Organisms of the so-called HACEK group, (Haemophilus, Actinobacillus actinomycetemcomitans, Cardiobacterium hominis, Eikenella species, Kingella kingae) which are part of the upper respiratory tract and oropharyngeal flora, infect abnormal cardiac valves, causing subacute NVE and cause PVE that occurs a year or more after valve surgery. In NVE, the HACEK organisms have been associated with large vegetations and a high incidence of systemic emboli. These organisms are fastidious and slow growing; when they are suspected, blood cultures should be incubated for 3 weeks. P. Aeruginosa is the gram-negative bacillus that most commonly causes endocarditis. Pseudomonal IE involves normal and abnormal valves on both sides of the heart and often causes valve destruction and heart failure.

The rickettsia C. burnetii infects humans after inhalation of desiccated materials from infected animals or contact with infected parturient animals. At variable intervals after acute infection by C. burnetii (Q. fever), persons with abnormal mitral or aortic valves who have not been able to eradicate the organism develop subacute IE with typical manifestations and often with valve dysfunction causing heart failure. The diagnosis is typically based on high IgG and IgA antibody titers to phase I C. burnetii antigens.