Which of the following characteristics would most likely rule out Pseudomonas as the identity of an unknown bacterial isolate?

• A. The bacterium is motile by means of polar flagella.
• B. The bacterium is oxidase positive.
• C. The bacterium is Gram-positive.
• D. The bacterium produces a green pigment.

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the typical characteristics of Pseudomonas species. Pseudomonas are generally Gram-negative bacteria, motile by polar flagella, oxidase positive, and often produce a green pigment called pyocyanin.
2. Step 2: Analyze each characteristic given in the problem in relation to Pseudomonas traits: motility by polar flagella is consistent with Pseudomonas, oxidase positivity is also typical, and green pigment production aligns with known Pseudomonas pigments.
3. Step 3: Focus on the Gram stain characteristic. Since Pseudomonas are Gram-negative, a Gram-positive result would contradict this fundamental trait and thus rule out Pseudomonas as the identity of the isolate.
4. Step 4: Conclude that the characteristic which most likely rules out Pseudomonas is the bacterium being Gram-positive, because this is a key differential feature in bacterial classification.
5. Step 5: Summarize that when identifying bacteria, Gram stain results are critical initial tests that help narrow down possible genera, and a Gram-positive result excludes Gram-negative genera like Pseudomonas.