What test differentiates S. pneumoniae from viridans streptococci?

The Optochin disk test is a method specifically designed to differentiate Streptococcus pneumoniae from viridans streptococci, such as Streptococcus mitis and Streptococcus sanguinis. S. pneumoniae is susceptible to optochin, meaning that a culture showing a zone of inhibition around the optochin disk indicates the presence of S. pneumoniae. In contrast, viridans streptococci are resistant to optochin and will not show a significant zone of inhibition.

The Bacitracin disk test is primarily used to differentiate Group A Streptococcus (S. pyogenes) from other beta-hemolytic streptococci, so it is not applicable for distinguishing S. pneumoniae from viridans streptococci. The CAMP test is used to identify Group B Streptococcus (S. agalactiae) by detecting its synergistic hemolytic activity when combined with Staphylococcus aureus, which is also irrelevant for this differentiation. The Bile esculin test helps identify enterococci and Group D streptococci but does not provide differentiation between S. pneumoniae and viridans streptococci.

Thus, the Optochin disk test is