Probiotics and Prebiotics

Mechanism

Mechanisms of cellular action in the intestinal epithelium

Probiotic microorganisms (mainly from the genera Lactobacillus, Bifidobacterium and the yeast *Saccharomyces boulardii*) exert their effects through three integrated mechanisms:

1. Competitive Exclusion by Binding Sites: They transiently colonize the mucus layer and selectively adhere to the cell surface receptors of enterocytes, physically preventing the anchoring and colonization of invasive pathogens (such as *Clostridioides difficile* or enteropathogenic strains of *E. coli*).
2. Synthesis of Bacteriocins and Organic Acids: The consumed prebiotics (oligofructose, inulin) are fermented to release Short Chain Fatty Acids (SCFAs), mainly acetate, propionate and butyrate. Butyrate acts as the main energy substrate for colonocytes and lowers intraluminal pH to an acidic environment hostile to pathogens. Likewise, they synthesize direct antimicrobial compounds called bacteriocins.
3. Immune Regulation of the Lamina Propria: They stimulate the differentiation of antigen-presenting cells that induce the synthesis of regulatory T lymphocytes (T_reg), increasing the secretion of anti-inflammatory cytokines such as interleukin 10 (IL-10) and promoting the active synthesis of secretory immunoglobulin A (IgAs) in the mucosa.

Pharmacokinetics

Pharmacokinetics

• Transit and Survival: Oral probiotics must be formulated in capsules resistant to the stomach acid environment to ensure their survival when passing through the stomach and the subsequent action of duodenal bile salts.
• Colonization: They do not permanently colonize the adult intestinal microbiota; Its complete clearance from feces typically occurs between 5 and 10 days after stopping its intake, so its administration must be continuous during therapy.

Indicators and dose

Clinical Indications Based on Evidence

• Prevention of Antibiotic Associated Diarrhea (AAD): The use of broad-spectrum antibiotics alters the protective native microbiota. Co-administration of strains such as *Saccharomyces boulardii* or *Lactobacillus rhamnosus GG* significantly reduces the incidence of AAD. Its administration should be spaced at least 2 hours from the dose of bacterial antibiotic.
• Adjunctive Treatment in Irritable Bowel Syndrome (IBS): Improvement of symptoms of bloating and abdominal pain.

Security

Risk of Sepsis or Bacteremia due to Translocation in Critical Patients

Despite being considered safe in the general population, the administration of probiotics containing live strains of bacteria or yeast is strictly contraindicated in patients admitted to intensive care units with severe active immunosuppression, presence of central venous catheters or short bowel syndrome. In these patients, the intestinal barrier may be compromised, allowing the direct translocation of probiotic strains into the general circulation with the consequent development of bacteremia, fungemia or septic shock that is difficult to control.