Apply aseptic technique
Work so that nothing from the room gets into your culture and nothing from your culture gets onto you: bench, hands, lids, loops and waste.
- Microbes are already on every surface: Aseptic technique only makes sense once you accept that the bench, your hands and the air already carry organisms that will grow if given agar.
- A contaminated sample looks normal: Nothing warns you at the moment contamination happens. That is why the technique is a habit rather than a reaction.
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
Now use the technique to explain a result that went wrong, and to store and discard a culture correctly.
A student streaks a plate from a single patient sample. After incubation the plate shows two clearly different colony types. Which is the STRONGEST technique-related explanation to rule out before reporting a mixed infection?
Reviewed- A.The plate was labelled on the lid instead of the base
- B.The incubator was set slightly too warm
- C.The same loop was used for a second sample without being replaced
- D.The plate was left in the incubator one hour too long
Show the worked solution ▾
Answer: C. The same loop was used for a second sample without being replaced
- Step 1: Ask what could ADD an organism: A second colony type means a second organism reached the agar.
- Step 2: Find the step that moves organisms: A reused loop carries cells from the first sample into the second, which puts a second organism on the plate.
Why it's right: A reused loop physically carries organisms from one sample into another, which is the one option that ADDS a second organism to the plate. Until that is ruled out, a second colony type is not evidence of a mixed infection.
- A: Labelling the lid risks mixing up which plate is which if lids are swapped, which is a real error, but it does not put a new organism on this agar.
- B: Temperature changes how fast things grow and which organisms are favoured. It cannot introduce a species that was not there.
- D: Extra incubation makes colonies larger and can merge them. It does not create a second, visibly different colony type.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
Plates are placed in the incubator upside down, with the agar on top. What does that prevent?
Reviewed- A.The colonies from sliding off the agar
- B.The agar from drying out completely
- C.The lid from falling off during handling
- D.Condensation from dripping onto the agar and smearing the colonies together
Show the worked solution ▾
Answer: D. Condensation from dripping onto the agar and smearing the colonies together
- Step 1: Find the water: Warm agar releases moisture, which condenses on the cooler lid.
- Step 2: Follow gravity: Right way up, that water drips down onto the growth surface. Inverted, it collects on the lid below and never reaches the agar.
Why it's right: Warm agar releases moisture that condenses on the lid. Inverted, the lid is underneath, so the condensation cannot fall onto the growth surface and run colonies together, which would destroy the isolation the streak was for.
- A: Colonies grow into the agar surface and do not slide. Agar is a firm gel, not a liquid.
- B: Inverting does not slow drying; a sealed or wrapped plate does. Drying is a storage problem, not an orientation one.
- C: Lids are held by the rim either way up. Inversion is about the water, not about grip.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
Fill these in as you work through the lesson.
- Aseptic technique (keeping unwanted microbes out):
- Contamination (something got in that should not have):
- Sterile (no living organisms at all):
- Pure culture (one species only):
Disinfect the bench you start and you finish, and treat every culture as if it could make you ill.
- Name one step that protects the culture from you, and one that protects you from the culture.
- Your plate grows two different colony types when the sample should have held one. Give two points in the procedure where that could have happened.
- Why is a plate incubated upside down?
You lift a plate lid to streak. Hold it at an angle just above the plate, like a roof, rather than setting it down face-up on the bench. Face-up, the inner surface collects whatever settles out of the air, and you then put that surface back over your agar.