Question

An experimenter is attempting to investigate the effect of a new antibiotic on \(E .\) coli. He plates cells and administers one milliliter of the antibiotic. Which of the following is an appropriate negative control in this experiment? (A) A plate with no cells that was coated with one milliliter of antibiotic. (B) A plate with \(E .\) coli and no additional treatment. (C) A plate with \(E .\) coli and one milliliter of isotonic saline. (D) \(\quad\) A plate of epithelial cells treated with one milliliter of antibiotic.

Short answer

B: A plate with E. coli and no additional treatment.

Step-by-step solution

Understand the Purpose of the Negative Control

A negative control is used to ensure that the outcome of the experiment can be attributed to the treatment itself and not to other variables. In this context, the negative control will help determine whether the effects observed on E. coli are specifically due to the antibiotic.

Analyze Each Option

Evaluate each option to see if it meets the criteria of a negative control, which should not produce the effect being tested (antibiotic action on E. coli).

Evaluate Option A

Option A has no cells and one milliliter of antibiotic. This setup cannot show the effect of the antibiotic on E. coli as there are no E. coli cells present.

Evaluate Option B

Option B contains E. coli but no antibiotic or any treatment. This plate will show normal growth of E. coli without any interference, serving as a control for bacterial growth conditions.

Evaluate Option C

Option C involves E. coli and isotonic saline. Saline doesn’t have antibacterial properties, so any effect can be compared to the antibiotic’s effect. This controls for the presence of liquid in the experiment, but not specifically the antibiotic.

Evaluate Option D

Option D uses epithelial cells with one milliliter of antibiotic. As the cells are different (epithelial vs. E. coli), this setup is not suitable as a control in this experiment about E. coli.

Choose the Most Appropriate Negative Control

The most effective negative control here would be option B. This option shows the normal growth of E. coli without any treatment, thus providing a baseline (comparison) to observe changes when the antibiotic is added.

Key concepts

Experimental Controls

In scientific experiments, controls are essential to ensure reliable and accurate results.
A control is an element that remains constant and unchanged to measure the effect of the variables being tested.
There are generally two types of controls: positive and negative.
A positive control shows an expected change, indicating that the experimental setup is working.
A negative control, conversely, shows no change, ensuring that the results can be attributed to the experimental variable alone.
For example, if testing a new antibiotic's effects on E. coli bacteria, you would use a negative control without the antibiotic. This setup shows how E. coli normally grows without interference, helping to pinpoint any changes directly attributable to the antibiotic.

E. Coli Experiments

E. coli, or Escherichia coli, is a common bacterium found in the intestines of humans and animals.
It is frequently used in laboratory experiments because it is easy to grow and manipulate.
These properties make it ideal for various scientific studies, including genetics, microbiology, and antibiotic testing.
Experiments with E. coli often involve growing the bacteria on plates and exposing them to different treatments to observe changes.
For example, scientists may measure bacterial growth, colony formation, and susceptibility to antibiotics.
Understanding these responses helps researchers develop new medicines and better comprehend bacterial behavior.

Antibiotic Testing

Antibiotic testing aims to determine the efficacy of compounds in stopping or slowing bacterial growth.
Antibiotics are critical in treating bacterial infections, and their effectiveness needs rigorous evaluation before use.
During such tests, scientists plate bacteria, apply the antibiotic, and observe the results.
Key measurements include bacterial death, reduced growth, or changes in morphology.
For accurate results, controlled conditions are crucial.
For instance, one might use different concentrations of the antibiotic to find the optimal dosage effectively killing the bacteria with minimal side effects.

Scientific Method

The scientific method is a systematic approach to investigating phenomena, acquiring new knowledge, or correcting previous knowledge.
It involves several critical steps:

• Observation: Noticing a phenomenon or problem.


• Question: Formulating a question or hypothesis about the observation.


• Experimentation: Testing the hypothesis through controlled experiments.


• Analysis: Analyzing the data obtained from the experiments.


• Conclusion: Drawing conclusions based on the analysis, supporting or refuting the hypothesis.

Using the scientific method ensures that results are repeatable, reliable, and unbiased.
For students learning this method, practicing with controlled experiments like E. coli and antibiotics offers invaluable insights into applying scientific principles to real-world problems.