Infective Endocarditis - Microbiology

Overview of the Microbial Spectrum

• Fuster and Hurst's The Heart, 15th Ed., p. 1028

Classification by Valve Type and Clinical Setting

• Frameworks for Internal Medicine

1. Native Valve IE (NVIE)

Streptococci

• Viridans group streptococci (VGS) - normal oropharyngeal flora; classic cause of subacute community-acquired IE following dental procedures. Key species: S. sanguinis, S. mutans, S. mitior, S. gallolyticus
• S. gallolyticus (formerly S. bovis) - bacteremia with this organism should prompt colonoscopy, as GI malignancy, polyps, or inflammatory disease are associated in up to 60% of cases
• Nutritionally variant streptococci (Abiotrophia and Granulicatella spp.) - require specialized culture media; have diminished susceptibility to penicillins/cephalosporins and often need aminoglycoside combination therapy
• S. pneumoniae IE - rare but acute and fulminant; causes severe valve damage with high mortality
• Group B streptococci (S. agalactiae) - associated with obstetric complications, diabetes, liver disease, alcoholism, and injection drug use

Staphylococci

• S. aureus - the dominant pathogen in acute IE; possesses virulence factors (fibronectin-binding proteins, coagulase) allowing adherence to endothelium and evasion of host defenses; 80-90% of staphylococcal IE is due to S. aureus. Risk is elevated with hemodialysis, diabetes, HIV, and recent surgery
• Coagulase-negative staphylococci (CoNS) - mainly S. epidermidis; principally affect prosthetic/damaged valves and intracardiac devices rather than intact native endothelium
• S. lugdunensis - a CoNS that behaves like S. aureus clinically, with high rates of perivalvular extension and metastatic spread

Enterococci

• Cause ~5-10% of subacute bacterial endocarditis
• Normal GI/genitourinary flora; bacteremia follows GU or GI instrumentation
• E. faecalis is more common and virulent than E. faecium in native valve disease
• High-level antibiotic resistance is increasingly common in healthcare-associated strains
• Frequently associated with prosthetic materials and indwelling catheters

2. Prosthetic Valve IE (PVE)

• The Washington Manual of Medical Therapeutics; Bailey & Love's Surgery, 28th Ed., Table 59.3

3. IV Drug Use (IVDU)-Associated IE

• S. aureus is responsible for >50% of cases; causes predominantly right-sided (tricuspid valve) endocarditis
• Viridans streptococci, Enterococcus, gram-negative bacilli, fungi also occur
• Eikenella corrodens (a HACEK organism) - linked to needle/skin licking; infection is often a standalone surgical indication
• Pseudomonas aeruginosa - rare, more common in PWID

4. HACEK Organisms

• Fastidious, slow-growing gram-negative rods; normal oropharyngeal flora
• Account for ~2% overall IE; 5-10% of community-acquired cases
• May take 3-4 weeks on standard culture (accelerated to 5-7 days with BACTEC systems)
• Classically form large, friable vegetations prone to embolization
• Particularly affect immunocompromised patients, prosthetic valve recipients, and IVDU

5. Fungi

• Candida (52%) and Aspergillus (24%) are the dominant fungal pathogens
• Seen in immunocompromised hosts and patients with indwelling devices
• Bulky vegetations - prone to large vessel embolization and even valve obstruction
• Aspergillus is almost never culture-positive from blood; diagnosis relies on serology/PCR
• Treatment nearly always requires combined medical and surgical management including prosthetic valve removal; chronic suppressive antifungal therapy is needed
• Mortality is extremely high

6. Culture-Negative IE (~10% of cases)

Key Clinical Associations Summary

• Primary sources: Fuster and Hurst's The Heart 15th Ed.; Washington Manual of Medical Therapeutics; Bailey & Love's Surgery 28th Ed.; Frameworks for Internal Medicine