Confirming a presumptive result with a second reaction
Take a screening result that only suggests an organism, add an independent reaction that tests a different property, and keep the two readings apart when you report.
- A biochemical test reads one property: Confirmation only adds anything if the second reaction reads a different property, so you first have to see that each reaction answers one narrow question.
- Steps in a protocol have an order for a reason: Some readings can be repeated and some destroy the tube. Knowing which is which is what makes the sequence matter.
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
Work the read sequence PLTW Activity 4.1.2 uses, and say why each reading sits where it does.
Three readings are available on a coliform panel: a colour change, a long wave lamp check, and a reagent that stains the tube. In what order should they be taken, and why?
Reviewed- A.Colour, then lamp, then reagent, because the reagent cannot be undone
- B.Reagent first, so the stain has the longest possible time to develop fully
- C.Any order at all, since each of the three reads a separate property
- D.Lamp first, then reagent, then colour, so the darkest reading comes early
Show the worked solution ▾
Answer: A. Colour, then lamp, then reagent, because the reagent cannot be undone
- Step 1: Sort the readings into repeatable and one-way: Looking at a tube and shining a lamp at it change nothing. Adding a reagent changes the tube permanently.
- Step 2: Put the repeatable ones where they can still be repeated: Once the reagent is in, the earlier readings are gone. So they have to come first.
Why it's right: Two of the three readings are non-destructive and one is not, so the non-destructive pair is taken while the tube can still be re-read, and the irreversible one goes last.
- B: Putting the irreversible step first destroys both other readings before they are taken.
- C: The three readings are not interchangeable in sequence, because one of them alters the tube for the other two.
- D: The lamp check is fine early, but running the reagent before the colour reading throws that observation away.
Aligned to Microbiology Testing and Technology · reading level ~grade 10
A group has reagent left over and adds two drops to a tube that stayed straw yellow all along and showed nothing under the lamp. A red ring appears. How should this be handled?
Reviewed- A.Repeat the lamp reading on the same tube to settle the disagreement
- B.Report the tube as coliform positive but not E. coli positive
- C.Report the tube as E. coli positive, since the red ring is a real observation
- D.Treat the ring as uninterpretable, and note that the reagent was misapplied
Show the worked solution ▾
Answer: D. Treat the ring as uninterpretable, and note that the reagent was misapplied
- Step 1: Ask what the reagent step was supposed to confirm: It is applied to tubes that already screened positive. On a tube that screened negative there is nothing for it to confirm.
- Step 2: Decide what the observation supports: A reading taken outside the conditions the method specifies does not carry the meaning the method assigns to it.
Why it's right: The confirmation step is defined only for tubes that already screened positive, so a ring produced outside those conditions cannot be read as a confirmation and the deviation belongs in the notes.
- A: The lamp reading cannot be repeated meaningfully now: the reagent has already stained the contents of the tube.
- B: The colour reading, not the reagent, is what would support a coliform call, and that reading was negative.
- C: The ring is a real observation and still not evidence of the target, because it was produced outside the conditions the reading is defined for.
Aligned to Microbiology Testing and Technology · reading level ~grade 10
- A technician photographs a rack before adding any reagent, because that is the last moment the untouched readings still exist.
Fill these in as you work through the lesson.
- Presumptive (suggested, not yet established):
- Confirmed (backed by a second, independent reaction):
- Screening test (wide net, deliberately easy to trigger):
- Independent reaction (reads a different property):
A screening reaction gives a result. It becomes only when a second reaction that reads a different agrees with it.
- Why would a laboratory design a first test that it knows will sometimes fire on the wrong organism?
- What would you lose if the confirmation measured the same property as the screen?
- Why is it a waste to run the confirmation on a tube that showed nothing at all?
A tube changes colour, then glows under a long wave lamp, then shows a red ring after reagent is added. Write the reading as two sentences: what the first two observations made you suspect, and what the third one established.