Which of the following best demonstrates mutualism between certain types of bacteria and humans?

• A. Bacteria competing with human cells for nutrients
• B. Pathogenic bacteria causing strep throat in humans
• C. Bacteria living on human skin without affecting the host
• D. Gut bacteria synthesizing vitamin K for human absorption

Answer: (D)

Verified step by step guidance

1. Understand the concept of mutualism: Mutualism is a type of symbiotic relationship where both organisms involved benefit from the interaction.
2. Analyze each option to see if both bacteria and humans benefit: For example, bacteria competing with human cells for nutrients benefits bacteria but harms humans, so it is not mutualism.
3. Consider pathogenic bacteria causing strep throat: This harms humans and benefits bacteria, so it is not mutualism but parasitism.
4. Look at bacteria living on human skin without affecting the host: This is an example of commensalism, where bacteria benefit but humans are neither helped nor harmed.
5. Identify gut bacteria synthesizing vitamin K: Here, bacteria produce vitamin K which humans absorb and use, benefiting humans, while bacteria gain a stable environment and nutrients, demonstrating mutualism.