Which of the following best describes a commensal bacterium?

• A. A bacterium that benefits from its host without harming or helping the host
• B. A bacterium that lives independently of any host organism
• C. A bacterium that causes disease in its host
• D. A bacterium that provides a benefit to its host, such as producing vitamins

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the term 'commensal' in microbiology. Commensal bacteria are those that live in or on a host organism and derive benefit from the host without causing harm or providing any benefit to the host.
2. Step 2: Review the options given and identify the key characteristics of each: whether the bacterium benefits the host, harms the host, or lives independently.
3. Step 3: Recognize that a bacterium that causes disease is termed 'pathogenic', so this option does not describe a commensal bacterium.
4. Step 4: Identify that a bacterium providing benefits to the host, such as producing vitamins, is considered 'mutualistic', not commensal.
5. Step 5: Conclude that the correct description of a commensal bacterium is one that benefits from its host without harming or helping the host.