Infective Enocarditis Microbiology for theory exam so give me a side heading and detaied explanation

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Infective Endocarditis (IE) - Microbiology

1. Introduction

Infective endocarditis (IE) is a microbial infection of the endocardial surface of the heart, most commonly the cardiac valves, that leads to the formation of vegetations composed of thrombotic debris, fibrin, platelets, and microorganisms. Although fungi, rickettsiae, and chlamydial species can cause endocarditis, the vast majority of cases are caused by bacteria.

The most common causative organisms worldwide are staphylococci, streptococci, and enterococci. The frequency of each depends heavily on the clinical setting (community vs. healthcare, native vs. prosthetic valve, IV drug use).

2. Classification of IE by Clinical Course and Microbiology

Type	Course	Causative Organisms
Acute IE	Rapid, destructive; high morbidity and mortality	Staphylococcus aureus, Group A streptococci, Streptococcus pneumoniae
Subacute IE	Insidious onset; weeks to months; good prognosis with treatment	Viridans group streptococci, HACEK organisms, Enterococci, CoNS

3. Causative Organisms - Detailed Classification

3a. Gram-Positive Cocci (Most Common Group)

(i) Staphylococci

Staphylococcus aureus is currently the single most common cause of IE in most high-income countries, particularly in healthcare settings and IV drug users (IVDU).

Causes acute IE - attacks healthy as well as deformed/damaged valves
Major pathogen in healthcare-associated IE (IV catheters, haemodialysis lines, pacemakers, cardiovascular implantable electronic devices)
In IVDU: most common organism; predilects the tricuspid valve (right-sided IE)
High complication rate: perivalvular abscess, purulent pericarditis, systemic emboli, metastatic infection
MRSA (methicillin-resistant S. aureus) is increasingly prevalent and complicates management

Coagulase-Negative Staphylococci (CoNS) - primarily S. epidermidis:

Most common organism in prosthetic valve endocarditis (PVE), especially early PVE (<2 months post-surgery)
Predilection for hardware/biomaterial; forms biofilms
Also seen in nosocomial/device-related IE
Treated with vancomycin (for methicillin-resistant strains) + rifampicin + gentamicin

(ii) Streptococci

Viridans Group Streptococci (VGS):

Formerly the most common overall cause of IE; still the leading cause in developing countries and community-acquired IE on damaged or deformed valves
Normal commensals of the oral cavity; enter bloodstream during dental procedures, oral trauma, or routine activities (brushing teeth)
Cause classic subacute IE - insidious onset, protracted course
Common species: S. mitis, S. sanguis, S. mutans, S. salivarius, S. milleri group
Most commonly infect the mitral and aortic valves
Generally penicillin-sensitive (some intermediate/resistant strains exist)
Frequency decreasing in industrialized nations with rising healthcare-associated IE

Streptococcus bovis (now S. gallolyticus):

Causes subacute IE; entry is from the gastrointestinal tract
Important association: S. bovis bacteremia and IE are strongly associated with underlying colorectal malignancy (colon cancer or polyps) - colonoscopy is mandatory
S. bovis group II (S. pasteurianus) has the strongest colorectal cancer association

Nutritionally Variant Streptococci (NVS) / Abiotrophia and Granulicatella spp.:

Formerly called "satelliting streptococci" or nutritionally deficient streptococci
Require pyridoxal (vitamin B6) supplementation in culture media to grow
Cause subacute IE, often culture-negative on standard media
High relapse rates compared to typical VGS

Non-Viridans / Beta-Hemolytic Streptococci (Groups A, B, C, G):

Group B (S. agalactiae): seen in elderly, diabetics, IV drug users; causes acute aggressive IE
Group A (S. pyogenes): uncommon, acute IE
Non-viridans streptococci cause ~10% of late PVE

