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Difficulty: MediumDetermining Control Groups and Baseline Conditions

A team of biomedical researchers investigated the inhibitory effect of a plant-derived polyphenol, compound CP, on bacterial biofilm formation by *Pseudomonas aeruginosa* on silicone catheter surfaces. The researchers prepared four different culture setups in microtiter plate wells and measured biofilm mass after incubation at 37∘C37^\circ\text{C} for 24 hours:

- Setup I: Silicone catheter disc + nutrient broth + *P. aeruginosa* + 0 μg/mL0\text{ }\mu\text{g/mL} compound CP (vehicle solvent only)
- Setup II: Silicone catheter disc + nutrient broth + *P. aeruginosa* + 50 μg/mL50\text{ }\mu\text{g/mL} compound CP
- Setup III: Silicone catheter disc + nutrient broth (no bacteria added) + 0 μg/mL0\text{ }\mu\text{g/mL} compound CP
- Setup IV: Nutrient broth only (no catheter disc, no bacteria added)

Based on the experimental design, match each culture setup (Setups I–IV) to its specific experimental role or baseline function.

  • Setup INegative control establishing baseline biofilm growth in the absence of the experimental inhibitor
  • Setup IIActive experimental group evaluating the specific antimicrobial effectiveness of compound CP
  • Setup IIISterility control verifying that nutrient broth and catheter materials are free of initial microbial contamination
  • Setup IVMedia background baseline used to calibrate spectrophotometric instruments and subtract liquid absorbance

Answer

Setup I matches the negative control for baseline biofilm growth; Setup II matches the active experimental treatment group; Setup III matches the sterility control; Setup IV matches the media background baseline.
Setup I establishes baseline bacterial biofilm growth in the absence of compound CP (negative control). Setup II tests the effect of the active antimicrobial chemical (experimental group). Setup III verifies that incubation media and substrates are uninfected (sterility control). Setup IV provides the optical background reading of the broth alone (media baseline).

Step-by-Step Solution

1
Identify the culture setup containing the manipulated independent variable.
Setup II is the only group tested with 50 μg/mL50\text{ }\mu\text{g/mL} compound CP, making it the active experimental treatment group.
An experimental group receives the specific treatment condition being evaluated.
2
Identify the culture setup establishing growth performance without the test chemical.
Setup I contains bacteria, broth, and catheter material with 0 μg/mL0\text{ }\mu\text{g/mL} compound CP, serving as the negative control.
A negative control isolates the effect of the independent variable by measuring baseline output when the treatment is absent.
3
Identify the setup designed to test for non-inoculated bacterial presence.
Setup III includes catheter substrate and growth broth without added bacteria, acting as a sterility control.
Sterility controls verify that materials and media do not harbor unintended contaminants prior to the experiment.
4
Identify the setup measuring background signal from reagents alone.
Setup IV contains only the liquid medium, functioning as a media background baseline.
Background baselines allow researchers to adjust optical detection instruments by subtracting signal contributions from non-biological reagents.

Key Concept

Distinguishing between negative controls, active experimental groups, sterility controls, and baseline conditions
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Topics

Determining Control Groups and Baseline Conditions

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