Enterobacteriaceae biochemistry — Which genera show a positive phenylalanine deaminase (PPA) test result?

Introduction / Context:
The phenylalanine deaminase (PPA) test detects an organism’s ability to deaminate phenylalanine to phenylpyruvic acid, which forms a green complex with ferric chloride. This test is particularly useful in clinical microbiology to help differentiate members of Enterobacteriaceae that may otherwise share overlapping biochemical profiles.

Given Data / Assumptions:

• We focus on three closely related genera often grouped by their PPA positivity.
• Test interpretation is based on development of a green color after addition of ferric chloride.
• Other Enterobacteriaceae, such as Escherichia and Klebsiella, are typically negative.

Concept / Approach:
Proteus, Providencia, and Morganella consistently yield positive PPA reactions and are sometimes collectively referenced as the PPM group. Their shared ability reflects similar metabolic pathways for aromatic amino acid deamination. Recognizing this cluster speeds identification from routine biochemical panels and directs appropriate antimicrobial testing given their characteristic resistance patterns.

Step-by-Step Solution:

Verification / Alternative check:
Standard identification flowcharts place PPA early to separate the PPM group from other Enterobacteriaceae that are lactose non-fermenters but PPA negative.

Why Other Options Are Wrong:

• Each single-genus option is incomplete; all three are positive.

Common Pitfalls:
Misreading weak color changes; ensure adequate incubation and fresh ferric chloride to avoid false negatives.

Final Answer:
All of these