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Why Can't Agar Grow All Microorganisms?

Agar, a derivative of red algae, mixes with water to form a translucent springy mass that bacteria cannot readily dissolve. Because it stays semisolid and resists bacterial decomposition, agar is used as a foundation for microbial cultures. However, as bacteria cannot digest agar, researchers must introduce growth media into the agar gel as they prepare Petri dishes for cultures. By itself, agar plates do not support a wide variety of bacterial growth.

Extremophiles
- The bacteria and archaea that scientists describe as extremophiles -- organisms that thrive at extreme temperatures, pressures and acidity levels that other organisms would find lethal -- have unusual growth requirements. Agar melts at 185 degrees Fahrenheit, a temperature at which some extreme thermophiles must live; liquid agar would serve no purpose as a substrate for these organisms. Agar does not tolerate the extreme acidity of acidophiles, either; the porous gel medium will not set in the highly acidic environment that some species prefer.
Restrictive Growth Media
- Scientists use a range of specific growth media in agar gel to support certain types of colonies. These restrictive plates permit only particular species to grow on a specialized food source, telling researchers at a glance whether a certain strain is present or absent in a culture. Such plates can test for highly specific pathogens depending on the nature of the growth medium. If nothing grows on the plate after swabbing the agar's surface with the culture, then that species does not exist within the culture.
Antimicrobial Agents
- Researchers mix antimicrobial agents into nutrient-rich agar plates to gauge the antimicrobial solutions' efficacy. An agar plate that fails to grow bacterial cultures despite offering an inviting nutrient supply and environment contains an effective killer of bacteria. Alexander Fleming noted the apparent antimicrobial effect of bread containing a type of Penicillium mold; although Fleming did not realize it at the time, his accidental discovery and subsequent observations led to the eventual creation of penicillin by Howard Florey and Ernst Chain.
Pure Agar
- Agar alone supports few species of microbes. Most bacteria cannot eat it without a supplemental nutrient broth mixed into the gel during plating. Agar's resistance to microbial contamination makes it ideal for incubating highly specific strains of bacteria. Its predecessor in the lab plate, gelatin, readily hosts a number of microbes; this generality made it difficult for scientists to isolate strains. As agar allows researchers to select for species, it has become the preferred medium in Petri dishes.

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