Optochin sensitivity test, Taxos P - History, Purpose, Principle, Procedure, Results, Interpretation
Introduction to Optochin sensitivity test
The optochin sensitivity test is an important parameter used to identify Streptococcus pneumoniae from other α-hemolytic streptococci species in a laboratory setting.
History of Optochin sensitivity test
Optochin or ethylhydrocupreine hydrochloride is a derivative of quinine that is completely soluble in water Morgenroth and Levy. They introduced it in 1911 to treat pneumococci infection. The chemical agent inhibits the growth of pneumococci in vitro in a very diluted state.
Historically, in the 1950s, the Optochin sensitivity test was developed for differentiating Streptococcus pneumoniae from other α-hemolytic streptococci 1915. However, the application of this method did not reach laboratory guidelines until the mid-1950s.
Purpose of Optochin sensitivity test
The main purpose of this test is to determine the effect of optochin on microorganisms. Streptococcus pneumoniae, which is a major cause of pneumonia, is sensitive to Optochin while other alpha-hemolytic streptococci (aka viridans streptococci) are resistant to its growth-inhibiting abilities.
In vitro, a minimum inhibitory concentration (MIC) of 1:10,000,000 will show a clear zone of inhibition (ZOI) for, and at 1:500,000, Optochin is bactericidal.
The optochin sensitivity test is widely used in a form of filter paper discs that have been impregnated with ethylhydrocupreine hydrochloride.
Principle of Optochin sensitivity test
Optochin works by interfering with ATPase and the production of adenosine triphosphate (ATP) in the test organisms.
For the Optochin sensitivity test, Optochin impregnated disc is placed on the lawn culture of the test organism and incubated at 37°C for 18-24 hours.
If the ZOI is 14-16mm is observed, the test organism is considered susceptible and is the presumptive identification of Streptococcus pneumoniae.
If the test organism is alpha-hemolytic streptococci species, no or less than 14mm ZOI is seen.
Fig: Streptococcus mitis vs S. pneumoniae (Source: CDC)
Procedure of Optochin sensitivity test
The procedure of the optochin sensitivity test is as follows:
Around 3-4 well, isolated colonies of the test α-hemolytic colonies are picked up by inoculating loop.
The isolate is streaked onto one-half of a TSA-5% sheep blood agar plate. The streaked culture must be spread evenly like a lawn.
With the help of sterile forceps, an optochin disk is placed onto the streaked surface and pressed on the disk gently to adhere to the disc to the agar plate.
The known culture of Streptococcus spp. can be used by the above procedure on the other half of the TSA plate.
The culture is incubated at 35°C-37°C for 18-24 hours.
ZOI is measured for the test organism
Result, Interpretation of Optochin sensitivity test
The result of the optochin sensitivity test can be interpreted as:
Positive: ZOI equal to/greater than 14mm in diameter with 6mm disk. (sensitive)
Negative: ZOI less than 14mm in diameter with 6mm disk. (resistant)