This sounds like a really great science project.
It’s not easy to answer your question. Identifying bacteria can be very time consuming, and probably beyond the scope of your science experiment. Here are some suggestions for a preliminary classification of the bacteria growing on the plates.
The nutrient agar is a general all purpose growth medium that would allow the growth of most aerobic bacteria. If you are testing the same samples on all three media, I would expect to find more colonies on this medium compared to the other two media.
The other two media are selective and do not allow the growth of all bacteria. Mannitol salt agar contains a high concentration of sodium chloride and mannitol, a sugar alcohol, which cannot be used as a carbon source by all bacteria. This medium inhibits Gram-negative bacteria, and normally, only pathogenic Staphylococci will grow and ferment the mannitol and turn the phenol red indicator from red to yellow.
The MacConkey agar with MUG contains bile salts and crystal violet, which inhibits the growth of Gram-positive bacteria. And it contains the sugar, lactose, which helps to distinguish between major groups of Gram-negative bacteria and is used to help identify enterohaemorrhagic E. coli from food and other samples.
For identification, I recommend that you describe the colonies on the plates. Here is information from the Science Buddies website on how to interpret colonies growing on agar plates:http://www.sciencebuddies.org/science-f ... ates.shtml
Since you will be working with unknown bacteria, you should be doing this experiment in a microbiology laboratory, and if so, then you might be able to do a Gram stain to confirm that the bacteria are Gram-negative or Gram-positive, and you could include a microscopic description to classify the shape of the bacteria (bacillus or coccus).http://www.google.com/search?q=shape+of ... afe=active
Please do review the information for doing a project involving potentially hazardous biological agents to make sure your project complies with all of the rules for doing a project like this.http://www.sciencebuddies.org/science-f ... ents.shtml
Please post again if you have any other questions.