Read a plate without opening it
Two days in an incubator changes what a plate is. Learn why the reading happens through the lid and why the materials list gives you a magnifying glass instead of anything that opens it.
- What is on a plate changes while it incubates: You streak a thin film of liquid culture. Two days later the same plate carries visible colonies, which is many times the material you put on it.
- You can see through a lid: Everything a colony description records is visible from outside. Opening a plate buys you nothing you did not already have.
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
Now say what actually changed over 48 hours, and rule out the shortcut that involves your nose.
After 48 hours in the incubator, why is opening the plate a bigger deal than it was on the day you streaked it?
Reviewed- A.It is not a bigger deal, because it is exactly the same plate it was on Monday
- B.The lid seals itself to the base over 48 hours and tears the agar
- C.The agar has dried out and will crack as soon as the lid is lifted
- D.There is a large population on it now that was not there on Monday
Show the worked solution ▾
Answer: D. There is a large population on it now that was not there on Monday
- Step 1: Compare what was on the plate then and now: A thin invisible smear, against colonies you can see across the room.
- Step 2: Ask what an open lid releases: Whatever is on the plate, and there is much more of it now.
Why it's right: The plate has spent two days doing exactly what it was set up to do. What comes out of the incubator carries far more material than what went in, so the same action has a much larger consequence.
- A: It is not the same plate in the way that matters. The whole point of incubation is that something changed on it.
- B: Lids sit loose on the base and are meant to be lifted. Nothing seals over two days.
- C: Plates do dry slowly, and a cracking lid is not what the instruction not to open them is protecting against.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
A group wants to smell their plate to help work out which organism grew. What is wrong with that?
Reviewed- A.Smell is too faint to be useful on a plate that has grown for only 48 hours
- B.It needs the lid off, and you never draw air from a culture toward your face
- C.Nothing is wrong, since describing the smell is a recognised way to characterise a culture
- D.It would contaminate the plate for whoever has to read it after you
Show the worked solution ▾
Answer: B. It needs the lid off, and you never draw air from a culture toward your face
- Step 1: Ask what smelling requires: The lid off and your face close to the agar.
- Step 2: Ask what you are then breathing: Air that has just come off a two day old culture.
Why it's right: Smelling means opening the plate and then deliberately pulling air from it into your airway. That is the one direction you spend the whole activity trying to avoid, so there is no careful way to do it.
- A: Some cultures do produce noticeable odours, which is what makes the idea tempting rather than what makes it wrong.
- C: Trained microbiologists do sometimes note odour, and they do it under controlled conditions that a classroom bench does not provide.
- D: Contamination of the plate is a real cost of opening it, and it is the smaller of the two problems here.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
- A student who wants more detail reaches for the magnifying glass rather than the lid.
Fill these in as you work through the lesson.
- Colony (a visible pile of cells grown from one starting cell):
- Incubate (hold warm so growth happens):
- Closed lid (the reading condition, not a suggestion):
- Aerosol (fine droplets released when a plate is disturbed):
Take the plates out of the incubator but do not them. Count, measure and sketch through the , using the magnifying the materials list gives you. Never draw air from a plate toward your .
- Say what changed on the plate during the 48 hours.
- List what you can record without lifting the lid.
- Explain what is wrong with smelling a plate.
Your plate comes out with a dozen separate colonies. You hold the magnifying glass over the closed lid, count them, measure the widest one against the ruler through the plastic, and sketch the shapes in coloured pencil.