
Depending on the selected EnteroScan® profile, molecular analysis may be combined with additional markers that broaden the assessment of the intestinal environment and digestive function. These include:

• Short-Chain Fatty Acids (SCFAs): Their measurement in stool provides information about metabolites produced primarily through microbial fermentation of dietary fibers and other substrates.
• Fecal Calprotectin: A marker of intestinal inflammation, primarily used in the investigation and monitoring of inflammatory bowel diseases.
• Fecal Pancreatic Elastase: Helps assess exocrine pancreatic function, particularly when there are indications of malabsorption or digestive disorders.
   

Conventional Laboratory Tests
Depending on symptoms and medical history, conventional laboratory tests may also be required, such as a complete blood count (CBC), C-reactive protein (CRP), ferritin, vitamin B12, and folate. These tests do not directly assess the gut microbiome but can help investigate associated conditions, such as systemic inflammation, anemia, and potential nutritional deficiencies or malabsorption disorders.

Endoscopic and Imaging Examinations
When specific clinical indications are present, further investigation may include colonoscopy, gastroscopy (upper gastrointestinal endoscopy), or magnetic resonance enterography. These examinations do not assess microbiome composition; they evaluate the anatomy and condition of the digestive tract and investigate structural or inflammatory diseases that may coexist with gastrointestinal symptoms.

Therapeutic Approaches

Modification of the gut microbiome is not, in itself, a therapeutic objective. Select interventions based on the underlying condition and their evidence-based clinical effectiveness.

A. Conventional Therapeutic Approaches
Pharmacological treatment depends on the diagnosis. Appropriate antibiotics are used for confirmed bacterial infections, while inflammatory bowel diseases may require anti-inflammatory medications, immunomodulators or biological therapies.
Fecal microbiota transplantation (FMT) is used in selected cases of recurrent Clostridioides difficile infection. It is not an established treatment for generalized dysbiosis or irritable bowel syndrome.

B. Diet, Lifestyle and Complementary Interventions
Diet and lifestyle influence the gut microbiome's composition and function. The Mediterranean diet, rich in dietary fiber and polyphenols, supports metabolic health and promotes microbial processes such as short-chain fatty acid production. Regular physical activity, adequate sleep, and stress management also help maintain intestinal health.
Probiotics and prebiotics may benefit some people in specific circumstances, while botanical extracts and polyphenols are being investigated for their potential effects on the microbiome. Supplement use should be individualized, considering scientific evidence, individual tolerance, and potential contraindications.

Conclusion

Modern gut microbiome research is shifting its focus from identifying microorganisms alone to investigating their metabolic functions and interactions with the human body.

Specialized tests now enable gut microbiome profiling and, depending on the selected panel, the assessment of functional and biochemical markers. This information may form part of an individualized laboratory investigation when an appropriate clinical indication exists.

The true value of modern analyses lies not only in the quantity of data they generate, but also in their scientifically grounded interpretation.

References

1. Porcari S, et al. International consensus statement on microbiome testing in clinical practice. Lancet Gastroenterol Hepatol. 2025 Feb;10(2):154-167. doi: 10.1016/S2468-1253(24)00311-X. Epub 2024 Dec 5.
2. Mukherjee A, Breselge S, Dimidi E, Marco ML, Cotter PD. Fermented foods and gastrointestinal health: underlying mechanisms. Nat Rev Gastroenterol Hepatol. 2024 Apr;21(4):248-266. doi: 10.1038/s41575-023-00869-x. Epub 2023 Dec 11.
3. Peery AF, Kelly CR, Kao D, Vaughn BP, Lebwohl B, Singh S, Imdad A, Altayar O; AGA Clinical Guidelines Committee. AGA Clinical Practice Guideline on Fecal Microbiota-Based Therapies for Select Gastrointestinal Diseases. Gastroenterology. 2024 Mar;166(3):409-434. doi: 10.1053/j.gastro.2024.01.008.
4. Yang SY, Han SM, Lee JY, Kim KS, Lee JE, Lee DW. Advancing Gut Microbiome Research: The Shift from Metagenomics to Multi-Omics and Future Perspectives. J Microbiol Biotechnol. 2025 Mar 26;35. doi: 10.4014/jmb.2412.12001.
5. Mann ER, Lam YK, Uhlig HH. Short-chain fatty acids: linking diet, the microbiome and immunity. Nat Rev Immunol. 2024 Aug;24(8):577-595. doi: 10.1038/s41577-024-01014-8. Epub 2024 Apr 2. PMID: 38565643.
6. Miyamoto K, Sujino T, Kanai T. The tryptophan metabolic pathway of the microbiome and host cells in health and disease. Int Immunol. 2024 Nov 14;36(12):601-616. doi: 10.1093/intimm/dxae035.
7. Lin X, Xia L, Zhou Y, Xie J, Tuo Q, Lin L, Liao D. Crosstalk Between Bile Acids and Intestinal Epithelium: Multidimensional Roles of Farnesoid X Receptor and Takeda G Protein Receptor 5. Int J Mol Sci. 2025 Apr 29;26(9):4240. doi: 10.3390/ijms26094240.

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