optochin

Optochin, named for the antibiotic ethylhydrocupreine hydrochloride, is a small molecule used primarily as a diagnostic agent in microbiology to differentiate Streptococcus pneumoniae from other alpha-hemolytic streptococci. In the optochin susceptibility test, a disk impregnated with optochin is placed on a blood agar plate inoculated with a suspected isolate. After incubation at about 35-37°C in a CO2-enriched atmosphere for 18-24 hours, the presence of a zone of inhibition around the disk indicates optochin susceptibility, a trait typical of S. pneumoniae; lack of inhibition suggests non-pneumococcal viridans streptococci. A commonly used interpretive guideline is that a zone diameter of 14 mm or greater indicates susceptibility under standard testing conditions.

The test is a rapid, initial tool for identifying respiratory isolates and is usually performed alongside other tests, such as bile solubility or biochemical assays, to confirm pneumococcal identity. Its accuracy can be influenced by incubation conditions, media quality, and organism variability. Although highly useful, the optochin test is not infallible: a small number of S. pneumoniae strains may be resistant, and some viridans streptococci may appear susceptible, which can lead to false results. Consequently, optochin testing is viewed as a supplementary diagnostic method rather than a definitive, standalone test, and results are interpreted within the broader clinical and laboratory context.

Today, optochin testing is mainly of historical and practical diagnostic value rather than therapeutic relevance.