Guide to Agar Plates

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Guide to Agar Plates
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The stuff that makes up the gel in a petri plate almost always consists of water, agar, and a solution of desired nutrients. Agar is composed of about five different polysaccharides. Four of them are anionic (and thus not fit for use in electrophoresis), and the fifth is non-ionic and is called agarose. Only in electrophoresis will you likely ever use purified agarose, which is very expensive. What the desired nutrient solution is depends on what you want the agar preparation to do for you. Hundreds of different agar-nutrient mixes are commercially available in powdered form. These are listed in the Difco Manual.* Usually students are on the pathway seeking instant gratification and thus want a growth medium upon which their bacteria will go as fast as possible. This is usually in the form of powder called nutrient agar. But several other types will be found on this webpage.

  1. Prehistoric microbiology. This is for those of you don't have available the fundamental ingredients - agar, glass or plastic petri plates, or nutrient solution. Remember, bacteriologists before Dr. Petri invented his, didn't have them either and had to make do with other things.
  2. Nutrient Agar plates. These plates will grow the largest number of different types of microbes - fungi and bacteria. Yet, not all bacteria can grow on these. Some find it too rich, and others find it deficient. The nutrient in this is beef broth, and some extracts from yeast.
  3. Tryptone plates. This is nice, clear yellowish agar that lacks everything but amino acids. It is made of a trypsin digest of milk protein (casein). You may have an experient in which you are testing vitamins, or various sugars. Those then can be added in various amounts.
  4. MacConkey plates. This is an agar upon which only Gram-negative bacteria can grow. What is more is that E.coli will grow into red colonies, as there is a pH indicator present. It should be mentioned that MacConkey agar powder comes in two versions: one with the sugar lactose in it, and one without any added sugars. Since E.coli ferments sugars to acids (thus the red color), one can add one of the many different kinds of sugars** to this sugar-free MacConkey agar and see if red colonies develop. If you get red colonies, you know the E.coli strain you are using can use that sugar.

* A Difco Manual should be found in any bacteriological laboratory. Pre-university students should be able to find one in their local hospital's bacteriology lab. Just ask the technician.

** Just to name a few: sucrose (table sugar), maltose, glucose, fructose, ribose, xylose, arabinose, galactose, mannose, sorbitol, mannitol....

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