Question

Agar is used as a culture medium because of it.
A. Provides nourishment
B. Remains as solid jelly
C. Resistant to microorganic breakdown
D. Both B and C

Answer 1

Hint: Agar is a solidifying agent obtained from a specific form of seaweed. It has found major use in microbiology as it is used as a solidifying agent to plate culture medium. The main advantage of agar is that it remains solid even at room temperature.

Complete answer: The culture medium is of several types but it can be broadly classified into a solid, liquid, and semi-solid medium. The solid culture medium is primarily employed for plating, culturing and studying microorganisms, mainly bacteria and fungi. Many solidifying agents are available, such as alginate, gelatin, etc. but only agar is used worldwide because of its many properties. Agar is made up of a mixture of two polysaccharides known as agarose and agaropectin. The peculiar nature of these polysaccharides is that they are resistant to bacterial decomposition and maintain their structure even at room temperature. This is beneficial because we need the target organism to only utilize the provided culture medium and prevent the growth of the unwanted organism. The plates also need to be incubated at room temperature for bacterial growth and we need a medium capable of maintaining its structural integrity even at that temperature.
Thus, the correct answer is option D.

Note: Agar is a mixture of two polysaccharides, agarose, and agaropectin obtained from seaweed. It remains solid even at room temperature, unlike gelatin and alginate. Agar plates are made up of culture medium, which is solidified by the agar, and the microorganism only utilizes the medium and not the agar.