Which of the following is not a Gram-positive bacterium?

Introduction / Context:
Rapid Gram classification informs empiric therapy and lab workflows. While some taxa have staining nuances, many genera are reliably categorized as Gram-positive or Gram-negative based on cell envelope architecture.

Given Data / Assumptions:

• Candidates: Streptococci, Pseudomonas, Mycobacteria.
• We seek the genus that is not Gram-positive.
• Conventional Gram stain interpretation applies.

Concept / Approach:

Streptococci are Gram-positive cocci in chains/pairs. Mycobacteria are acid-fast due to mycolic acids; on Gram staining they often appear poorly or irregularly stained but phylogenetically group with Gram-positive actinobacteria (monoderm envelope, no classic outer membrane). Pseudomonas, however, is unequivocally Gram-negative (diderm outer membrane with LPS) and a classic non-fermenting rod. Therefore, Pseudomonas is the best answer to “not Gram-positive.”

Step-by-Step Solution:

Verification / Alternative check:

Culture characteristics (oxidase-positive, non-fermenter), outer membrane porins, and susceptibility patterns all align Pseudomonas with Gram-negative bacteria. Ziehl–Neelsen staining confirms the acid-fast nature of Mycobacteria rather than Gram-negativity.

Why Other Options Are Wrong:

• Streptococci: clearly Gram-positive.
• Mycobacteria: acid-fast; not considered classic Gram-negative.
• None of these: incorrect because one option (Pseudomonas) is truly not Gram-positive.

Common Pitfalls:

• Misclassifying Mycobacteria as Gram-negative due to poor Gram staining; envelope architecture places them with Gram-positive lineage.
• Ignoring biochemical traits that correlate with Gram status.

Final Answer:

Pseudomonas