M13: Microbiology of Infective Endocarditis Flashcards
1. List common bacterial causes of IE 2. List host factors predisposing IE 3. Know the pathological features of IE 4. Describe antibiotic therapy for common causes of IE 5. Understand why cross-speciality liaison is important in treatment 6. Issues around preventative measures against IE
What is infective endocarditis
An infection of the endocardium
What is the endocardium
Membrane layer of endothelial cells lining the heart that is continuous with artery and vein lining and also forms the valve cusps
What infective organisms are present in IE
- Oral Streptococci
- Staph. aureus
- Strept. viridians
- HACEK organisms
What are HACEK organisms
- Haemophilus
- Aggregatibacter
- Cardiobacterium
- Eikenella
- Kingella
What is acute IE
Aggressive, rapidly destructive form of IE which occurs on previously normal heart valves involving Staph. aureus and occurs few days after bacteraemia
This tends to be necrotising, ulcerative and invasive
What is subacute IE
Insidious onset with low virulence organisms e.g. Strept. viridans on previously abnormal heart valves, people with rheumatic fever or prosthetic valves
This is less destructive and vegetations show evidence of healing
Factors predisposing to IE
- Rheumatic fever
- Ageing
- Degenerative heart disease
- Congenital defects (esp. turbulent flow)
- Mitral valve prolapse
- IVDU
Which part of the endocardium does IE typically affect
Heart valves
Septal defects
Mural endocardium in left ventricular aneurysm
What can IE complicate
Cardiac abnormalities e.g.
- Arteriovenous shunts
- Coarctation of aorta (narrowing)
- Developmental defects
What is a ventricular aneurysm and when can this occur
A ballon-like swelling in the wall of heart which can develop post MI
What are IE vegetations
Characteristic lesions occurring in IE found on the valve leaflets and when blood flows form a high to low pressure chamber
Describe how IE occurs
- Bacteraemia occurs
- Micro-organisms are deposited on and adhere to an endothelial breach which has developed a platelet thrombus
- Micro-organisms are encased in additional layers of fibrin and platelets which protects them from phagocytic cells
- Results in formation of a vegetation
Why do the following different types of IE occur
- On previously normal valves
- On previously abnormal valves
- On prosthetic valves
- Iatrogenically caused
Normal valves: due to bacteraemia - S. aureus
Abnormal valves: native valve defects - S. viridans
Prosthetic valves: late onset/ within first few months of new valve inserted
Iatrogenic: resulting from hospital treatment e.g. cannula, IVDU
Why are oral streptococci virulent in IE (S. viridans)
Because they can bind to fibronectin (protein on endocardium)
Produce extra-cellular polysaccharide
Bind to platelets (platelet aggregation associated proteins) produced by streptococci = PAAP
Why are staphylococcus virulent in IE (S. aureus)
They have microbial surface components recognising adhesion matrix molecules = MSCRAMMS
They can evade the immune system via
- leukocidin subunit
- superantigen-like protein
How is IE diagnosed
Hard to diagnose - done though Duke Criteria = cross-speciality liaison (lab and clinical studies)
What is the Duke Criteria (MAJOR)
- Blood culture = cx organisms from 2 sep. cultures, persistently positive cultures for any cx microbe
- Endocardial involvement evidence = +ve on echocardiogram for IE, vegetations or abscesses, new damage to artificial valve, new regurgitation
What is the Duke Criteria (MINOR)
- Predisposition
- Fever >37 degrees
- Vascular phenomena = arterial embolism, septic pulmonary infarcts
- Immunological phenomena = glomerulonephritis
- Echocardiogram with cx IE not meeting major criteria
- Microbiological = +ve blood culture but not meeting major criteria, antibody response indicating active infection with cx IE microbes
What is a definite IE diagnosis using the Duke Criteria
2 major
1 major + 3 minor
5 minor
What is a possible IE diagnosis using the Duke Criteria
1 major + 1 minor
3 minor
How are blood cultures processed for IE investigations
- Incubate 5 days at 37 degrees
- After growth detection, do gram stain
- Clue about ID: gram positive cocci in chains likely to be strep.
- Culture on solid media for full ID
How is IE treated
Because the vegetation is impenetrable by phagocytes, bacteriocidal antibiotics are needed in synergistic combination IV for at least 2 weeks
Needs isolation of the micro-organism and surgical backup
What treatment is given for acute presentation of IE
Flucloxacillin
Gentamicin
What treatment is given for indolent presentation of IE
Penecillin or Amicillin/amoxicillin
Gentamicin
What treatment is given for indolent presentation of IE when patient presents with penicillin allergy
Vancomycin
Rifampicin
Gentamicin
What are the possible causes of persistent fever in IE patients
- Abscess at aortic root/ prosthetic valve ring
- Drug hypersensitivity
- IV line infection
- Other supervening nosocomial
