A Gram stain of a sputum sample shows pink, rod-shaped bacteria. Which of the following best explains the pink color of these organisms?

Explanation

## Interpretation of Pink (Gram-Negative) Bacteria ### Gram-Negative Staining Mechanism **Key Point:** Pink color in a Gram stain indicates a **Gram-negative bacterium**. This color arises because the organism loses the crystal violet-iodine (CV-I) complex during the decolorization step and subsequently takes up the safranin counterstain. ### Step-by-Step Mechanism 1. **Crystal violet application** → All bacteria (Gram-positive and Gram-negative) initially stain purple 2. **Iodine (mordant)** → Forms a large CV-I complex that precipitates in the cell 3. **Acetone-alcohol (decolorizer)** → Dissolves the CV-I complex in Gram-negative cells because: - Thin peptidoglycan layer (~10% of cell wall) - High lipid content in outer membrane - Decolorizer is lipophilic and penetrates easily 4. **Safranin (counterstain)** → Pink dye enters the now-colorless Gram-negative cell and binds to nucleic acids and proteins ### Why Gram-Negative Cells Lose CV-I **High-Yield:** The **thin peptidoglycan layer** in Gram-negative bacteria cannot retain the large CV-I complex. The acetone-alcohol dissolves the lipid-rich outer membrane, allowing the complex to escape. Gram-positive bacteria, by contrast, have a **thick peptidoglycan (50–90% of cell wall)** that traps the CV-I complex and resists decolorization. ### Comparison: Gram-Positive vs. Gram-Negative | Feature | Gram-Positive | Gram-Negative | | --- | --- | --- | | **Peptidoglycan thickness** | Thick (50–90% of wall) | Thin (~10% of wall) | | **Outer membrane** | Absent | Present (lipid-rich) | | **CV-I complex retention** | Retained (purple) | Lost (decolorized) | | **Safranin uptake** | Minimal (remains purple) | Uptake (becomes pink) | | **Final color** | Purple | Pink | **Clinical Pearl:** Pink rods in sputum are classically **Gram-negative rods** such as *Pseudomonas aeruginosa*, *Klebsiella pneumoniae*, or *Escherichia coli*. This morphology and staining pattern are critical for initial organism identification and empiric antibiotic selection. ### Mnemonic **"Thin = Pink"** — Thin peptidoglycan in Gram-negative bacteria → Pink color after safranin uptake.