Which organism is alpha-hemolytic, optochin resistant, commonly causing subacute endocarditis on damaged heart valves and dental caries?

• A. Streptococcus pneumoniae
• B. Staphylococcus epidermidis
• C. Streptococcus viridans
• D. Streptococcus agalactiae

Answer: (C)

Alpha-hemolysis with resistance to optochin points to the viridans group of streptococci. These organisms are normal inhabitants of the mouth and can form dental biofilms, with Streptococcus mutans being a common culprit in dental caries. They also have a role in subacute endocarditis on already damaged heart valves, often due to Streptococcus sanguinis and related species that adhere to surfaces via dextran-containing biofilms. The key distinction is that viridans streptococci are alpha-hemolytic but optochin resistant, whereas Streptococcus pneumoniae, though also alpha-hemolytic, is optochin sensitive. The other options do not fit this pattern: the pediatric group B streptococcus is beta-hemolytic, and Staphylococcus epidermidis is a catalase-positive cocci not described by this alpha-hemolysis/optocin resistance pattern. Therefore, the organism described is Streptococcus viridans.