Bacterial Infective Endocarditis

Abnormal Hemodynamics and Valve Turbulence

• High-velocity jets from regurgitant or stenotic lesions create endothelial injury and platelet-fibrin deposition
• Jet lesions directly damage valve tissue, creating a nidus for bacterial seeding
• Abnormal flow patterns increase vulnerability in left-sided lesions (mitral > aortic > tricuspid > pulmonary)

Bacterial Adherence and Colonization

• PAMP-TLR signaling: Pathogen-associated molecular patterns on bacterial surfaces bind toll-like receptors on endothelial cells
• Adhesin expression: Staphylococcal protein A, streptococcal M protein, and fimbriae promote bacterial attachment to damaged endocardium
• Fibronectin and collagen binding: Microorganisms express adhesins specific for extracellular matrix proteins exposed by endothelial injury
• Viridans group streptococci preferentially adhere to platelet-fibrin complexes

Vegetation Formation and Bacterial Biofilm Development

• Extracellular polysaccharide matrix: Bacteria produce glucan, dextran, or capsular material that encases organisms
• Biofilm protection: High-density bacterial aggregates exhibit phenotypic resistance to antibiotics (100-1000 fold reduced susceptibility) and host immune mechanisms
• Coagulation cascade activation: Tissue factor expression by infected endocardium triggers thrombin generation, promoting fibrin incorporation into vegetations
• Platelet aggregation: Bacterial adhesins and tissue damage stimulate platelet recruitment, enlarging vegetations

Immune and Inflammatory Responses

• Complement activation: Both classical and alternative pathways activated by bacterial antigens; C3a and C5a recruitment of neutrophils and macrophages
• Antibody formation: Development of antistaphylococcal, antistreptococcal, and anti-cardiac autoantibodies (cross-reactive epitopes)
• Immune complex deposition: Circulating immune complexes deposit in kidneys (glomerulonephritis), joints (arthritis), and skin (vasculitis), causing vascular inflammation
• TNF-α and IL-6 production: Proinflammatory cytokines drive systemic inflammatory response and organ dysfunction

Septic Embolism and Metastatic Seeding

• Fragments of infected vegetation embolize to coronary arteries, cerebral circulation, lungs, kidneys, and spleen
• Septic arteritis: Bacterial seeding within arterial walls, particularly at branch points, can lead to mycotic aneurysm formation
• Congestive heart failure: Large vegetations impair valve function; aortic regurgitation most common cause of acute hemodynamic decompensation
• Valve perforation and rupture: Progressive infection destroys valve architecture, causing acute regurgitation

Microbiology by Clinical Setting

• Streptococcus viridans (40-50%): Alpha-hemolytic group; commensal of oral flora; higher risk with poor dental hygiene, recent dental procedures, or underlying valvular disease
• Staphylococcus aureus (20-30%): Most common cause in injection drug users; high virulence and propensity for acute, aggressive disease; both MSSA and MRSA seen
• Enterococcus species (5-10%): Gram-positive, catalase-negative; associated with biliary/urinary tract procedures; Enterococcus faecalis > faecium
• HACEK group organisms (5%): Haemophilus, Aggregatibacter (formerly Actinobacillus), Cardiobacterium, Eikenella, Kingella; fastidious gram-negative rods; frequent dental source
• Streptococcus bovis/gallolyticus (5%): Associated with colonic neoplasia and polyps; warrants colonoscopy
• Nutritionally variant streptococci (Abiotrophia, Granulicatella): Fastidious; require supplemented media

• Early PVE (<2 months post-op): Staphylococcus epidermidis and aureus, gram-negative rods, fungi; typically related to surgical contamination
• Late PVE (>12 months post-op): Similar organisms to NVE (viridans streptococci), reflecting circulating bacteremia acquisition

