Telling contamination from a finding
Use sterility checks to spot a signal that came from your own bench rather than from the sample, and decide what the run is still allowed to report.
- Sterile means nothing living is in it: A sterility check only works if you expect exactly nothing from it, so the word has to mean nothing living rather than mostly clean.
- Every signal came from somewhere: The whole skill is asking where a signal entered, and that question only opens once you stop treating a reading as automatically belonging to the sample.
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
Trace where a stray organism entered, and decide what a compromised run can still support.
A group transfers five samples using one pipette tip for all five. Sample three comes back positive. Which statement about that positive is correct?
Reviewed- A.It proves the tip was contaminated before the first transfer was made
- B.It cannot be assigned to sample three, since earlier samples may have carried over
- C.It is trustworthy, since the tip touched only water and never any surface
- D.It is trustworthy for sample three, but it leaves samples four and five unreadable
Show the worked solution ▾
Answer: B. It cannot be assigned to sample three, since earlier samples may have carried over
- Step 1: Follow the tip through the order of transfers: The tip visited samples one and two before it reached sample three, and whatever was in them travelled with it.
- Step 2: Ask which source the positive can be assigned to: With more than one possible origin and no way to separate them, the positive belongs to no single sample.
Why it's right: Anything carried on the tip from the two earlier samples arrives in sample three, so the positive has several possible origins and cannot be assigned to one of them.
- A: A tip contaminated at the start would be one explanation among several, and nothing in the result singles it out.
- C: Water samples are exactly what carries organisms here. Touching only water is not protection.
- D: The carry-over runs forward from the first transfer onward, so sample three is already affected, not just the ones after it.
Aligned to Microbiology Testing and Technology · reading level ~grade 10
The tube that received no sample changed colour overnight, and so did two sample tubes. What is the right handling of this rack?
Reviewed- A.Report the two positives with a note that the third tube was probably mislabeled
- B.Report all three as positive, since the third one behaved like the others
- C.Report no positives from this rack, record what happened, and repeat the panel
- D.Report the two sample positives and leave the third tube out of the report
Show the worked solution ▾
Answer: C. Report no positives from this rack, record what happened, and repeat the panel
- Step 1: Read the tube that should have stayed unchanged: It changed, so a route into the tubes existed that had nothing to do with any sample.
- Step 2: Apply that finding to the rest of the rack: The same route was available to every tube handled the same way, so no positive on the rack can be attributed to its sample.
Why it's right: A signal in the tube that received no sample shows a route open to the whole rack, so every positive on it has an alternative explanation and the run has to be repeated with the event recorded.
- A: Mislabeling is a guess. The observation is a change in a tube that should not have changed, and that is what gets recorded.
- B: The third tube had no sample in it, so calling it positive would be reporting a result for water that was never tested.
- D: Dropping the inconvenient tube from the report removes the one observation that says the other two cannot be trusted.
Aligned to Microbiology Testing and Technology · reading level ~grade 10
- A laboratory voids a batch and files a deviation record, so the same reagent lot is not quietly used again the following week.
Fill these in as you work through the lesson.
- Sterility check (a tube expected to stay empty):
- Cross-contamination (carrying material between samples):
- Turbidity (cloudiness from suspended cells):
- Deviation note (the written record of what went wrong):
If the tube that should hold shows growth, the run has a somewhere in it, and every reading from that has to be treated as unreliable until the source is found.
- Why does one pipette tip reused across two samples put both of their results at risk, not just the second?
- What is the difference between throwing a run out and writing down what happened to it?
- Uninoculated medium in the fridge has gone cloudy. What does that tell you about every plate poured from that batch?
Your sterile water tube changed colour overnight along with three of your samples. Write what you can and cannot report from this rack, and name two specific places the contamination could have entered.