Re: Are there vegetable and meat based agars made for culturing E. coli?

Date: Fri Oct 29 19:42:26 1999
Posted By: Lynn Bry, MadSci Admin
Area of science: Microbiology
ID: 940196919.Mi

Message:

Hi Whitney -

Your idea of looking at whether the bacterium Escherichia coli prefers meats or veggies for growth is an interesting one. In fact, a few scientists have won Nobel Prizes by studying what bugs like to eat and how they do it. Let me make a few suggestions, however..

When attacking a problem such as the growth of a microbe on different "foodstuffs," it is best to be as specific as possible. "Meats" and "vegetables" vary significantly with regards to the amount of nutrients in them, nutrients including proteins, carbohydrates, fats, vitamins and minerals. However, many factors vary between the two groups, let alone among members of a group, such as chicken meat, versus beef or pork.

I'd recommend comparing bacterial growth on different kinds of microbiological media - agar plates may be the easiest with which to work. The Reagents/Suppliers section of the Try the links in the MadSci Library for more information on Microbiology. that sell agar media. Just so you know, agar is a complex carbohydrate that comes from seaweed. Most species of bacteria cannot digest it, so it serves as a good medium and support for holding water and nutrients that the bugs can digest.

In addition to the agar base, minimal media, or a media that contains only basic requirements for many types of bacteria, has some salts, a few vitamins and minerals, and:

a source of carbon such as the sugar glucose. The carbon source is used by the microbe to make energy, just like we do.
A source of nitrogen such as ammonium chloride (NH₄Cl). All organisms need nitrogen. The amino acids which make up the proteins in our body contain nitrogen.

Many places sell minimal media already poured into Petrie plates. The companies that do should also list what they put into their media.

You can also purchase enriched media which contains other things in addition to the minimal base. Some things added to enriched media include:

Yeast extract: A breakdown of components from yeast cells. The extract is rich in amino acids, vitamins and minerals from yeast cells.
Peptone: Another additive rich in amino acids.

In addition, you can add specific nutrients to minimal agar to assess their effect on bacterial growth. For instance, you could obtain a solution of a particular amino acid, sugar, or vitamin. In microbiology labs we tend to place a small amount of the liquid, usually 0.1 milliliters on top of the agar plate, then spread it around with what looks like a glass hockey stick (which has been sterilized). If you don't have such available, you can place a drop of liquid on your plates, and let it soak in for 24 hours or more so it diffuses into the agar. Keep in mind that you would need to do this under *sterile conditions* so you don't contaminate your plates with bacteria from the surrounding environment.

I hope I have provided you with some background that will now let you pursue aspects of factors that affect bacterial growth. For starters, you could consider comparing the amount of a growth of a given number of bacteria on minimal plates versus on enriched media. You comparison could include the number of colonies that grow as well as how "big" they get over 1, 2 and 3 days - measure their size by measuring the diameter of a few colonies on each plate.

Lynn Bry, MD/PhD
Dept. Clinical Pathology
Brigham & Women's Hospital
Harvard Medical School
Boston, MA 02115

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