• Prior antibiotics (most common cause)
• Fastidious organisms: HACEK, Coxiella burnetii (Q fever), Bartonella, Brucella, Legionella
• Fungi: Candida, Aspergillus, Histoplasma (especially immunocompromised)
• Tissue infections: Diagnosable only by valve culture or PCR

Major Risk Factors for IE

• Rheumatic heart disease (most common globally; mitral stenosis highest risk)
• Mitral valve prolapse with regurgitation (prolapse alone without MR is low-risk)
• Congenital heart disease (ventricular septal defect, patent ductus arteriosus, tetralogy of Fallot)
• Degenerative valve disease: Aortic stenosis, sclerotic changes with increasing age
• Bicuspid aortic valve: Predisposes to endocarditis even without significant hemodynamic obstruction
• Prosthetic valves: Both mechanical and bioprosthetic; early >late period risk
• Aortic regurgitation: Any cause increases risk independent of etiology

• Poor oral hygiene and dental disease: Most common source for viridans streptococci
• Dental procedures (extraction > prophylaxis/cleaning): Bacteremia occurs in up to 90% of extractions
• Injection drug use: Repeated skin flora inoculation; tricuspid valve most commonly affected
• Urinary tract infection/instrumentation: Enterococci and gram-negative rods
• Biliary tract procedures: Ascending cholangitis risk
• Vascular access devices: Central lines, peripheral catheters, dialysis fistulas
• Skin and soft tissue infection: Boils, abscesses, cellulitis with Staphylococcus

• Age: Both very young and elderly at higher risk
• Immunosuppression: HIV/AIDS, chemotherapy, calcineurin inhibitors
• Male gender: 2-3 fold higher risk
• Hemodialysis: Chronic kidney disease patients; Staphylococcus predominates
• Nosocomial acquisition: Healthcare-associated IE increasingly common (15-20% of cases)
• Cardiac surgery history: Particularly within 2 months (early PVE risk)

Constitutional Symptoms (Nonspecific, Often Insidious)

• Fever (90% of cases): Low-grade in subacute disease, spiking in acute; may be absent in elderly, immunocompromised, or with prosthetic valves
• Malaise and fatigue: Prolonged illness over weeks to months
• Night sweats: Often drenching; related to inflammatory cytokine release
• Weight loss: Cachexia from chronic infection
• Myalgias and arthralgias: Immune complex-mediated; large joints most common

Cardiac Manifestations

• New or changing cardiac murmur (80%): Most sensitive finding for IE
• New aortic regurgitation (early diastolic decrescendo murmur): Most common; suggests valve vegetation or destruction
• New mitral regurgitation (holosystolic apical murmur): Vegetation on anterior mitral leaflet or chordae rupture
• Absent in tricuspid endocarditis (right-sided IE)
• Congestive heart failure (20-40%): Acute onset from valve rupture or perforation; poor prognostic sign requiring urgent surgical intervention
• Arrhythmias: Atrial fibrillation, heart block (suggests septal abscess involvement)
• Pericardial friction rub: Purulent pericarditis from septic emboli to pericardium

Embolic Manifestations (30-40% of cases)

• Septic emboli to brain: Stroke (10-20%), most common complication; mycotic aneurysm rupture may cause hemorrhagic stroke; can occur during or after treatment
• Splenic infarction: Left upper quadrant pain; occur silently in up to 40%
• Renal infarction: Flank pain, hematuria; microinfarcts contribute to glomerulonephritis
• Mesenteric embolism: Abdominal pain; risk of bowel ischemia and necrosis
• Limb ischemia: From septic emboli to peripheral arteries (rare, <5%)

Vascular Phenomena (Duke Criteria)

• Osler nodes (5-10%): Painful nodules on fingertips/toe pads; small immune complex vasculitis at capillaries (NOT septic emboli)
• Janeway lesions (5-10%): Painless erythematous macules on palms/soles; septic microemboli with vasculitis
• Splinter hemorrhages (5-15%): Linear hemorrhages under nails; thought to be septic emboli (specificity only ~50% as other causes common)
• Petechiae (10-20%): Small red dots on extremities, oral mucosa, conjunctiva; immune vasculitis
• Subconjunctival hemorrhages: Petechial bleeding in conjunctiva

