Bacitracin & Optochin Susceptibility

The bacitractin and optochin sensitivity tests identify whether an organism is susceptible or resistant to optochin and bacitracin.  A blood agar plate is streaked for confluent growth with the organism to be tested, and paper disks that have been infused with either bacitracin or optochin are applied to the surface of the agar. The plate is then incubated. If the organism grows up to the edge of the disk, it is resistant to the antimicrobial compound infusing the disk.  If there is a zone around the edge of the disk where the organism has not grown, the organism is susceptible to the antimicrobial in the disk. 

Bacitracin is a true antibiotic in that it is an antimicrobial compound which is naturally produced by a microorganism. It is produced by Bacillus lichenformis and acts to interrupt the formation of the bacterial cell wall.  It is not effective on bacteria that do not have cell walls and are not actively growing.  The bacitracin test is useful for differentiating β-hemolytic Group A streptococci from β-hemolytic non-Group A streptococci.  This is important because most streptococcal diseases are caused by Group A streptococci.  The bacitracin test can also be used to differentiate the bacitracin-resistant Staphylococcus from the bacitracin-susceptible Micrococcus

Optochin is also known as ethylhydrocupreine; it is a chemical that inihibits pneumococci but does not affect other α-hemolytic streptococci.