## Distinguishing Pseudomonas aeruginosa from E. coli ### Key Structural & Biochemical Differences **Key Point:** Pyoverdine (yellow-green fluorescent pigment) and pyocyanin (blue-green pigment) are pathognomonic for *Pseudomonas aeruginosa* and are absent in *E. coli*. These pigments are produced under iron-limiting conditions and are hallmark virulence factors. ### Comparison Table | Feature | *Pseudomonas aeruginosa* | *E. coli* | | --- | --- | --- | | **Pigment production** | Pyoverdine (fluorescent), pyocyanin (blue-green) | None | | **Flagella** | Single polar (monotrichous) | Peritrichous (multiple) | | **Glucose fermentation** | Non-fermenter (oxidative only) | Fermenter (acid + gas) | | **LPS** | Present (Gram-negative) | Present (Gram-negative) | | **Oxidase test** | Positive | Negative | ### High-Yield Clinical Pearl **High-Yield:** The combination of: - Non-fermenting Gram-negative rod - Positive oxidase test - **Pigment production** (especially pyocyanin—blue-green colonies on culture plates) ...is virtually diagnostic of *P. aeruginosa* in the clinical microbiology lab. Pyocyanin is so characteristic that its presence on a MacConkey or blood agar plate immediately suggests *P. aeruginosa*. ### Why This Matters **Clinical Pearl:** *P. aeruginosa* is an opportunistic pathogen in immunocompromised hosts, cystic fibrosis patients, and wound infections. The pigments themselves contribute to virulence by generating reactive oxygen species and chelating iron, thereby evading host immunity. ### Mnemonic **Mnemonic:** **PEP** — *Pseudomonas* = **P**igments (pyoverdine, pyocyanin), **E**nvironment (water-loving, biofilm-forming), **P**olar flagellum (single). [cite:Jawetz, Melnick & Adelberg's Medical Microbiology 28e Ch 16]
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