Mucocutaneous Manifestations

• Oral manifestations: Poor dentition, aphthous ulcers, gingival erythema
• Clubbing of nails: Rare; seen in chronic, untreated infection
• "Stigmata" of chronic IE: Weight loss, pallor, splenomegaly, hepatomegaly

Renal Manifestations

• Acute glomerulonephritis: Hematuria, proteinuria (immunocomplex mediated); usually resolves with effective antibiotic therapy
• Acute kidney injury: From septic emboli or severe vasculitis
• Microscopic hematuria: Seen in 50% of patients

Right-Sided Endocarditis (IVDU)

• Pulmonary septic emboli: Pleurisy, hemoptysis, "septic pneumonia" with wedge-shaped infiltrates
• Absence of left-sided murmurs: Tricuspid regurgitation murmur (holosystolic at left lower sternal border, increases with inspiration) may be subtle
• Lower incidence of embolic phenomena: Right-sided emboli preferentially lodge in lungs

Fever Pattern

• Acute IE (S. aureus, pneumococci): High fever, rapid clinical deterioration over days to weeks
• Subacute IE (viridans streptococci, enterococci): Low-grade fever, indolent course over weeks to months
• Atypical presentations: Elderly, immunocompromised, or early-treated patients may lack fever

Clinical Suspicion and Diagnostic Algorithm

• IE should be suspected in any patient with persistent bacteremia, fever + new cardiac murmur, fever + prosthetic valve, septic emboli, or injection drug use with fever
• Absence of fever does not exclude IE (5-10% afebrile, especially elderly or prosthetic valve)

Blood Cultures

• Gold standard: Obtain 3-4 sets before antibiotics (each 10 mL from separate venipuncture at different sites/times)
• Timing: Culture should be taken over 1-2 hours before initiating empiric therapy; do NOT delay antibiotics for blood cultures in acute, hemodynamically unstable patients
• Interpretation:
• Positive cultures: Same organism in multiple sets diagnostic; contaminating organisms (e.g., coagulase-negative staphylococci) require ≥2 positive cultures from separate draws
• Negative cultures: In 5-10% of cases; causes include prior antibiotics, fastidious organisms, fungi, or atypical bacteria (HACEK, Bartonella, Coxiella)
• Recommendation: Blood cultures positive in 85-90% of NVE, lower in PVE; repeat cultures if initial negative and high clinical suspicion

Laboratory Findings

• CBC: Anemia (60-70%), from chronic infection; leukocytosis (mild to moderate)
• ESR/CRP: Markedly elevated, non-specific but supports inflammatory diagnosis
• Rheumatoid factor: Positive in 50% (immune complex formation)
• Cryoglobulins: May be elevated
• Prothrombin time (PT) and partial thromboplastin time (PTT): Prolonged in severe disease (DIC, liver involvement)
• Renal function: Elevated creatinine from glomerulonephritis or embolic infarction
• Urinalysis: Hematuria and proteinuria (50% with RBC casts)
• Liver function tests: Mild elevation from septic emboli or hepatitis

Echocardiography (TTE and TEE)

• Vegetation: Oscillating mass attached to valve; echogenic, often mobile
• Sensitivity: 50-60% for NVE (limited by acoustic windows), lower for PVE and right-sided lesions
• Valve regurgitation: New or worsening aortic/mitral regurgitation
• Chamber enlargement: Chronic volume overload
• Abscess: Low sensitivity on TTE; appears as areas of decreased echogenicity or disruption of normal anatomy
• Perforation/rupture: Flail leaflet segments, large eccentric regurgitant jets

