Antimicrobial Assays for Probiotics
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Overview
An important function of probiotics is to protect the host gastrointestinal tract from pathogens. Inhibition of adherence of pathogenic bacteria to their receptors reduces intestinal colonization and thus alters the pathogenic process. The ability to inhibit the pathogen growth can be evaluated by coculture experiments as well as by antimicrobial assays with cell-free culture supernatant, while interference with the adhesion to/invasion of intestinal cells can be studied by adhesion and invasion inhibition assays. The antimicrobial substances mainly produced by lactic acid bacteria (LAB) can be divided into low molecular weight and high molecular weight compounds. The first group also named the non-bacteriocin group, includes organic acids, hydrogen peroxide, diacetyl, acetaldehyde, acetoin, and carbon dioxide. The second group of antimicrobial compounds produced by LAB, i.e., high molecular mass substances, includes proteinaceous molecules referred to as bacteriocins.
Mechanisms of Antimicrobial Activity of Probiotic
The antimicrobial activity of probiotics is strain specific, but the mechanisms of interaction between probiotics and pathogenic microorganisms can be divided into direct and indirect mechanisms.
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Competitive exclusion through competition for adhesion sites.
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Competition for substrates and limited resources.
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Synthesis of antimicrobial substances.
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Inhibition of toxin expression in pathogens.
Fig.1 Mechanisms of action of probiotics. (Silva, 2020)
Antimicrobial Activities of LAB-derived Exopolysaccharide (EPS)
EPS can promote the accumulation of secondary metabolites in the culture medium and may adversely affect both Gram-positive and negative pathogens.
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Antifungal Activity
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Anti-biofilm Activity
EPS may interfere with biofilm activity by modifying the bacterial cell surface, hindering the initial attachment of bacterial cells to the surface, or acting as signaling molecules to down-regulate the expression of genes involved in biofilm formation.
Antimicrobial Assays at Creative Biolabs
A variety of methods can be used to evaluate or screen extracts or pure compounds for in vitro antimicrobial activity. The best-known and most basic methods are disk diffusion and broth or agar dilution. Other methods were specifically used for antifungal testing.
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Reference
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Silva, D.R.; et al. Probiotics as an alternative antimicrobial therapy: Current reality and future directions. Journal of Functional Foods. 2020, 73: 104080.
For Research Use Only. Not intended for use in food manufacturing or medical procedures (diagnostics or therapeutics). Do Not Use in Humans.
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