Which bacteria shows sensitivity to optochin?

Streptococcus pneumoniae is known for its sensitivity to optochin, which is a key characteristic used to distinguish it from other alpha-hemolytic streptococci, particularly the viridans group of streptococci. When cultured on blood agar, S. pneumoniae will show a zone of inhibition around an optochin disk, indicating its susceptibility, which is a critical test in the laboratory diagnosis of pneumonia and other infections caused by this pathogen.

In contrast, viridans streptococci are resistant to optochin and typically do not exhibit a zone of inhibition. The other options, Staphylococcus aureus and Escherichia coli, are not relevant in this context, as they are not streptococci and do not interact with optochin in a meaningful way related to infections caused by S. pneumoniae. Therefore, the identification of optochin sensitivity is a cornerstone in differentiating S. pneumoniae from other bacteria in clinical microbiology.