A 32-year-old man with a history of intravenous drug use presents with fever, new cardiac murmur, and splinter hemorrhages. Blood cultures are obtained. Which confirmatory test is most appropriate to identify Streptococcus pyogenes as the causative organism of infective endocarditis?

## Confirmatory Identification of S. pyogenes in Blood Culture **Key Point:** Gram stain, culture on blood agar, bacitracin sensitivity, and PYR (pyrrolidonyl arylamidase) test constitute the gold standard phenotypic identification of S. pyogenes from blood culture isolates in clinical microbiology laboratories. ### Identification Algorithm for S. pyogenes **High-Yield:** S. pyogenes is identified by a stepwise approach: 1. **Gram stain:** Gram-positive cocci in chains 2. **Culture:** Beta-hemolytic colonies on blood agar (clear zone of hemolysis) 3. **Bacitracin sensitivity:** Inhibition zone ≥10 mm (S. pyogenes is bacitracin-sensitive; S. agalactiae is resistant) 4. **PYR test:** Positive (S. pyogenes hydrolyzes L-pyrrolidonyl-β-naphthylamide; S. agalactiae is negative) ### Differential Identification: S. pyogenes vs. S. agalactiae | Feature | S. pyogenes | S. agalactiae | |---|---|---| | **Hemolysis** | Beta (clear) | Beta (clear) | | **Bacitracin** | **Sensitive** | Resistant | | **PYR test** | **Positive** | Negative | | **CAMP test** | Negative | Positive | | **Lancefield group** | **A** | B | | **Hyaluronic acid capsule** | Yes (mimics host) | Yes | **Clinical Pearl:** Bacitracin sensitivity and PYR positivity together are pathognomonic for S. pyogenes among beta-hemolytic streptococci. This combination is rapid (results in 24–48 hours) and cost-effective, making it ideal for clinical decision-making in endocarditis. ### Why This Approach is Superior in Clinical Context **Tip:** In infective endocarditis, rapid identification of the causative organism guides targeted antibiotic therapy. Bacitracin and PYR tests provide results within 24–48 hours, whereas molecular methods (16S rRNA, MALDI-TOF) may not be available in all laboratories and are not necessary for routine clinical identification. ### Diagnostic Flowchart ```mermaid flowchart TD A[Blood culture positive]:::outcome --> B[Gram stain]:::action B --> C{Gram-positive cocci in chains?}:::decision C -->|Yes| D[Culture on blood agar]:::action D --> E{Beta-hemolytic?}:::decision E -->|Yes| F[Bacitracin sensitivity test]:::action F --> G{Bacitracin sensitive?}:::decision G -->|Yes| H[PYR test]:::action H --> I{PYR positive?}:::decision I -->|Yes| J[S. pyogenes confirmed]:::outcome I -->|No| K[S. agalactiae or other]:::outcome G -->|No| K E -->|No| L[Non-beta-hemolytic streptococcus]:::outcome ``` [cite:Robbins 10e Ch 8; Jawetz, Melnick & Adelberg's Medical Microbiology 28e Ch 15]