There has been a disproportionate focus on the frequency of bacteremia associated with dental procedures rather than on the species of bacteria recovered from blood cultures. Studies suggest that more than 700 species of bacteria, including aerobic and anaerobic Gram-positive and Gram- negative microorganisms, may be identified in the human mouth, particularly on the teeth and in the gingival crevic- es.24,37 40 Approximately 30% of the flora of the gingival crevice is streptococci, predominantly of the viridans group. Of the more than 100 oral bacterial species recovered from blood cultures after dental procedures, the most prevalent are viridans group streptococci, the most common microbiolog- ical cause of community-acquired native valve IE in non in- travenous drug users.21 In healthy mouths, a thin surface of mucosal epithelium prevents potentially pathogenic bacteria from entering the bloodstream and lymphatic system. Anaer- obic microorganisms are commonly responsible for periodon- tal disease and frequently enter the bloodstream but rarely cause IE, with fewer than 120 cases reported.41 Viridans group streptococci are antagonistic to periodontal pathogens and predominate in a clean, healthy mouth.42 Few published studies exist on the magnitude of bactere- mia after a dental procedure or from routine daily activities, and most of the published data used older, often unreliable microbiological methodology. There are no published data that demonstrate that a greater magnitude of bacteremia, compared with a lower magnitude, is more likely to cause IE in humans. 