MadSci Network: Microbiology

Re: I am trying to see which food will grow bacteria best, bread, raw meat, or

Area: Microbiology
Posted By: Prudence Risley, Collective Enigma Elucidator
Date: Mon Jan 27 00:23:44 1997

Well Melissa -- it sounds (smells?) like all your experiments have been successful :)

'Grow bacteria,' based on your description, is fairly non-descript. You have left different foods open to colonization from anything that might be in the air, or already on your pieces of food. Different microbes like to colonize different things - so I'll guess you probably have very different collections of organisms living on your different foods. Many things determine what micrboes will colonize any 'open habitat' - termperature, pH, and the availability of things the microbes can eat. The bread is rich in *carbohydrates* (like sugars such as sucrose, or table sugar). Meat is rich in *protein*, while the apple has some of both, and a lot more *acid* than the bread or the meat.

The 'mold' is a fungus. You probably also have many species of bacteria and yeasts growing on the food. To observe the mold under the microscope, take some of the strands, called hyphae, place them on a slide and cover it with a glass coverslip - you should be able to observe their structure under 40-60X objectives (I'm guessing you have 40X & 60X objectives, not 400X and 600X!). Incidentally, since you don't know *what* sorts of critters you have grown, I'd recommend wearing *gloves* when you handle the specimens, and be certain to carefully wash your hands before and after working with them.

The "slime" on the apple and peice of meat will be more difficult to visualize. Yeast cells are fairly large and can be seen at 40-60X magnification. However, it helps a lot if you can stain them with a dye to make them stand out. I'd recommend asking your teacher if your school has the dyes needed to perform a Gram stain. They can be purchased pre-made from places such as Carolina Biological Supply. It's easy to do, but you need the necessary reagents. The bacteria will be nearly impossible to see at 40-60X unless you have stained them.

The reagents in a Gram stain include

	Crystal Violet
	Lugol's Iodine (mostly a solution of potassium iodide - KI).
	Safrannin, a red dye.
	70% ethanol solution for washing and rinsing the slides.

PROCEDURE: **Wear gloves; Have your teacher help you.**
	Collect some of your 'slime' on the end of a toothpick, e.g.
	Mix it with ~ 5mL of water, preferably sterile, but you can use
	 tapwater if you have nothing else.
	Place a drop of this solution on a glass slide. Let it dry at room
	Light a *Bunsen Burner* in your school lab - have your teacher
	   help you. Run the glass slide over the tip of the flame a few times.
	   This action heat fixes any organisms to the surface of the
	Let the slide cool.
	Place it over some metal container, or other surface that you don't mind
	   spilling indelible dyes on.
	Add the crystal violet solution - cover the entire slide with it.
	Let it sit for 30 seconds - minute.
	Add the Lugol's Iodine, to the crystal violet on the slide, swirl a little
	    with your had to mix.
	Let sit 30 sec - minute.
	Decant the solution into your container.
	**Wash with the 70% ethanol to get rid of most of the dye.**
	Rinse with water, preferably from a chemistry squirtbottle.
	Cover the slide with the safrannin dye (this is a 'counterstain').
	Let sit 30sec - minute.
	Decant and rinse with water.
	Gently blot the slide and let it dry.
You can then look at it under the microscope. If it worked you should see a lot of purple and red spots. Some bacteria form a special complex with the crystal violet dye - they trun purple and are called Gram positive bacteria. Everything else counterstains with the red safrannin dye, called Gram negative. It's best to view them under oil immersion which is 100X magnification. Ask your teacher if any microscopes at school have lenses for this.

I'd strongly recommend talking to your teacher about the dyes. If the school doesn't have them, they may be able to locate some.

As to whether the bacteria should move -

Some species of bacteria are highly motile while other are not. It depends on what colonized your foods. For more micro information, I'd recommend visiting the Microbe Zoo


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