Question

Anaerobic bacteria are intrinsically resistant to

• A. Beta lactams
• B. Aminoglycosides
• C. Chloramphenicol
• D. Metronidazole

Hint:

Always remember: "Aminoglycosides need $O_2$ to enter."
Since anaerobes live in $O_2$-free environments and lack aerobic respiratory chains, they are always intrinsically resistant to all aminoglycosides.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Intrinsic resistance is an inherent characteristic of a bacterial species, dependent on its structural or physiological properties, making a specific class of antibiotics ineffective against it.
Detailed Explanation:
- Aminoglycosides (e.g., gentamicin, amikacin) require an active, oxygen-dependent transport process to cross the bacterial inner membrane and reach their ribosomal targets inside the cell.
- This active transport system (specifically the Energy-Dependent Phase I or EDP-I) is driven by the proton motive force generated by the respiratory electron transport chain, which requires molecular oxygen.
- Because obligate anaerobic bacteria lack respiratory chains that utilize oxygen as the terminal electron acceptor, they cannot generate the electrical potential required to transport aminoglycosides across their cell membrane.
- Consequently, aminoglycosides cannot enter the cells of anaerobic bacteria, rendering them intrinsically resistant.
- In contrast, beta-lactams, chloramphenicol, and metronidazole do not require oxygen-dependent transport mechanisms.

Step 2: Final Answer:
Anaerobic bacteria are intrinsically resistant to aminoglycosides.