Intestinal Bacterial Strain: Role in the Gut Microbiome

What Is an Intestinal Bacterial Strain?

An intestinal bacterial strain refers to a clearly defined type or lineage of bacteria that inhabits the human gastrointestinal tract. In microbiology, bacteria are classified into phyla, classes, orders, families, genera, and species. In everyday usage, the term is often applied to individual bacterial species or genetically uniform lineages within a species that colonize the gut.

The human gut is home to an estimated 10 to 100 trillion microorganisms, including bacteria, viruses, fungi, and archaea. Together, these are referred to as the gut microbiome or gut microbiota. The various intestinal bacterial strains exist in a complex balance and influence a wide range of bodily functions.

The Most Important Intestinal Bacterial Phyla

A few dominant phyla account for the majority of bacteria in the human gut, while enormous diversity exists at deeper taxonomic levels:

• Firmicutes: This phylum includes many gram-positive bacteria such as Lactobacillus and Clostridium. Firmicutes are important for fermenting carbohydrates and extracting energy from food.
• Bacteroidetes: A gram-negative phylum with key representatives such as Bacteroides and Prevotella. These bacteria help digest complex polysaccharides and produce short-chain fatty acids.
• Actinobacteria: This phylum includes the well-known genus Bifidobacterium, which is especially abundant in the infant gut and has probiotic properties.
• Proteobacteria: Contains potentially pathogenic bacteria such as Escherichia coli. In a healthy microbiome, they are present only in small amounts.
• Verrucomicrobia: Includes Akkermansia muciniphila, a bacterium that strengthens the gut lining and is associated with healthy metabolism.

Functions of Intestinal Bacterial Strains

The various intestinal bacterial strains perform vital functions in the human body:

• Digestion and nutrient absorption: Many dietary fibers cannot be digested without bacterial assistance. Gut bacteria ferment these substances and produce short-chain fatty acids such as butyrate, propionate, and acetate, which serve as an energy source for intestinal cells.
• Immune system support: Approximately 70 to 80 percent of the human immune system is located in the gut. Intestinal bacterial strains help train and regulate immune responses, distinguish between self and non-self, and protect against infections.
• Protective barrier: Beneficial bacterial strains colonize the gut lining and prevent harmful pathogens from taking hold, a process known as colonization resistance.
• Vitamin production: Certain intestinal bacterial strains synthesize vitamins such as vitamin K2 and several B vitamins, including biotin, folate, and vitamin B12.
• Gut-brain axis: Intestinal bacteria produce neurotransmitters and precursor molecules such as serotonin, which can influence mood and the nervous system via the so-called gut-brain axis.

Dysbiosis: Imbalance of Intestinal Bacterial Strains

Dysbiosis describes a disrupted balance of intestinal bacterial strains, in which beneficial bacteria decrease and potentially harmful ones increase. Factors that promote dysbiosis include:

• Use of antibiotics
• Unbalanced, low-fiber diet
• Chronic stress
• Physical inactivity
• Excessive alcohol consumption
• Certain medications (e.g., proton pump inhibitors, NSAIDs)

Dysbiosis is associated with various conditions, including inflammatory bowel diseases (Crohn's disease, ulcerative colitis), irritable bowel syndrome, obesity, type 2 diabetes, allergies, and even mental health disorders such as depression.

Intestinal Bacterial Strains in Medicine and Nutrition

The significance of specific intestinal bacterial strains for health has sparked growing interest in targeted approaches to promote a healthy gut flora:

• Probiotics: Live microorganisms that, when consumed in adequate amounts, confer a health benefit. Common probiotic strains include Lactobacillus acidophilus, Bifidobacterium longum, and Saccharomyces boulardii.
• Prebiotics: Non-digestible dietary fibers such as inulin or fructooligosaccharides that serve as food for beneficial intestinal bacterial strains and promote their growth.
• Synbiotics: Combined products containing both probiotics and prebiotics.
• Fecal Microbiota Transplantation (FMT): The transfer of stool from a healthy donor to a recipient to restore a healthy microbiota, particularly in cases of recurrent Clostridioides difficile infection.

Diagnostics

The composition of intestinal bacterial strains can today be analyzed using modern molecular biology methods. In particular, 16S rRNA sequencing and metagenomic analyses are used to obtain a comprehensive picture of the gut microbiome. Such analyses are used both in research and increasingly in clinical practice, as well as by commercial providers.

References

1. Thursby E, Juge N. Introduction to the human gut microbiota. Biochemical Journal. 2017;474(11):1823-1836. doi:10.1042/BCJ20160510
2. World Health Organization (WHO). The human microbiome. WHO Technical Report, 2019. Available at: https://www.who.int
3. Sender R, Fuchs S, Milo R. Revised Estimates for the Number of Human and Bacteria Cells in the Body. Cell. 2016;164(3):337-340. doi:10.1016/j.cell.2016.01.013