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Streptococci are the leading cause of infective endocarditis (IE), accounting for 30%-65% of cases. Among these, streptococci viridans (alpha-hemolytic streptococci), commonly found in the mouth and gut, are the most frequent cause of community-acquired native valve endocarditis (NVE). Staphylococci contribute to 30%-40% of IE cases, with coagulase-positive Staphylococcus aureus being the primary culprit in IE linked to intravenous drug abuse and nonvalvular cardiovascular device infections. Recent studies indicate that S. aureus is now the most common cause of IE, while the prevalence of viridans streptococci has declined. Other less common IE-causing agents include the HACEK group (Haemophilus, Actinobacillus, Cardiobacterium, Eikenella, Kingella), Pseudomonas aeruginosa, Corynebacterium, Bacteroides fragilis, and fungi.

Streptococci are the most common cause of IE 30%-65%, of which streptococci viridans (alpha-hemolytic streptococci), which are normal constituents of the oral flora and gastrointestinal tract, remain the most common cause of community acquired NVE.  Staphylococci are the cause of at least 30%-40% of cases of IE; mostly coagulase-positive Staphylococcus aureus which is the most common pathogen in IE associated with IV drug abuse, it is also the most common pathogen in nonvalvular cardiovascular device infections.  In some recent studies, S. aureus has emerged as the most common cause of IE and rates of viridans streptococci have decreased.  Other microbial agents that less commonly cause IE such as the HACEK group (Haemophilus, Actinobacillus, Cardiobacterium, Eikenella, Kingella), Pseudomonas aeruginosa, Corynebacterium, Bacteroides fragilis, and fungi.