What it is:Infective endocarditis (IE) is infection of the heart valves (or other endocardial surfaces, including implanted defibrillators and pacemakers), almost always bacterial. Bacteremia is the broader, simpler problem underneath it: bacteria circulating in what should be a sterile bloodstream, confirmed by a true-positive blood culture. Every case of IE involves bacteremia. Not every bacteremia becomes IE.
The core problem:Damaged or turbulent endocardium collects platelets and fibrin, bacteremia seeds that surface, and the resulting vegetation is a self-sealing shelter: dense fibrin packed with bacteria that neither your immune system nor a normal antibiotic dose can penetrate well.
What you do about it:Find the organism, then hit it with high-dose, bactericidal, parenteral antibiotics for weeks, not days. Surgery removes what antibiotics can't reach. For plain bacteremia without endocardial involvement, the job is shorter: eradicate the organism, find and control the source, and stop.
Think of the vegetation as a walled fortress built out of the patient's own clotting system. Bacteria hiding inside it are metabolically sluggish and physically shielded, which is exactly why treatment needs bactericidal(not just bacteriostatic) drugs, high doses, and an extended duration, even against organisms that look fully susceptible on a culture plate.
Three overlapping ways to describe the same infection, and each one changes what you expect and how long you treat.
| Axis | Categories | Why it matters |
|---|---|---|
| Clinical tempo | Acute vs subacute | Acute bacterial endocarditis is fulminant: high fevers, systemic toxicity, and possible death within days to weeks if untreated. Subacute IE is indolent, usually on top of prior valvular disease. |
| Valve involved | Native valve endocarditis (NVE) vs prosthetic valve endocarditis (PVE) | PVE within the first year after valve surgery is mostly staphylococcal (S. aureus and coagulase-negative staph), and durations run longer across the board. |
| Diagnostic certainty | Definite IE vs possible IE (Modified Duke criteria) | Drives whether you commit to a full extended antimicrobial course. |
Endocarditisis a diagnosis (infection of the endocardium/valve). Bacteremiais a lab finding (positive blood culture). IE always produces bacteremia because the vegetation continuously sheds bacteria into the blood, but a positive culture alone doesn't mean someone has endocarditis. That distinction is why the chapter, and this document, covers both.
Everything about IE treatment, the drug choice, the huge doses, the multi-week duration, traces back to how the vegetation forms and what it does to antibiotic access.
Because bacteria are enclosed within valvular vegetations and fibrin deposits, they are physically insulated from both host defenses and drug penetration. That single fact explains the entire treatment philosophy: large parenteral doses (not oral, you need to force bactericidal concentrations into an avascular clot), bactericidal drugs specifically (a bacteriostatic drug just holds the line while immune cells that can't reach the vegetation do nothing), and weeks of therapy even for a fully susceptible organism, because killing bacteria embedded in fibrin is slow no matter how good the drug is.
The vegetation continuously sheds organisms into the bloodstream, which is why IE produces a continuousbacteremia rather than an intermittent one, and why 90 to 95 percent of patients with IE have positive blood cultures. It's also why persistent positive cultures during therapy are such an alarming sign: the vegetation is still actively seeding.
| Tier | Factors |
|---|---|
| Highest risk | Prosthetic heart valve or prior history of IE |
| Elevated risk | Congenital heart disease, advanced age, chronic IV access, diabetes mellitus, acquired valvular dysfunction (eg, rheumatic heart disease), cardiac implantable electronic device, chronic heart failure, mitral valve prolapse with regurgitation, IV drug abuse, HIV infection, poor dentition or oral hygiene |
Presentation is variable and often nonspecific, which is exactly why it gets missed early. Fever is the single most common finding, present in over 90% of patients.The mitral and aortic valves are affected most often. Onset is usually insidious, worsening gradually rather than announcing itself.
| Finding | Looks like | Mechanism |
|---|---|---|
| Osler nodes | Purple, painful, tender papules on finger and toe pads | Not specific for IE |
| Janeway lesions | Hemorrhagic, painless plaques on palms and soles | Embolic process |
| Splinter hemorrhages | Thin, linear hemorrhages under the nail bed | Not specific for IE, trauma causes these too |
| Petechiae | Small, red, painless hemorrhagic spots, clustered on skin, mouth, or eyelids | Microembolic/vascular phenomenon |
| Clubbing of fingers | Chronic finding | Reflects a more indolent, longer-standing course |
| Roth spots | Retinal hemorrhages with a pale center | Usually seen in IE, but not exclusive to it |
| Emboli | Variable, depends on where they land | Direct fragmentation of the vegetation, drives complications |
Osler nodes hurt, Janeway lesions don't.Osler on the pads of the fingers and toes, tender. Janeway on the palms and soles, painless and embolic. Both show up in subacute disease; neither is exclusive to IE.
