Bacteria That Cause Infective Endocarditis
The most common bacteria causing infective endocarditis are Staphylococcus aureus (the single most frequent pathogen overall), viridans group streptococci (particularly from oral flora), and Enterococcus species, with coagulase-negative staphylococci being important in prosthetic valve infections. 1
Primary Causative Organisms
Staphylococci
- Staphylococcus aureus is now the most frequent cause of infective endocarditis in both native and prosthetic valve infections, representing a shift from traditional patterns 1
- Coagulase-negative staphylococci (CNS) are particularly important in prosthetic valve endocarditis, with oxacillin resistance being common 1
- Staphylococcus lugdunensis (a CNS species) can cause native valve IE with an aggressive clinical course 1
- Staphylococci are the primary cause of early postoperative infections after cardiac valve surgery 2
Streptococci
- Viridans group streptococci account for 37-50% of endocarditis cases and are the classical oral flora pathogens 1, 2
- Streptococcus bovis is associated with gastrointestinal pathology 1
- Streptococcus pneumoniae, S. pyogenes, and Groups B, C, and G streptococci cause endocarditis less commonly 1
- Oral streptococci are the most common organisms in culture-negative endocarditis following prior antibiotic treatment 1
Enterococci
- Enterococcus faecalis is documented as a cause of IE, particularly associated with genitourinary and gastrointestinal procedures 1
- Enterococcus gallinarum has also been identified 1
HACEK Group Organisms
The HACEK group represents fastidious gram-negative bacteria that cause culture-negative endocarditis 1:
- Haemophilus parainfluenzae and H. aphrophilus 1
- Actinobacillus actinomycetemcomitans 1
- Cardiobacterium hominis 1
- Eikenella corrodens 1
- Kingella kingae 1
Fastidious and Culture-Negative Organisms
These organisms require special culture techniques or serological diagnosis 1:
Intracellular Bacteria
- Bartonella species (B. quintana most common, followed by B. henselae) - may account for 3% of IE cases and is the most commonly seen fastidious organism 1
- Coxiella burnetii (Q fever) 1
- Brucella species 1
- Chlamydia species 1
- Tropheryma whipplei (Whipple's disease agent) 1
- Legionella species 1
These fastidious organisms collectively account for up to 5% of all IE cases 1
Additional Oral Flora Bacteria
Extensive oral flora bacteria documented in bacteremia studies that can cause IE include 1:
- Gemella species 1
- Abiotrophia defectiva and Granulicatella species (formerly nutritionally variant streptococci) - these are technically difficult to culture and harder to cure than typical viridans streptococci 1
- Rothia species (R. dentocariosa, R. mucilaginosus) 1
- Lactobacillus species 1
- Neisseria species (non-pathogenic oral species) 1
- Corynebacterium species 1
- Capnocytophaga 1
Anaerobic Bacteria
- Peptostreptococcus species 1
- Prevotella species 1
- Fusobacterium species 1
- Propionibacterium species (including P. acnes) 1
Fungi
- Non-Candida fungi are recognized as rare causes of endocarditis, often associated with prosthetic cardiac devices and central venous catheters 1
Important Clinical Context
The classical IE pathogens (S. aureus, Streptococcus spp., and Enterococcus spp.) share specific virulence characteristics: they can adhere to damaged valves, trigger local procoagulant activity, and survive within infected vegetations by resisting platelet microbicidal proteins 1
A critical pitfall: In culture-negative endocarditis with prior antibiotic exposure, empiric therapy should cover oxacillin-susceptible staphylococci, viridans group streptococci, and enterococci for at least 6 weeks 1