Probiotics are living microorganisms. Consumers eat them in dairy products and value them for their health benefits. A companion article (see The Probiotic Nature of Normal Microflora on Page 41) discusses the role of probiotics and normal microflora in maintaining homeostasis in human intestines and on human skin and mucosa. This column discusses mechanisms by which probiotics provide therapeutic activity in the intestines, where most of the research has been done, and on the skin, where more research is needed.
One goal of this column is to point out that many of the identified actions of probiotics are already being performed by the normal microflora of skin and mucous membranes. A second goal is to suggest that formulators recognize opportunities for making products that modulate the normal microflora. This will help maintain the health and/or improve the appearance of skin and mucous membranes.
Figure 1a illustrates schematically how bacteria or toxins may interact with a cell. By attaching to a cell membrane receptor, they initiate a sequence of intracellular reactions that result in the release of nuclear factor kappa-beta (NF-κß) from its inactive form with its inhibitor (NF-κß-I) to produce the active molecule that migrates to the nucleus and directs synthesis of inflammatory cytokines (IL-1, TNFα, IL-8) and initiates apoptosis.
Figure 1b shows probiotic bacteria interacting with the cell to prevent the release of inflammatory cytokines and to down-regulate the inflammatory process by directing the cell to produce cytokines that decrease inflammation.
Probiotics in the Intestines
It must be realized that the mechanisms described here require a large number of probiotic microorganisms to be effective.
Blocking attachment sites and competing for food: The first step in infection is the attachment of a microorganism to a target cell. It has been believed for several decades that probiotics protect intestinal epithelial cells by competing with pathogens for mucosal adherence sites. Adherence by probiotics blocks pathogens from attaching to their target cells so they are stopped before they can initiate an infection.
Also, probiotic microorganisms are believed to compete with other microorganisms for nutrients in the gastrointestinal tract. This competition limits the ability of pathogens to grow.
Producing antimicrobial peptides: Probiotics may also stimulate epithelial cells to produce antimicrobial peptides such as defensins. The defensins are cationic peptides that are effective against Gram-positive and Gram-negative bacteria. Wehkamp et al.1 found that probiotic bacteria strongly induce the expression of human β-defensin-2 (hBD-2) in Caco-2 intestinal epithelial cells in a time- and dose-dependent manner. They found that treatment with a NF-κß inhibitor blocked hBD-2 induction.
Such stimulation of innate defenses through up-regulation of inducible antimicrobial peptides enhances the mucosal barrier to potentially harmful bacteria in the GI tract.
Cathelicidins are small cationic peptides that have broad specificity against bacteria, fungi, parasites and viruses. They are thought to kill bacteria by disruption of cell membrane integrity and/or block protein synthesis.
Nizet and Gallo2 reviewed cathelicidin antimicrobial peptides and their role in defense against invasive bacterial infection. Humans have a single human cathelicidin; it is known as LL-37 because it begins with two leucines and has 37 amino acid residues. Nizet and Gallo indicated that disregulated cathelicidin expression may contribute to chronic inflammatory disorders. Short-chain fatty acids have been reported to up-regulate colonocyte production of cathelicidin LL-37,3 so it is possible that lactic acid production by probiotic bacteria in the intestine may up-regulate production of cathelicidins by intestinal epithelial cells.
Modulating cellular physiology: Interaction of bacteria with epithelial cells can modulate cellular physiology. Drakes et al.4 studied probiotic modulation of dendritic cells and found that there was substantial enhancement of interleukin-10 (IL-10) release after three days of stimulation with bacterial probiotic cocktail VSL#3. IL-10 is a cytokine that down-regulates inflammatory responses, so this report helps to explain how probiotics work in situ to promote immunological quiescence.
Probiotics may inhibit phosphorylation of I-κß, the enzyme that releases NF-κß. Phosphorylation is needed to convert I-κß to the active form of the enzyme so it can cleave NF-κß-I and release NF-κß. Inhibition of the activation of this enzyme by probiotics prevents the production and release of inflammatory cytokines (IL-1, TNFα, IL-8), and it may induce the formation and release of non-inflammatory cytokines (e.g., IL-10) and heat shock proteins (hsps) that down-regulate cellular responses and reduce apoptosis.