Without antimicrobial therapy (and surgery when indicated), IE is usually fatal. With proper management, most patients recover. Factors linked to higher mortality: heart failure, increasing age, resistant organisms (gram-negatives, fungi), left-sided IE caused by S. aureus, paravalvular complications, healthcare-acquired infection, and PVE.
Bacteremia can look completely different from IE. Findings may include fever, chills, rigors, altered hemodynamics, shock, coagulation abnormalities, cutaneous findings like ulcerations, and a documented or suspected primary source of infection.
Patients with bacteremia may be completely normothermic with a normal white blood cell count. Neither leukocytosis nor fever (≥38°C), alone or together, reliably predicts the presence of bacteremia.A calm-looking vital sign panel does not clear a patient.
The hallmark of IE is continuous bacteremia from the vegetation shedding organisms, so 90 to 95 percent of patients with IE have positive blood cultures. This is both the most important diagnostic clue and the main tool for tracking response to therapy.
Echo should be performed in every patient suspected of IE. It identifies vegetations and complications and directly informs surgical decision-making.
| Modality | Sensitivity for vegetations | Trade-off |
|---|---|---|
| TTE(transthoracic) | 40%–65% | Noninvasive, first-line, but can miss smaller or posterior vegetations |
| TEE(transesophageal) | 90%–100% | More sensitive, but invasive; used when TTE is negative but suspicion stays high, or in higher-risk patients |
Diagnosis is anchored to the Modified Duke criteria (updated in 2023 by the Duke-International Society for Cardiovascular Infectious Diseases). Two major findings carry the most weight: persistent bacteremiawith a typical organism, and echocardiographic evidenceof endocardial involvement. Predisposing cardiac conditions, fever, the vascular and immunologic stigmata covered above, and cultures that don't meet the major threshold all fold in as supporting (minor) findings. The combination sorts patients into definite IEor possible IE, and in practice, blood cultures plus echo are the two tests that actually make that call.
Bacteremia is identified purely by a true-positive blood culture, there's no equivalent scoring system. The diagnostic work shifts to finding the primary source (and any secondary, metastatic foci) rather than characterizing valve involvement.
Surgical removal of infected tissue and valve repair/replacement is an important adjunct in both NVE and PVE, not a last resort. Indications include heart failure, persistent bacteremia, persistent or growing vegetation, recurrent emboli despite prolonged antibiotics, valve dysfunction, paravalvular extension (abscess), or IE caused by resistant organisms.Antibiotics alone can't fix mechanical valve destruction or sterilize an abscess.
Streptococci are a common cause of IE, and most isolates are viridans group streptococci. MIC to penicillin should be determined for every viridans strep isolate, since it directly dictates the regimen.
| Susceptibility | Regimen | Duration |
|---|---|---|
| Fully susceptible (MIC ≤0.12 mcg/mL) | Penicillin G or ceftriaxone alone | 4 weeks |
| Penicillin G or ceftriaxone + gentamicin | 2 weeks | |
| Vancomycin (penicillin-allergic only) | 4 weeks | |
| Relatively resistant (MIC >0.12–0.5 mcg/mL) | Penicillin G or ceftriaxone + gentamicin (first 2 weeks), then penicillin/ceftriaxone alone | 4 weeks total |
| PVE, viridans strep / S. bovis | Extend the applicable regimen | 6 weeks |
Even with a fully susceptible organism, avoid the 2-week combo regimen in: most patients over 65, children, anyone with eighth cranial nerve impairment, CrCl <20 mL/min, known cardiac or extracardiac abscess, or infection with Abiotrophia, Granulicatella, or Gemella species. These patients need the full 4-week course.
Immediate-type penicillin hypersensitivity → vancomycin. And when vancomycin is used for viridans strep, don't add gentamicin, that combination isn't recommended here (contrast this with staph and enterococcal regimens, where combination therapy is often the point).
