## Confirmatory Investigation for S. pneumoniae **Key Point:** Blood culture is the gold standard for diagnosing invasive pneumococcal infection (bacteremia/sepsis). When positive, it provides both organism isolation and susceptibility testing. ### Why Blood Culture + Optochin/Bile Solubility? **High-Yield:** The combination of: 1. **Optochin sensitivity** — S. pneumoniae is α-hemolytic AND optochin-sensitive (inhibition zone ≥14 mm); Viridans streptococci are optochin-resistant 2. **Bile solubility** — S. pneumoniae lyses in bile (deoxycholate); Viridans streptococci do not These two tests together are pathognomonic and definitively differentiate pneumococcus from other α-hemolytic streptococci. ### Diagnostic Algorithm ```mermaid flowchart TD A[Gram-positive diplococci<br/>α-hemolytic on blood agar]:::outcome --> B{Optochin sensitive?}:::decision B -->|Yes| C{Bile soluble?}:::decision B -->|No| D[Viridans streptococcus]:::outcome C -->|Yes| E[S. pneumoniae confirmed]:::action C -->|No| F[Streptococcus bovis]:::outcome ``` **Clinical Pearl:** In community-acquired pneumonia with lobar consolidation and Gram-positive diplococci on sputum Gram stain, blood culture is preferred because: - Isolates susceptibility to penicillin, cephalosporins, and fluoroquinolones can be determined - Bacteremia indicates invasive disease requiring parenteral antibiotics - Sputum culture may be contaminated with oral flora **Tip:** Remember the mnemonic **POBS** for pneumococcus identification: - **P**enicillin sensitive (historically; now variable) - **O**ptochin sensitive - **B**ile soluble - **S**ome are encapsulated (mucoid colonies) [cite:Jawetz, Melnick & Adelberg's Medical Microbiology 27e Ch 16]
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