optochinresistant

optochinresistant refers to a phenotype in which bacteria do not show inhibition around an optochin disk in standard susceptibility testing. In clinical microbiology, optochin testing is most often used to distinguish Streptococcus pneumoniae from viridans group streptococci, with S. pneumoniae typically being optochin sensitive and many viridans strains being resistant. However, optochin resistance can occur in pneumococci as well, and some viridans isolates may display variable results, making interpretation dependent on the broader diagnostic context.

Testing is performed on a confluent lawn of the organism on a blood or chocolate agar plate by placing an optochin-impregnated disk and incubating at 35-37°C with elevated CO2 for 18-24 hours. After incubation, the zone of inhibition around the disk is measured. Interpretive breakpoints can differ by guidelines, but many laboratories regard a zone diameter of 14 mm or greater as susceptible (optochin sensitive) and 13 mm or less as resistant (optochin resistant). Local guidelines should be consulted for exact cutoffs.

The mechanism by which optochin affects pneumococci is not completely resolved, but the compound is believed to interact with the bacterial cell membrane to inhibit growth. Resistance can arise from alterations in membrane properties or other factors that reduce optochin uptake or activity. Because optochin resistance can complicate species identification, results are typically corroborated with additional tests such as bile solubility, pneumococcal antigen testing, or molecular methods.

In practice, recognizing an optochin-resistant result helps prevent misidentification of viridans streptococci as Streptococcus pneumoniae. It highlights the importance of using optochin testing as part of a comprehensive identification algorithm rather than as a sole criterion.