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Nutrient Agar Powder - Preparation & Equipment Use

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)


Agar Powder
distilled water
flask or beaker
glass stir rod
lab thermometer
sterile petri dishes (plastic or glass)
flame or boiling mixture
heat resistant hand protection

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


  1. Measure agar and distilled water into clean flask or beaker.

  2. Recipe: Agar + Distilled Water = Yield  
    23 g
    11.5 g
    9.2 g
    4.6 g
    + Distilled Water
    1000 ml
    500 ml
    400 ml
    200 ml
    = Yield
    50 plates
    25 plates
    20 plates
    10 plates
  3. Flame sterilize a clean glass stir rod to stir the medium as it melts.
  4. While wearing heat resistant hand protection, hold the flask or beaker over the flame. Swish or stir the mixture constantly while heating.
  5. Boil the mixture for 1 minute. Remove from heat.
  6. Place a sterile lab thermometer. in the mixture and monitor the temperature until it falls to approximately 45 - 50° C or if a lab thermometer is not available, cover and let stand a few minutes.
  7. Pour enough melted agar into each sterile plastic petri dish to cover the bottom - about 1/8" to 1/4" deep. Replace the lid immediately.
  8. Place agar plates on a counter top to cool and set. Agar medium will set like stiff gelatin at room temperature.
  9. The agar medium is now ready for storage or use.
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.

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. 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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