How do Probiotics work?

Probiotics are live microorganisms that are supposed to have health benefits when consumed or applied to the body in adequate amounts. Scientific evidence suggests that specific strains of probiotic microorganisms confer health benefits on the host and are safe for human use.  Healthy individuals are using probiotics with the intended benefits of promoting gastrointestinal health and immunity and preventing urogenital infections, allergies, and even cancers.

Metchnikoff in the early 1900s suggested that beneficial bacteria could be administered to replace harmful microbes with useful ones.

Lactobacillus, Bifidobacterium, Escherichia, Enterococcus, and Saccharomyces are some of the most widely used probiotic microorganisms. Find more about probiotic microorganisms in this blog post.

There are several possible explanations of how probiotic microorganisms displace pathogens and enhance the development and stability of the microbial balance in the large intestine.

Probiotic mechanisms in the human gastrointestinal tract


Competition with pathogens for nutrients and adhesion sites

The beneficial bacteria prevent the colonization of pathogenic microorganisms by competitive inhibition for microbial adhesion sites. For example, Lactobacillus casei and Lactobacillus plantarum competitively inhibit the attachment of enteropathogenic Escherichia coli.

Inactivation of pathogenic bacterial toxins or metabolites

Some members of the intestinal microbiota influence the onset of carcinogenesis by producing enzymes, such as glycosidase, azoreductase, nitroreductase, and β-glucoronidase, which transform pre-carcinogens into active carcinogens. Human studies have shown that the ingestion of L. acidophilus or Lactobacillus casei results in reduced levels of the above enzymes in the stools of volunteers. It’s not yet confirmed that consumption of these probiotic microorganisms actually reduces the incidence of cancer.

Production of substances that inhibit pathogen growth

Probiotic microorganisms produce organic acids, fatty free acids, ammonia, hydrogen peroxide, and bacteriocins, all of which have antimicrobial activity. For example, L. casei produces a low-molecular-weight antibacterial substance that is inhibitory to both Gram-positive and Gram-negative enteric bacteria. Probiotics suppress the growth of pathogens also by inducing the host’s production of β-defensin and IgA.

Stimulation of nonspecific immunity

Oral administration of different probiotics have shown adjuvant-like effects on intestinal and systemic immunity. Several studies demonstrates enhanced phagocytic activity against intracellular pathogens and enhanced immunoglobulin A (IgA) responses against pathogenic viruses. Probiotics may be able to fortify the intestinal barrier by maintaining tight junctions and inducing mucin production.

In-vitro and in-vivo studies suggest that probiotics may modulate the immune response by promoting endogenous host defense systems. Probiotic bacteria can modify various immune parameters, including humoral, cellular, and nonspecific immunity

  1. Enhance the activity of natural killer cells in the elderly
  2. Induces mucus production
  3. Activate macrophage by lactobacilli signaling
  4. Stimulate secretory IgA and neutrophils
  5. Inhibit release of inflammatory cytokines, etc.

Observed Health Benefits

Prevention of Dirrhea

Consumption of probiotics microorganisms such as Lactobacillus rhamnosus GG, Bifidobacterium lactis BB-12 and Lactobacillus reuteri SD2222 has shown a significant reduction in incidence or duration of certain diarrheal illnesses such as rotavirus diarrhea. Prophylactic use in hospitalized children has also resulted in a reduction in the risk of acquiring nosocomial diarrhea.
Mechanisms that have been proposed for this protective effect include competitive blockage of receptor sites (resulting in inhibition of virus adherence and invasion), enhancement of the host immune system, and production of substances that inactivate virus particles.

Reductions of Atopic Diseases (Allergy) in Children

A clinical study showed that giving L. rhamnosus GG to pregnant women for 4 weeks before delivery, and then to the newborn infant, caused a significant reduction in the occurrence of early atopic disease. Though the precise mechanism is not yet known, it is suggested that promotion of gut barrier function via the restoration of normal microbiota, enhancement of gut-specific IgA response, and production of certain cytokines by these organisms might have played the role for this.

Decrease in Genitourinary Tract Infections

Absence of vaginal lactobacilli increases the risk of sexually transmitted infection and carriage of Neisseria gonorrhoeae and Chlamydia trachomatis. Different studies show that oral intake of probiotic lactobacilli reduces the risk of urinary tract infection, bacterial vaginosis, and candidiasis. Though the exact mechanism is not known, it has been suggested that ingested lactobacilli could ascend from the rectal skin to the vagina, or alternatively, prevent the ascent of pathogens.

Side Effects

People are using probiotic microorganisms from ancient times to prepare various fermented foods and there are no reports of adverse side effects. Probiotics are generally assumed to be safe in healthy people, though in some people they may cause gas.

There have been some reports of probiotics causing problems among vulnerable patients. Saccharomyces fungemia secondary to the use of the probiotic has been described for critically ill patients. To ensure patient safety, proper assessment of risk versus benefit of probiotics administration must be made and probiotics should be properly handled during administration.

References and Further Readings

  1. Probiotics: facts and myths. Senok, A.C. et al. Clinical Microbiology and Infection, Volume 11, Issue 12, 958 – 966

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

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