Staphylococci, especially S. aureus, are the most common overall cause of IE, driven by IV drug abuse, more frequent central and peripheral catheter use, and more valve replacement surgery. Coagulase-negative staph (usually S. epidermidis) is a prominent cause of PVE specifically.
| Scenario | Regimen | Duration |
|---|---|---|
| MSSA, left-sided IE | Nafcillin or oxacillin | 6 weeks |
| Mild, delayed penicillin allergy | Cefazolin (avoid if immediate-type hypersensitivity) | 6 weeks |
| Immediate-type penicillin allergy | Vancomycin (or daptomycin 6 mg/kg/day) | 6 weeks |
| MRSA or methicillin-resistant CoNS | Vancomycin (daptomycin 6 mg/kg/day is a recommended alternative) | 6 weeks |
Vancomycin kills S. aureus slowly and is generally regarded as inferior to the penicillinase-resistant penicillins for MSSA. Use it only when true penicillin allergy forces the switch. If a penicillin-allergic patient fails vancomycin, consider penicillin desensitization rather than just pushing on.
IV drug abuse:60%–70% of IE in people who inject drugs is caused by S. aureus, though other organisms turn up more in some geographic areas. The standard regimen shortens here: a 2-week course of nafcillin, oxacillin, or daptomycin, without an aminoglycoside.If vancomycin is the chosen agent instead, you go back to the standard 6-week course, the short regimen is specific to the nafcillin/oxacillin/daptomycin options.
Prosthetic valve endocarditis:PVE occurring within 2 months of cardiac surgery is usually caused by staphylococci implanted at the time of surgery, and methicillin resistance is common. Vancomycin is the cornerstone. Because of the high morbidity, mortality, and refractoriness of staph PVE, combination therapy is the norm, not the exception.
Enterococci are the third leading cause of IE and behave differently from strep or staph in ways that change the whole strategy:
| Scenario | Regimen | Duration |
|---|---|---|
| Ampicillin/penicillin/vancomycin-susceptible, symptoms <3 months, native valve | Ampicillin + gentamicin, or penicillin G + gentamicin | 4 weeks |
| Symptoms >3 months, or PVE | Same combinations | 6 weeks |
| CrCl <50 mL/min (baseline or on a gentamicin-containing regimen) | Ampicillin + ceftriaxone (as effective as ampicillin + gentamicin, avoids aminoglycoside toxicity) | 6 weeks |
| Can't tolerate penicillin/ampicillin | Vancomycin + gentamicin | 6 weeks |
| Gentamicin-resistant strain | Streptomycin in place of gentamicin, if CrCl >50, CN VIII intact, and rapid strep levels are available | per above |
| β-lactamase-producing strain (often E. faecium) | Ampicillin-sulbactam + gentamicin | 6 weeks |
| Intrinsic penicillin resistance | Vancomycin + gentamicin | 6 weeks |
| E. faecium resistant to penicillin, aminoglycosides, AND vancomycin | Linezolid or daptomycin | >6 weeks |
For E. faecium resistant to penicillin, aminoglycosides, and vancomycin, antimicrobial cure rates can be under 50%, and bacteriologic cure may only come with valve replacement. These patients need a multidisciplinary team (cardiology, cardiovascular surgery, infectious disease, clinical pharmacy). Note the aminoglycoside dosing quirk here too: relatively low serum concentrations are actually adequate for enterococcal synergy, a gentamicin peak around 3–4 mcg/mL, so don't chase the higher peaks you'd use for other purposes.
All doses assume normal renal function; adjust for renal impairment. Doses reflect adult treatment dosing for IE, not the prophylaxis regimens (see below).
