Normal flora defend the host by what mechanism?

• A. Competing with pathogens for attachment sites and nutrients.
• B. Producing toxins that kill the host.
• C. Increasing susceptibility to infection.
• D. They have no defensive role.

Answer: (A)

Normal flora defend the host primarily through competition for attachment sites and nutrients on skin and mucosal surfaces. By occupying binding sites and consuming available nutrients, they limit the ability of pathogens to attach, grow, and establish an infection—this is known as colonization resistance. Some resident microbes also produce substances that inhibit invaders, but the main protective mechanism being tested is this competitive exclusion. The other options aren’t accurate: normal flora don’t typically kill the host, and they do have a defensive role through occupying niches and limiting pathogen access.