Which of the following bacteria are not reliably identified using a Gram stain?

• A. Bacillus subtilis
• B. Escherichia coli
• C. Mycobacterium tuberculosis
• D. Staphylococcus aureus

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the principle of Gram staining. Gram stain differentiates bacteria based on the structure of their cell walls, classifying them as Gram-positive (thick peptidoglycan layer) or Gram-negative (thin peptidoglycan layer with outer membrane).
2. Step 2: Review the characteristics of each bacterium listed: Bacillus subtilis (Gram-positive rod), Escherichia coli (Gram-negative rod), Staphylococcus aureus (Gram-positive cocci), and Mycobacterium tuberculosis (acid-fast bacterium with a waxy, lipid-rich cell wall).
3. Step 3: Recognize that Mycobacterium tuberculosis has a unique cell wall containing mycolic acids, which makes it resistant to the conventional Gram stain and requires special staining techniques like the acid-fast stain (Ziehl-Neelsen stain) for reliable identification.
4. Step 4: Conclude that while Bacillus subtilis, Escherichia coli, and Staphylococcus aureus can be reliably identified using Gram stain due to their typical cell wall structures, Mycobacterium tuberculosis cannot be reliably identified by Gram stain.
5. Step 5: Summarize that the limitation of Gram staining lies in bacteria with atypical cell walls, such as Mycobacterium tuberculosis, which necessitates alternative staining methods for accurate detection.