Probiotics mainly inhibit the growth of harmful bacteria through the following mechanisms, which usually work synergistically:
1. Competitive exclusion
Competing for nutrients and space: Probiotics compete with harmful bacteria for binding sites and nutrients (such as carbohydrates and amino acids) on the intestinal mucosa, reducing the chance of harmful bacteria colonizing.
Formation of biofilm: Some probiotics can form a protective biofilm on the intestinal surface to physically block the invasion of harmful bacteria.
2. Change the intestinal environment
Acid production reduces pH value: Probiotics (such as Lactobacillus and Bifidobacterium) ferment to produce short-chain fatty acids (such as lactic acid and acetic acid), which lowers the local pH value and inhibits the growth of alkaline harmful bacteria (such as Salmonella and Escherichia coli).
Regulate oxygen concentration: Some probiotics consume oxygen in the intestines and create an anaerobic environment, which is conducive to the proliferation of anaerobic beneficial bacteria (such as bifidobacteria) and inhibits aerobic pathogenic bacteria.
3. Secrete antibacterial substances
Bacteriocins: Some probiotics secrete bacteriocins (such as nisin), which directly destroy the cell membrane of harmful bacteria or inhibit their protein synthesis.
Hydrogen peroxide: Some Lactobacilli produce hydrogen peroxide, which has an oxidative sterilization effect.
Organic acids: In addition to lowering pH, acetic acid, etc. can directly penetrate the cell membrane of harmful bacteria and interfere with their metabolism.
4. Interference with quorum sensing
Blocking signal transduction: Harmful bacteria coordinate the expression of virulence genes by secreting signaling molecules (quorum sensing). Probiotics can degrade these molecules or secrete competitive analogs to interfere with their communication and reduce virulence.
5. Enhance intestinal barrier function
Promote mucus secretion: stimulate intestinal cells to produce mucin, strengthen the mucus layer, and prevent harmful bacteria from adhering.
Tight junction proteins: Strengthen the tight junctions between intestinal epithelial cells and reduce the penetration of harmful bacteria and toxins (i.e., repair of "leaky gut").
6. Modulate immune response
Activate immune cells: Probiotics regulate immunity through dendritic cells, promote the release of secretory IgA (a type of antibody), and neutralize harmful bacteria.
Inhibit inflammatory pathways: downregulate pro-inflammatory factors (such as TNF-α), reduce inflammation's damage to the intestinal barrier, and indirectly inhibit the growth of harmful bacteria.
Key points in practical application
Strain specificity: The antibacterial mechanisms of different probiotic strains may focus on different aspects (for example, Lactobacilli focus on acid production, and Bifidobacteria focus on immune regulation).
Dosage and survival rate: It is critical that sufficient amounts of viable bacteria reach the intestinal tract, which is affected by gastric acid, storage conditions, etc.
Balance of flora: The long-term effect depends on the overall reconstruction of the intestinal microecology, rather than simply "sterilization".
Give an example
Clostridium difficile infection: Taking Saccharomyces boulardii can reduce its reproduction by secreting proteases to degrade toxins and compete for nutrients.
Candida overgrowth: Lactobacilli inhibit the hyphae formation of Candida albicans by producing acid and competing for adhesion sites.
Things to note
Probiotics are not a panacea, and serious infections need to be treated with medication. When selecting probiotics, you should refer to clinical evidence and select corresponding strains for specific problems (such as Lactobacillus rhamnosus GG for diarrhea, Lactobacillus reuteri to improve the adjuvant treatment of Helicobacter pylori).