• Sensitivity: 90-95% for detection of vegetations, much superior to TTE
• Key advantage: Better visualization of prosthetic valves, abscesses, and septal involvement
• Indications:
• All patients with prosthetic valves and suspected IE
• Suspected abscesses or severe complications
• Negative TTE with high clinical suspicion
• Persistent fever/positive cultures despite antibiotics
• Recurrent emboli
• Findings: Vegetations, annular abscesses, new regurgitation, paravalvular leaks, perforations
• TEE normal: Does not exclude IE (10% false negatives); diagnosis rests on clinical and microbiologic criteria

Modified Duke Criteria for Diagnosis of Infective Endocarditis

1. 1.Positive blood culture:

• Typical organism (viridans streptococci, S. bovis, HACEK group, S. aureus, Enterococcus) in 2 separate blood cultures, OR
• Coxiella burnetii or Bartonella detected by serology or PCR

1. 1.Imaging findings:

• TTE/TEE showing oscillating intracardiac mass on valve or supporting structure, vegetation, abscess, or new paravalvular leak, OR
• Abnormal activity on nuclear imaging

1. 1.Predisposing heart condition (valve disease, prosthetic valve, IVDU with right heart disease)

Immediate priorities

• Obtain blood cultures, then start antibiotics: per the AHA 2015 Infective Endocarditis scientific statement, 3 sets from separate sites precede therapy — but in septic shock, acute valvular failure, or embolic stroke, empiric therapy proceeds after cultures are drawn without waiting for results.
• Hemodynamic support: acute aortic or mitral regurgitation causes flash pulmonary edema poorly tolerated by a non-dilated ventricle. Afterload reduction and diuresis temporize; intra-aortic balloon counterpulsation is contraindicated in severe AR. Surgical consultation should be simultaneous, not sequential.

Empiric therapy (culture pending)

• Glycopeptide + a beta-lactam: vancomycin covers MRSA and enterococci; ceftriaxone (native valve) or cefepime/gentamicin plus rifampin (prosthetic valve) broadens gram-negative and staphylococcal biofilm coverage. All therapy is parenteral and bactericidal — biofilm-embedded organisms are metabolically shielded, so bacteriostatic agents fail.

Pathogen-directed therapy (AHA/IDSA)

• MSSA: antistaphylococcal penicillin (nafcillin) or cefazolin — narrow beta-lactams outperform vancomycin; de-escalate promptly.
• MRSA: vancomycin, or daptomycin as an alternative.
• Viridans streptococci / S. gallolyticus: penicillin G or ceftriaxone, typically 4 weeks.
• Enterococcus faecalis: ampicillin plus ceftriaxone, or ampicillin plus gentamicin, for synergistic killing.
• Prosthetic valve staphylococcal IE: vancomycin or nafcillin plus rifampin (penetrates biofilm) plus gentamicin for the initial weeks.
• Total duration is generally 6 weeks for prosthetic valve, enterococcal, and staphylococcal disease.

Surgery (ACC/AHA 2020 Valvular Heart Disease Guideline)

• Early surgery indications: heart failure from valve dysfunction; perivalvular abscess, fistula, or new heart block; fungal or highly resistant organisms; persistent bacteremia/fever despite appropriate antibiotics; prosthetic valve dehiscence; recurrent emboli with a persistent large mobile vegetation.

Avoid

• Anticoagulation or aspirin added solely to shrink vegetations — no embolic benefit and increased intracranial hemorrhage.
• Aminoglycosides for native-valve staphylococcal IE — nephrotoxicity without benefit.
• Oral or outpatient step-down in unstable, abscess-complicated, or non-adherent patients.

