Why is agar not gelatin is most commonly used as a solidfying agent of culture media?
1) Most microbes, especially those of interest to human pathology, like to be grown around human body temperature (37C), at which point gelatin is a liquid. Agar is a solid at this temperature. 2) Even if the microbes are grown at a lower temperature, many of them like to feed on the gelatin, which can liquefy it. Agar, on the other hand, is not eaten by bacteria–only the nutrients mixed with the agar are consumed. 3) Agar does not melt until it is boiled to 100C, and does not solidify until it cools at 50C (unlike most materials, whose melting point and solidifying point are the same–think of water, which melts and freezes at 0C), so heat-sensitive nutrients such as blood can be added when the agar is still liquid but relatively cool, preventing the nutrients from denaturing or becoming otherwise damaged.
