Probiotics assimilated in many lactic acid bacteria, they attach themselfes on smooth surfaces of gut and others and decrease pH value to acidic levels where pathogenic bacteria can not grow.They balance and or inhibit pathogens by their produced metabolites such as actaldhyde, ethanol , diacetyl, CO2, classical antibiotics and bacteriocins or antimicrobial proteins.

http://www.sciencedirect.com/science/article/pii/S1319016413000819

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Article Role of Probiotics in health improvement, infection control ...