Presence of human microbiota (normal flora) defends the host by which mechanism?

• A. Coating mucosal surfaces
• B. Maintaining an acid environment
• C. Competing with potential pathogens
• D. Keeping phagocytes in the area

Answer: (C)

Normal flora defend the host primarily through colonization resistance: they outcompete potential pathogens for attachment sites and nutrients on mucosal surfaces, making it harder for invaders to establish themselves. This competitive exclusion means pathogens have fewer resources and fewer places to adhere, reducing the chance of infection. In addition, resident microbes can produce antimicrobial substances and alter the local environment in ways that further inhibit pathogens. While coating surfaces or acidifying the surroundings can help in some sites, the central protective mechanism is the competition for space and nutrients that keeps harmful invaders at bay.