Science Stuff


    Nutrient Agar in plastic petri dishes
    Inoculation Loop
    Note: Keep sterile petri dishes closed until ready to pour agar into them.
    Air-borne contaminants can easily invade an open petri dish.

Background Information:

    Agar is pronounced awger (sounds like fogger without the f). Agar is a 
    gelling agent extracted from red seaweed. Nutrient agar is a commonly
    used food medium for microbial cultures. Nutrient agar contains:
    • beef extract (provides carbohydrates, nitrogen, vitamins, salts) 
    • peptone (helps control pH) 
    • agar (a carbohydrate used as a solidifying agent) 

    • distilled water (an agent for distributing food materials to 
      growing colonies of micro-organisms)
Storage: Stack agar plates upside down in the refrigerator. Do Not Freeze! The purpose of placing the plates upside down is to prevent condensation from dripping down onto the agar surface which could then facilitate movement of organisms between colonies.

Setting Up Your Lab Area:

  1. Set up your lab in an un cluttered area.
  2. Keep air movement to a minimum. Air movement can bring unwanted airborne bacteria or molds into your petri dish when you have it open. Do not have a fan, air conditioner, or heater blowing air near your work area. Even the movement of other people or pets can move the air around you. Hold your breath while inoculating the agar or wear a nose and mouth mask.
  3. Keep your lab notepad close to record procedures and data.

Preparing the Plates

  1. If plates have been refrigerated, set them out and allow them to warm to room temperature.
  2. Sterilize the loop
    • To sterilize the loop, hold the handle with a pot holder and place the tiny looped wire in a flame until it turns bright red
    • Allow the loop to cool for 3 - 5 seconds before touching the collection area.
    • Resterilize the loop after each inoculation.
    • Do not allow the loop to touch any surface other than the collection area and the agar.
  3. Uncover each agar plate just long enough to inoculate the medium. hold the petri dish lid directly over the petri dish (or tilt the lid just enough to allow the loop inside) while inoculating the medium to help prevent contamination from air-borne particles. Do not allow the loop to touch the petri dish.
  4. Do not Dip the loop in the agar, let it glide over the surface.
  5. Make a pattern of inoculation lines (parallel lines, tic-tac-toe, zig-zag, initials, etc.) to help determine that what is growing is what you put there and not an air-borne contaminant.
  6. Place the cover back on the plate immediately.


  1. Place each petri dish inside a zip lock bag to prevent drying out and to control odors.
  2. Turn the plates upside down and put them in a warm place. The ideal temperature for incubation is 32° C or 90° F. Bacterial growth should start to become visible in about 2 -3 days.

Science Fair Tips and Ideas for Kids:

  1. Have someone take lots of pictures - some showing your face, some not showing your face.

  2. Picture ideas:
    • preparing materials for the experiment
    • setting up to do the experiment
    • doing the experiment, all the parts
    • keeping a log of the experiment
    • pictures of petri dishes with colonies
  3. Keep a log of everything you do that relates to your project. Don't forget dates and times.
  4. Label the bottom of each petri dish with a wax marker before inoculating them. Fewer mistakes are made when all the planning and preparation is done before starting the project.
  5. Describe and number colonies. Measure them daily in mm. with a metric ruler. A clear ruler is ideal
  6. Use a hand lens or microscope to help in describing colonies.
  7. List all your helpful resources; books, parents, teachers, web sites, and please don't forget us at Science Stuff.
  8. Use color pictures and illustrations on your display board.
  9. Be neat! A great project should have a great presentation! Measure and lightly mark where each item will go on your display board before you glue or tape it in place. Even with a great project, if your presentation is sloppy, you will lose points.

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