| Drug | Adult dose | Notes |
|---|---|---|
| Beta-lactams | ||
| Ampicillin | 2 g IV every 4 hours | May give as continuous infusion, 12 g IV/24h |
| Ampicillin-sulbactam | 3 g IV every 6 hours | |
| Aqueous crystalline penicillin G | 3–4 million units IV every 4 hours (dose depends on indication/MIC) | May give as continuous infusion, 12–24 million units IV/24h |
| Nafcillin or oxacillin | 2 g IV every 4 hours | May give as continuous infusion, 12 g IV/24h |
| Cefazolin | 2 g IV every 8 hours | |
| Ceftriaxone | 2 g IV or IM every 24 hours | 2 g every 12h for E. faecalis specifically |
| Cefepime | 2 g IV every 8 hours | |
| Glycopeptides / lipopeptides / oxazolidinones | ||
| Vancomycin | 15–20 mg/kg IV every 8–12 hours | Loading dose 25–30 mg/kg may be used; use actual body weight; single dose should not exceed 2 g |
| Daptomycin | ≥8 mg/kg IV every 24 hours | Doses up to 10–12 mg/kg/day used for resistant enterococcus; use actual body weight |
| Linezolid | 600 mg IV or orally every 12 hours | |
| Aminoglycosides | ||
| Gentamicin | 3 mg/kg IV/IM every 24h, or 1 mg/kg IV/IM every 8h | Once-daily dosing only for streptococcal infections; use ideal/adjusted body weight for divided dosing if actual weight >120% ideal |
| Streptomycin | 7.5 mg/kg IV/IM every 12 hours | Substitute for gentamicin-resistant enterococcal strains |
| Other | ||
| Ciprofloxacin | 400 mg IV every 12h, or 500 mg orally every 12h | Avoid in patients <18 years when possible |
| Doxycycline | 100 mg IV or orally every 12 hours | |
| Rifampin | 300 mg IV or orally every 8 hours | Never start until blood cultures have cleared, resistance risk |
| Population | What changes |
|---|---|
| IV drug use | S. aureus causes 60%–70% of cases. Shortened 2-week regimen with nafcillin, oxacillin, or daptomycin, without an aminoglycoside, is appropriate. Choosing vancomycin instead means reverting to the full 6-week standard course. |
| PVE within 2 months of surgery | Usually staphylococci implanted at the time of surgery; methicillin resistance is common. Vancomycin-based combination therapy is the default given the high morbidity and mortality of staph PVE. |
| Renal impairment (CrCl <50) | For enterococcal IE, ampicillin + ceftriaxone is the recommended substitution for a gentamicin-containing regimen, equally effective and avoids aminoglycoside toxicity. |
| Multidrug-resistant E. faecium | Cure rates with antibiotics alone may be under 50%. Needs a multidisciplinary team and possibly valve replacement. |
The goal is diminishing the chance of IE in high-risk patients undergoing procedures that cause transient bacteremia. The evidence base is genuinely thin, the literature lacks solid proof either way, and using antimicrobials for this purpose remains a matter of common practice more than settled science.
Prosthetic heart valve, prosthetic material used for valve repair, a prior diagnosis of IE, cardiac transplantation with subsequent valvulopathy, and specific congenital heart disease (unrepaired cyanotic CHD, the first 6 months after prosthetic material is used to repair a congenital defect, or repaired CHD with a residual defect at or next to prosthetic material). It is nota blanket recommendation for anyone with a heart murmur or mild valve disease.
Qualifying procedures:dental procedures involving perforation of the oral mucosa or manipulation of the gingival tissue or periapical region of the teeth; invasive respiratory procedures involving an incision or biopsy; and invasive procedures involving infected skin, skin structures, or musculoskeletal tissue.
| Regimen | Adult dose | When to use |
|---|---|---|
| Oral amoxicillin | 2 g | Standard first choice |
| IM or IV ampicillin | 2 g | Can't tolerate oral |
| IM or IV cefazolin or ceftriaxone | 1 g | Nonimmediate PCN allergy, can't tolerate oral; avoid if immediate-type hypersensitivity |
| Oral cephalexin | 2 g | Nonimmediate PCN allergy; other 1st/2nd gen cephalosporins may be substituted at an equivalent dose |
| Oral azithromycin or clarithromycin | 500 mg | Nonimmediate PCN allergy |
| IV or IM clindamycin | 600 mg | Nonimmediate PCN allergy, can't tolerate oral |
All of these are single, one-time doses given 30 to 60 minutes before the procedure.
| Parameter | When | Watching for |
|---|---|---|
| Blood cultures | Rechecked frequently after starting therapy until negative, then infrequently for the rest of the course | Sterilization should occur within a few days; continued positive cultures beyond the first few days suggests inadequate drug activity or inadequate dosing (vancomycin's response is characteristically slower) |
| Fever curve | Daily during the first week+ | Fever persisting beyond 1 week may mean ineffective therapy, an embolic event, an infected IV catheter, or a drug reaction, though fever can occasionally persist despite appropriate treatment |
| MICs from blood isolates | At diagnosis and any time cultures remain positive | MICs, not MBCs, are the standard for guiding therapy |
| Signs/symptoms and organ function | Throughout therapy | Overall response and drug toxicity |
| Serum drug concentrations | Vancomycin and aminoglycosides especially | Efficacy and toxicity, aminoglycosides need attention to eighth cranial nerve (hearing/balance) and renal function |