Cardiac (most common cause of death)

• Acute valvular regurgitation with heart failure — emergency: leaflet perforation or chordal rupture imposes acute volume load on a non-compliant ventricle; signaled by new decrescendo diastolic murmur, pulmonary edema, and narrow pulse pressure. Strongest indication for early surgery per the ACC/AHA valvular guideline.
• Perivalvular abscess with new conduction block — emergency: infection burrows into the aortic annulus adjacent to the AV node/His bundle; signaled by new first-degree AV block progressing to complete block, or persistent fever on appropriate antibiotics. Next step is TEE, not repeat TTE.
• Fistula or pseudoaneurysm: annular destruction creating a shunt; new continuous murmur.
• Purulent pericarditis / myocardial abscess: contiguous spread; friction rub, chest pain.

Embolic

• Ischemic stroke — emergency: vegetation fragment occludes a cerebral vessel; highest risk before and during the first week of antibiotics, falling sharply thereafter.
• Mycotic aneurysm rupture — emergency: septic arteritis weakens the vessel wall at branch points; presents as sudden headache, subarachnoid or intraparenchymal hemorrhage. Obtain vascular imaging before considering any anticoagulation or cardiac surgery.
• Splenic abscess: persistent LUQ pain and relapsing fever after adequate therapy; requires drainage or splenectomy — antibiotics alone often insufficient.
• Vertebral osteomyelitis/discitis and septic arthritis: metastatic seeding, especially S. aureus; new focal back pain.
• Right-sided IE: septic pulmonary emboli with wedge-shaped cavitating infiltrates and hemoptysis.

Immune-mediated

• Immune-complex glomerulonephritis: hematuria, RBC casts, low complement; typically resolves with cure of infection.

Treatment-related

• Vancomycin: acute kidney injury (dose/trough-dependent), infusion reaction.
• Gentamicin: nephrotoxicity and irreversible vestibulotoxicity — the reason it is avoided in native-valve staph IE.
• Daptomycin: CK elevation/myopathy and eosinophilic pneumonia.
• Rifampin: hepatotoxicity and potent CYP induction (warfarin failure).
• Beta-lactams: acute interstitial nephritis, drug fever mimicking treatment failure, Clostridioides difficile colitis; long-term catheter causes line infection and thrombosis.

• New AV block in a febrile bacteremic patient means perivalvular abscess: the single best next step is transesophageal echocardiography, and surgical consultation follows — not simply broadening antibiotics. A normal TTE never excludes an abscess.
• Streptococcus gallolyticus (bovis) bacteremia mandates colonoscopy: the association with colonic adenoma/carcinoma is the classic examiner's link. S. bovis in the blood means look in the bowel.
• Staphylococcus aureus is the answer for acute IE in injection drug use, with tricuspid involvement, septic pulmonary emboli, and often no audible murmur. Right-sided IE spares the systemic circulation, so Osler nodes and Roth spots are typically absent.
• De-escalate MSSA to nafcillin/oxacillin or cefazolin: vancomycin is inferior for methicillin-susceptible strains. Continuing vancomycin after susceptibilities return is a favorite wrong answer.
• Osler nodes are painful and immune-complex mediated; Janeway lesions are painless and embolic — the mnemonic "Osler = Ouch" separates them. Both count as minor Duke criteria only.
• Do not add anticoagulation or aspirin to shrink a vegetation: no reduction in embolism and increased intracranial hemorrhage, particularly with mycotic aneurysm. Existing anticoagulation for a mechanical valve is managed cautiously, not reflexively continued after a stroke.
• Antibiotic prophylaxis is narrow: per the AHA/ACC recommendation, amoxicillin 2 g orally one hour before dental procedures involving gingival manipulation applies only to prosthetic valves/prosthetic repair material, prior IE, certain unrepaired or recently repaired congenital heart disease, and cardiac transplant valvulopathy. Mitral valve prolapse, bicuspid aortic valve, and hypertrophic cardiomyopathy are not indications — a frequent distractor.
• Culture-negative IE with negative serial cultures: think prior antibiotics first, then Coxiella burnetii and Bartonella (diagnosed serologically or by valve PCR). Culture-negative vegetations without fever suggest nonbacterial thrombotic (marantic) endocarditis from malignancy or Libman–Sacks lesions in SLE.