(iii) Enterococci

Enterococcus faecalis is the most common species (>90% of enterococcal IE)
E. faecium less common but often more drug-resistant
Cause both native valve and prosthetic valve IE
Primarily affect elderly or debilitated patients, those with GI/GU tract manipulation or disease
Portal of entry: genitourinary (UTI, cystoscopy, prostate surgery) or GI tract
Present as subacute IE
Notable intrinsic resistance to many antibiotics including low-level resistance to aminoglycosides and cephalosporins; combination therapy (penicillin/ampicillin + aminoglycoside) is required for synergistic bactericidal activity
Vancomycin-resistant Enterococcus (VRE) is a growing concern

3b. Gram-Negative Bacteria

(i) HACEK Group Organisms

HACEK is an acronym for a group of fastidious, slow-growing, gram-negative bacilli that are normal commensals of the oral cavity and upper respiratory tract:

Letter	Organism
H	Haemophilus spp. (especially H. parainfluenzae)
A	Aggregatibacter spp. (formerly Actinobacillus actinomycetemcomitans)
C	Cardiobacterium hominis
E	Eikenella corrodens
K	Kingella kingae

Key features of HACEK IE:

Account for 0.8-6% of all IE cases
Require CO2-enriched atmosphere (5-10% CO2) for growth; do NOT grow on MacConkey agar
Cause slowly progressive subacute endocarditis (except Kingella, which may be more aggressive)
Occur in patients with pre-existing valvular disease or prosthetic valves, often after a recent dental procedure or nasopharyngeal infection
Most commonly affect the aortic and mitral valves
Occur in younger patients compared to non-HACEK IE
High embolic rate: systemic embolization in 28-71% of cases; large vegetations visible on echocardiography (up to 85%)
Aggregatibacter and Haemophilus - mitral valve vegetations; Cardiobacterium - aortic valve vegetations
Prognosis: excellent - significantly better than non-HACEK endocarditis
Diagnosis: cultures positive within first week with modern systems (BACTEC); PCR (16S rRNA gene amplification) and MALDI-TOF are useful adjuncts
Treatment: Ceftriaxone or levofloxacin (ampicillin-sulbactam also used); beta-lactamase production may occur in Haemophilus and Aggregatibacter

Individual HACEK organisms:

H. parainfluenzae: Most common HACEK species in IE; mitral valve; large vegetations; septic emboli including CNS emboli
Aggregatibacter actinomycetemcomitans: 62-100% of bacteremia cases have associated IE
Cardiobacterium hominis: aortic valve predilection; highly specific for IE
Eikenella corrodens: bacteremia rarely causes IE (0-6%)
Kingella kingae: more common in children; may cause aggressive IE

(ii) Other Gram-Negative Bacteria

Enterobacteriaceae (e.g., Klebsiella, E. coli, Enterobacter, Proteus): rare causes; seen in IVDU, immunocompromised, or neonates
Pseudomonas aeruginosa: associated with IVDU; causes aggressive acute IE; very difficult to treat
Both are rare but significant in the setting of IVDU or nosocomial IE

3c. Fungi

Rare cause of IE but associated with high morbidity and mortality
Risk factors: prolonged IV catheterization, IV drug use, prolonged antibiotic therapy, immunosuppression (organ transplant, HIV), parenteral nutrition, prosthetic heart valves
Most common fungi: Candida spp. (most common overall), Aspergillus spp.
Candida albicans and C. parapsilosis most frequent in IV drug users and prosthetic valve IE
Vegetations are typically large - high embolic risk
Diagnosis: blood cultures (often negative for Aspergillus); need special media
Treatment: antifungal therapy (amphotericin B, azoles, echinocandins) + often surgical valve replacement because medical therapy alone is usually insufficient
Prognosis: very poor

3d. Culture-Negative Endocarditis

Definition: IE in which no organism is isolated from blood cultures despite adequate sampling. Accounts for approximately 10% of all IE cases.

