Within this secluded focus, the buried microorganisms multiply as rap- idly as bacteria in broth cultures to reach maximal microbial densities of 108 colony-forming units per gram of vegetation within a short time on the left side of the heart, apparently uninhibited by host defenses in left-sided lesions. Right-sided vegetations have lower bacterial densities, which may be the consequence of host defense mechanisms active at this site, such as polymorphonuclear activity or platelet- derived antibacterial proteins. More than 90% of the micro- organisms in mature left- or right-sided valvular vegetations are metabolically inactive rather than in an active growth phase and are therefore less responsive to the bactericidal effects of antibiotics.20 Rationale for or Against Prophylaxis of IE Historical Background Viridans group streptococci are part of the normal skin, oral, respiratory, and GI tract flora, and they cause at least 50% of cases of community-acquired native valve IE not associated with intravenous drug use.21 More than a century ago, the oral cavity was recognized as a potential source of the bacteremia that caused viridans group streptococcal IE. In 1885, Osler22 noted an association between bacteremia from surgery and IE. Okell and Elliott23 in 1935 reported that 11% of patients with poor oral hygiene had positive blood cultures with viridans group streptococci and that 61% of patients had viridans group streptococcal bacteremia with dental extraction. 