Fastidious Gram-negative organisms classified as the HACEK group (Haemophilus spp., Actinobacillus actinomycetemcomitans, Cardiobacterium hominis, Eikenella corrodens and Kingella spp.) are rare causes of infective endocarditis.

In this series, we report six cases of endocarditis in Saudi patients occurring between 1990 and 1994 in our hospital, caused by two of the organisms in the HACEK group, i.e. Cardiobacterium hominis and Actinobacillus actinomycetemcomitans. The clinical features, predisposing factors and treatment of the patients are briefly described. The microscopic and cultural characteristics of the organisms are described together with the laboratory methods of diagnosis.

HACEK endocarditis was frequently associated with prosthetic heart valves or structural heart abnormalities. Dental caries or periodontal disease seems to be a predisposing factor. The prognosis of HACEK endocarditis is very good as clinical and bacteriological care were achieved with antibiotic therapy in all our cases except one who required mitral valve replacement. Contrary to previous reports we did not find all the organisms sensitive to penicillin and aminoglycosides. However, all our isolates were sensitive to amoxycillin, cefuroxime, ceftriaxone and ciprofloxacin.

Laboratory diagnosis of HACEK group of organisms requires a high index of suspicion and should be suspected in cases of endocarditis in which fastidious Gram-negative coccobacilli are isolated which fail to grow on MacConkey agar. Empiric therapy should be started in suspected cases with second generation cephalosporins or with ciprofloxacin, until antibiotic sensitivity results become available. Antibiotic therapy should be continued for 4–6 weeks. Progress and outcome was very good in the series.