Causes:

Cause	Details
Prior antibiotic therapy	Most common cause - organisms already suppressed
Coxiella burnetii	Q fever endocarditis; exposure to farm animals, cattle, unpasteurized milk; diagnosed by serology (phase I IgG antibody titer ≥1:800)
Bartonella henselae	Cat exposure (cat scratch disease); diagnosed by serology/PCR
Bartonella quintana	Body lice; seen in homeless patients
Brucella spp.	Exposure to goats, wild pigs, unpasteurized dairy products
Tropheryma whipplei	Whipple's disease; diagnosed by PCR
Legionella spp.	Environmental water exposure
Chlamydophila psittaci	Pet bird exposure
Abiotrophia / Granulicatella	NVS; require special culture media
Fungi (especially Aspergillus)	Often blood culture negative

Approach to culture-negative IE:

Notify the microbiology lab to use prolonged (>5 days) incubation and special culture techniques
Serological testing for Coxiella, Bartonella, Brucella
PCR on blood or resected valve tissue (16S rRNA gene amplification)
MALDI-TOF mass spectrometry on tissue
Careful history of animal and environmental exposures is key

4. Microbiology by Clinical Setting

Clinical Setting	Most Common Organisms
Community-acquired NVE	Viridans streptococci (50-60%), S. aureus
Healthcare-associated IE	S. aureus (dominant), CoNS, Enterococci, gram-negative bacilli
IV Drug Users (IVDU)	S. aureus (most common, esp. tricuspid valve); also Streptococci, Enterococci, Pseudomonas, Candida
Early Prosthetic Valve IE (<2 months)	S. aureus, CoNS (S. epidermidis), gram-negative bacilli, Enterococci, Fungi, Diphtheroids
Late Prosthetic Valve IE (≥2 months)	CoNS, S. aureus, Viridans streptococci, Enterococci, Mycobacterium chimaera
Nosocomial IE	S. aureus, CoNS, Enterococci (from lines, catheters, devices)
Developing countries	Viridans streptococci still dominant; rheumatic valve disease predisposes
Children (congenital heart disease)	Viridans streptococci, S. aureus, CoNS, Kingella kingae

5. Pathogenesis of Microbial Seeding

The sequence of events for IE development:

Endothelial damage (turbulent blood flow, instrumentation, IV catheters)
Non-bacterial thrombotic endocarditis (NBTE) - sterile platelet-fibrin vegetation forms
Transient bacteremia - organisms enter the bloodstream (dental procedure, invasive procedure, skin/GI/GU source, IVDU)
Microbial adherence - Staphylococci and streptococci adhere more avidly to vegetations via surface adhesins (e.g., fibronectin-binding proteins in S. aureus)
Microbial proliferation - organisms multiply within avascular vegetation, reaching 10^9 to 10^11 CFU/gram of tissue
Biofilm formation - especially CoNS on prosthetic material
Metastatic seeding to kidneys, spleen, brain, and other highly perfused organs

6. Special Microbiological Points for Theory Exam

S. aureus has surpassed viridans streptococci as the single most common cause of IE worldwide in high-income countries
Viridans streptococci remain the most common cause in developing countries and in patients with pre-existing valvular disease
S. bovis IE mandates colonoscopy to exclude colorectal malignancy
HACEK organisms are the most common gram-negative cause of IE and are characterized by fastidious growth requiring CO2
Enterococcal IE requires combination therapy for bactericidal effect
Fungal IE almost always requires surgery in addition to antifungal drugs
Culture-negative IE most commonly results from prior antibiotic therapy; Coxiella burnetii (Q fever) is the most common specific organism in serologically confirmed culture-negative IE
Right-sided IE (tricuspid valve) is the hallmark of IVDU; S. aureus is the dominant pathogen
Early PVE is typically nosocomial (S. aureus, CoNS); Late PVE resembles native valve IE (VGS, CoNS, Enterococci)
Organisms achieve high bacterial loads in vegetations (up to 10^11 CFU/g), and the avascular nature of vegetations makes antibiotic penetration and immune clearance difficult, necessitating prolonged parenteral therapy

Sources: Robbins & Kumar Basic Pathology; Goldman-Cecil Medicine; Harrison's Principles of Internal Medicine 22nd ed.; Frameworks for Internal Medicine; Tietz Textbook of Laboratory Medicine

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