Selective and differential media
Tell what a growth medium keeps out from what it makes visible, and read a plate knowing which organisms were never given a chance to appear on it.
- A medium is a set of conditions: Everything here follows from seeing a plate as a set of conditions rather than as neutral food, because conditions include and exclude.
- Colour on a plate can be a chemical result: Reading a differential plate means knowing that a colour was produced by the colony's own chemistry, not painted on by anybody.
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
Read a plate as a filter as well as a display, and keep the two readings apart when you write.
Two colonies on the same differential plate are the same shade. A student concludes the two colonies are the same species. Where does that reasoning break?
Reviewed- A.The two colonies would have to be measured with an instrument to compare
- B.Colour on a plate fades with time, so two shades are never comparable
- C.Several different organisms can carry out the same reaction and look alike
- D.Shade on a plate is set by how thickly the colony was streaked
Show the worked solution ▾
Answer: C. Several different organisms can carry out the same reaction and look alike
- Step 1: Ask what the shade is reporting: It reports one chemical behavior, such as whether a sugar was fermented.
- Step 2: Ask how many organisms share that behavior: More than one. So the appearance groups them together without separating them.
Why it's right: The appearance reports a behavior shared by more than one organism, so identical shade groups colonies into a behavioral family rather than naming a species.
- A: An instrument would measure the same shared behavior more precisely and still would not separate organisms that share it.
- B: Fading over days is real and does not touch the flaw here, which is that one appearance covers several organisms at any moment.
- D: A colony grows from a single cell wherever it lands, so how thickly the streak was made does not set an individual colony's shade.
Aligned to Culturing · reading level ~grade 10
A sample is streaked onto a plate whose inhibitors suppress most gram positive organisms, and the plate comes back empty. What may NOT be concluded?
Reviewed- A.That the sample contained no living organisms of any kind whatsoever
- B.That nothing capable of growing on this medium was present in the sample
- C.That no colony reached a visible size within the time the plate was held
- D.That the organisms this medium favours were absent or below detection
Show the worked solution ▾
Answer: A. That the sample contained no living organisms of any kind whatsoever
- Step 1: List what the plate could ever have shown: Only organisms able to grow under these conditions, in this time.
- Step 2: Check each conclusion against that list: Three of them stay inside it. One reaches past the plate to the whole sample.
Why it's right: Organisms the inhibitors suppressed leave the same blank plate as organisms that were never there, so the blank cannot clear the sample of everything living.
- B: This stays inside what the plate can report: it speaks only about organisms able to grow here.
- C: This is a statement about visible size within the hold, which is exactly what an empty plate observes.
- D: This names the favoured organisms specifically and adds the below detection alternative, so it does not overreach.
Aligned to Culturing · reading level ~grade 10
- A report on a selective plate names the medium, because the medium is half of what the result means.
Fill these in as you work through the lesson.
- Selective medium (stops some organisms growing):
- Differential medium (makes a reaction visible):
- Inhibitor (the ingredient doing the stopping):
- Indicator (the ingredient doing the showing):
An ingredient that stops growth makes the plate . An ingredient that changes appearance when a colony does a particular chemistry makes it . Something excluded by the first looks exactly like something that was never .
- Why would anybody deliberately stop most of the organisms in a sample from growing?
- If two colonies look different on a differential plate, what is that difference actually reporting?
- What does a plate with nothing on it fail to rule out?
A plate contains bile salts, which suppress many organisms, plus lactose and a pH indicator. Say which ingredient makes it selective and which makes it differential, then say what a pale colony and an empty plate each mean.