Why do most microorganisms fail to use agar as a carbon source?

Answer 1

Because the agar remains a solid anywhere below 42 degrees Celsius. A typical environment is never going to raise above 42 degrees therefore it will always remain solid even in the human body (37 degrees Celsius). Having that said, most microorganisms cannot digest agar unless it is liquid.

Answer 2

Most microorganisms fail to use agar as a carbon source because they lack the necessary enzymes to break down the complex polysaccharides present in agar into simpler sugars that can be used for energy. Agar is a complex mixture of agarose and agaropectin, which are difficult to metabolize for many microorganisms.