A 28-year-old woman from rural Maharashtra presents with acute pharyngitis, fever (39.2°C), and enlarged tonsils with exudate. Throat culture grows β-hemolytic cocci in chains. Regarding S. pyogenes, all of the following statements are true EXCEPT:

Explanation

## Laboratory Identification and Antibiotic Susceptibility of S. pyogenes **Key Point:** S. pyogenes is universally susceptible to penicillin and vancomycin. Resistance to vancomycin has NEVER been documented in clinical isolates of S. pyogenes — this statement is false and is the exception. ### Correct Identification Features | Feature | S. pyogenes | Clinical Use | |---------|-------------|-------------| | Gram stain | Gram-positive cocci in chains | Presumptive identification | | Hemolysis | β-hemolysis on blood agar | Presumptive identification | | Catalase | Negative | Differentiates from S. aureus (catalase-positive) | | Oxidase | Negative | Gram-positive bacterium | | Bacitracin | Sensitive (zone >10 mm) | Gold standard for presumptive ID | | PYR test | Positive (pyrrolidonyl aminopeptidase) | Confirmatory test | ### Antibiotic Susceptibility Profile **High-Yield:** S. pyogenes remains 100% susceptible to: - Penicillin G (first-line) - Amoxicillin (oral first-line) - Cephalosporins (first-line alternative) - Vancomycin (reserved for penicillin-allergic patients) - Macrolides (erythromycin, azithromycin — though resistance is increasing in some regions) **Warning:** There is NO vancomycin resistance in S. pyogenes. Any question claiming S. pyogenes is vancomycin-resistant is factually incorrect. Vancomycin resistance is seen in enterococci and some gram-positive anaerobes, but NOT in S. pyogenes. **Clinical Pearl:** In a penicillin-allergic patient with acute S. pyogenes pharyngitis, vancomycin is a reliable second-line agent. Vancomycin has been used for decades in severe invasive S. pyogenes infections (necrotizing fasciitis, streptococcal toxic shock syndrome) with consistent efficacy. **Mnemonic for S. pyogenes ID:** **BACPYR** - **B**acitracin sensitive - **A**ntigen group A (Lancefield grouping) - **C**atalase negative - **P**YR test positive - **Y**ellow pigment (optional, variable) - **R**ed blood cell hemolysis (β-hemolysis) [cite:Robbins 10e Ch 8; Harrison 21e Ch 161]