Re: How can I tell what kind of bacteria is growing in my agar dishes?

Hi Cristina -
Congratulations on growing some bacteria!

Observations of colonies can help deterine what kind or organism it is, but often is insufficient to determine the exact species. In fact in clinical microbiology labs we use the appearance of microbial colonies with biochemical tests, and in some cases genetic tests of their DNA, to help us determine what cells are in a given colony.

Depending on the plates you used you may be able narrow what kinds of microbes are present. In general agar media supports the growth of bacteria (cells without a nucleus) and some species of yeasts and molds (things with a nucleus that can make fuzzy bread). You can't culture viruses or most parasites on Petrie plates.

First you should find out the kinds of agar plate. If you used "blood agar" plates - nutrient agar with 5-10% sheep's blood or horse blood, you can determine if the colonies hemolyze the red cells in the media. Hemolysis is the process of breaking apart red blood cells, and only certain bacteria species do it. There are two types of hemolysis - alpha hemolysis which produces a greenish tinge around the colonies, and beta hemolysis which represents total breakdown of the red cells. You will see complete clearing around the colonies if they are beta-hemolytic. When determining hemolysis always hold the plate towards a light source and look only around a single, isolated colony, not in a crowded area of the plate. If you see clearing there is beta-hemolysis.

Beyond that look at the shape or morphology of the colony. Here are some characteristics to consider:

• Is it round or with jagged edges/feet? Many spcies of yeast produce "feet" or small projections extending from the colony's edge.
• Does it look dry or moist and mucoid?
• Is it pigmented? Some common colors you might see include: white, cream-colored, yellow, pink, green and bright red.
• If it's fuzzy, like bread mold, suspect that you do indeed have a fungus. I strongly recommend that you seal the plate with tape and dispose of it as you don't want to be exposed to high concentrations of mold spores.

• Staphylococcus species: White to creamy colored colonies. Colonies of Staphylococcus aureus are beta-hemoytic, and potentially disease-causing, so always be careful how you handle your plates. Use gloves and wash your hands thoroughly with soap and warm water when finished.
• Streptococcus species - often very small colonies that are whitish in color. Some produce a large zone of beta-hemolysis around the colony.
• Enterics: these include species such as Escherichia coli which lives happily in your intestines, and many disease-causing species such as Salmonella, Yersinia. Colonies are often whitish or translucent, appear mucoid, and frequently have a somewhat foul aroma. Believe it or not, microbiologists often use their noses to help identify bacteria - the bad smell is one indication something might be an enteric. However never open a plate to smell anything fuzzy or wrinkly & dry - they might be species that produce spores, something of which you don't want a nose-full!. If you do smell plates, hold them at least 4-6 inches from your face.
• Pseudomonads: these are common organisms found in the environment, particularly in moist areas. I guarantee that you have millions living on any veggies in your refrigerator, and possibly some in your mouth or any slime present in the bathroom. Psedomonads often appear similar to enterics on blood agar; many are hemolytic. One common species called Pseudomonas aeruginosa produces a very powerful grape-like or "corn-tortilla" smell (not all bacteria smell bad!).

For more information, try your local public library. Most microbiology textbooks have color plates that will show different colony morphologies. Some textbooks to try include the Manual of Clinical Microbiology and Diagnostic Microbiology. Ask your librarian to help you locate either text within your library or through inter-library loan with a university library.

Good luck with your project!

Lynn Bry, Moderator MadSci Network