What is Agar medium used for?

Agar medium is a gelatinous substance derived from seaweed that is commonly used in microbiology as a solid growth medium for culturing bacteria and other microorganisms. It provides a stable surface for colonies to grow while offering nutrients essential for the growth of heterotrophic organisms, which obtain their food by consuming organic matter.

In this context, using agar medium for heterotrophic plate count is particularly relevant because this method involves growing and counting organisms that feed on organic material in water samples. The agar allows for the selective growth of these heterotrophic bacteria, facilitating a reliable estimation of microbial populations in various samples, including drinking water and wastewater. The ability to visually count the colonies formed on agar plates makes it an effective choice for this purpose.

The other options do not apply to the primary use of agar medium. Biochemical oxygen demand involves quantifying the amount of oxygen consumed by microorganisms in a sample, which typically requires liquid media rather than solid growth media like agar. Gas chromatography is a technique for analyzing compounds in a gas phase rather than culturing microorganisms. pH measurement involves determining the acidity or alkalinity of a solution and does not involve any growth medium at all. This distinction underscores why agar is specifically suited for heterotrophic plate counts.