Which statement best describes how an agar plate is used in microbiology?

• A. To store bacteria cultures in a liquid medium.
• B. To grow bacteria on a solid, gel-like nutrient surface in a petri dish.
• C. To measure the diffusion of solutes in gels.
• D. To observe microbial motility in broth.

Answer: (B)

Growing bacteria on a solid surface is what an agar plate is designed to do. The agar medium is a gel-like nutrient mix that is poured into a petri dish and allowed to solidify. This solid surface lets individual bacterial cells multiply in place to form visible colonies, which makes it possible to isolate pure cultures and study colony characteristics such as shape, size, and color. The plate format keeps microbes in place for easy incubation and observation, unlike liquid media where organisms are suspended and evenly dispersed. While diffusion in gels and motility studies are relevant in other contexts, the standard agar plate is specifically used to grow organisms on a solid surface in a petri dish.