24th June 2026

Aetiology 

IE is caused by bacteraemia in association with valvular damage, and the subsequent formation of vegetations on valve leaflets and other endothelial structures. Damage may occur due to an underlying valvular condition, degeneration of the valve, the infective organism itself, or due to trauma (such as from foreign material). Damage to the endothelium leads to platelet and fibrin aggregation, providing a surface for the circulating microorganism to bind to. This leads to further aggregation of platelets and fibrin, eventually resulting in the formation of a vegetation.

IE is most commonly caused by bacterial infection, but fungal infections may also occur. In native-valve IE (NVIE), the commonest causative pathogens world-wide are staphylococci, streptococci, and enterococci. Other, rarer, pathogens may also be implicated. Right-sided IE (90% of which is related to IVDU) is usually caused by staphylococcus aureus, which accounts for up to 90% of cases. Table 1 summaries the microorganisms implicated in IE, and their associations. 

In prosthetic-valve IE (PVIE), the commonest responsible pathogens can be predicted depending on the length of time since the valve was placed:

• < 2 months post-procedure: staphylococcus aureus and coagulase-negative staphylococci.
• 2 – 12 months post-procedure: coagulase-negative staphylococci and streptococci.
• > 12 months post-procedure: the spectrum of causative organisms is similar to that in NVIE.

Additionally, in patients with prosthetic valves in situ, IE may develop due to seeding on the prosthetic valve from a bacteraemia of any other source, and this is important to keep in mind in unwell patients with prosthetic valves.

In some cases, patients with IE may have negative cultures (culture-negative IE). This may be due to patients receiving antibiotics prior to blood cultures being taken, but is also associated with specific microorganisms, and therefore additional serological testing may be required to identify the causative organism. Bacteria associated with culture-negative IE are:

• HACEK group (Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, Kingella)
• Coxiella burnetii
• Chlamydia species
• Bartonella species
• Legionella

Table 1: pathogens associated with IE and their characteristic associations