In a microbiology experiment testing the effect of an antibiotic on bacterial growth, which test tube(s) acts as a negative control?

• A. A test tube containing nutrient broth, bacteria, and antibiotic
• B. A test tube containing nutrient broth and bacteria but no antibiotic
• C. A test tube containing only antibiotic and nutrient broth
• D. A test tube containing only sterile nutrient broth without bacteria or antibiotic

Answer: (D)

Verified step by step guidance

1. Understand the purpose of a negative control in a microbiology experiment: it is used to show what happens when the experimental factor (in this case, bacteria or antibiotic) is absent, ensuring that any observed effect is due to the factor being tested.
2. Identify the components in each test tube: one has nutrient broth, bacteria, and antibiotic; another has nutrient broth and bacteria but no antibiotic; the last has only antibiotic and nutrient broth.
3. Recognize that a negative control should lack the bacteria to confirm that any growth observed is due to bacterial presence and not contamination or other factors.
4. Note that the test tube containing only sterile nutrient broth without bacteria or antibiotic serves as the negative control because it contains no bacteria to grow and no antibiotic to affect growth.
5. Conclude that this negative control helps verify that any bacterial growth in other tubes is due to the bacteria added and not from contamination or other variables.