M13: Microbiology of Infective Endocarditis

Question 1

What is infective endocarditis

Answer 1

An infection of the endocardium

Question 2

What is the endocardium

Answer 2

Membrane layer of endothelial cells lining the heart that is continuous with artery and vein lining and also forms the valve cusps

Question 3

What infective organisms are present in IE

Answer 3

• Oral Streptococci
• Staph. aureus
• Strept. viridians
• HACEK organisms

Question 4

What are HACEK organisms

Answer 4

1. Haemophilus
2. Aggregatibacter
3. Cardiobacterium
4. Eikenella
5. Kingella

Question 5

What is acute IE

Answer 5

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

Question 6

What is subacute IE

Answer 6

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

Question 7

Factors predisposing to IE

Answer 7

• Rheumatic fever
• Ageing
• Degenerative heart disease
• Congenital defects (esp. turbulent flow)
• Mitral valve prolapse
• IVDU

Question 8

Which part of the endocardium does IE typically affect

Answer 8

Heart valves
Septal defects
Mural endocardium in left ventricular aneurysm

Question 9

What can IE complicate

Answer 9

Cardiac abnormalities e.g.

• Arteriovenous shunts
• Coarctation of aorta (narrowing)
• Developmental defects

Question 10

What is a ventricular aneurysm and when can this occur

Answer 10

A ballon-like swelling in the wall of heart which can develop post MI

Question 11

What are IE vegetations

Answer 11

Characteristic lesions occurring in IE found on the valve leaflets and when blood flows form a high to low pressure chamber

Question 12

Describe how IE occurs

Answer 12

1. Bacteraemia occurs
2. Micro-organisms are deposited on and adhere to an endothelial breach which has developed a platelet thrombus
3. Micro-organisms are encased in additional layers of fibrin and platelets which protects them from phagocytic cells
4. Results in formation of a vegetation

Question 13

Why do the following different types of IE occur

• On previously normal valves
• On previously abnormal valves
• On prosthetic valves
• Iatrogenically caused

Answer 13

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

Question 14

Why are oral streptococci virulent in IE (S. viridans)

Answer 14

Because they can bind to fibronectin (protein on endocardium)

Produce extra-cellular polysaccharide

Bind to platelets (platelet aggregation associated proteins) produced by streptococci = PAAP

Question 15

Why are staphylococcus virulent in IE (S. aureus)

Answer 15

They have microbial surface components recognising adhesion matrix molecules = MSCRAMMS

They can evade the immune system via

• leukocidin subunit
• superantigen-like protein

Question 16

How is IE diagnosed

Answer 16

Hard to diagnose - done though Duke Criteria = cross-speciality liaison (lab and clinical studies)

Question 17

What is the Duke Criteria (MAJOR)

Answer 17

• 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

Question 18

What is the Duke Criteria (MINOR)

Answer 18

• 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

Question 19

What is a definite IE diagnosis using the Duke Criteria

Answer 19

2 major
1 major + 3 minor
5 minor

Question 20

What is a possible IE diagnosis using the Duke Criteria

Answer 20

1 major + 1 minor

3 minor

Question 21

How are blood cultures processed for IE investigations

Answer 21

1. Incubate 5 days at 37 degrees
2. After growth detection, do gram stain
3. Clue about ID: gram positive cocci in chains likely to be strep.
4. Culture on solid media for full ID

Question 22

How is IE treated

Answer 22

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

Question 23

What treatment is given for acute presentation of IE

Answer 23

Flucloxacillin

Gentamicin

Question 24

What treatment is given for indolent presentation of IE

Answer 24

Penecillin or Amicillin/amoxicillin

Gentamicin

Question 25

What treatment is given for indolent presentation of IE when patient presents with penicillin allergy

Answer 25

Vancomycin
Rifampicin
Gentamicin

Question 26

What are the possible causes of persistent fever in IE patients

Answer 26

• Abscess at aortic root/ prosthetic valve ring
• Drug hypersensitivity
• IV line infection
• Other supervening nosocomial