Nutrient Agar Powder -
Preparation & Equipment Use
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)
Note: Keep sterile petri dishes closed until ready to pour agar into them. Air-borne contaminants can easily invade an open petri dish.
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.
- Measure agar and distilled water into clean flask or beaker.
Recipe: Agar + Distilled Water = Yield
|+ Distilled Water
- Flame sterilize a clean glass stir rod to stir the medium as it melts.
- While wearing heat resistant hand protection, hold the flask or beaker
over the flame. Swish or stir the mixture constantly while heating.
- Boil the mixture for 1 minute. Remove from heat.
- 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.
- 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.
- Place agar plates on a counter top to cool and set. Agar medium will
set like stiff gelatin at room temperature.
- The agar medium is now ready for storage or use.
Preparing the Plates
- If plates have been refrigerated, set them out and allow them to
warm to room temperature.
- Sterilize the loop
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.
Do not Dip the loop in the agar, let it glide over the surface.
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.
Place the cover back on the plate immediately.
- 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
- Resterilize the loop after each inoculation.
- Do not allow the loop to touch any surface other than the collection
area and the agar.
- 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:
- Have someone take lots of pictures - some showing your face, some
not showing your face.
Keep a log of everything you do that relates to your project. Don't
forget dates and times.
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.
Describe and number colonies. Measure them daily in mm. with a metric
ruler. A clear ruler is ideal.
Use a hand lens or microscope to help in describing colonies.
List all your helpful resources; books, parents, teachers, web sites,
and please don't forget us at Science Stuff.
Use color pictures and illustrations on your display board.
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.
- 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
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