Optochin sensitivity test: Difference between revisions
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* A test applied to alpha-hemolytic streptococci in order to presumptively identify '''[[Streptococcus pneumoniae]]''', which is susceptible to optochin (although not all strains are susceptible) |
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= Optochin sensitivity test = |
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* Apply an optochin or P disc to an SBA plate with freshly-streaked inoculum, then incubate at 35-37°C with 5% CO<sub>2</sub> for 18 to 24 hours |
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** Using a standard 6 mm disc, a zone of 14 mm or more indicates susceptibility and suggests [[Streptococcus pneumoniae]] |
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** Any zone of inhibition is uninterpretable, and should be followed by a bile solubility test |
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== Further reading == |
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[https://microbeonline.com/streptococcus-pneumoniae-pneumococcus-disease-properties-pathogenesis-and-laboratory-diagnosis/ ''Streptococcus pneumoniae''] strains are sensitive to the chemical optochin (ethylhydrocupreine hydrochloride). Optochin sensitivity allows for the presumptive identification of alpha-hemolytic streptococci as ''S. pneumoniae'', although some pneumococcal strains are optochin-resistant. Other alpha-hemolytic streptococcal species are optochin-resistant. |
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* Microbe Online. [https://microbeonline.com/optochin-test-principle-procedure-expected-results-and-quality-control/ Optochin Sensitivity Test: Principle, Procedure, Results and Quality Control]. |
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[[Category:Diagnostics]] |
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== Procedure == |
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Optochin (P) disks (6 mm, 5 µg) can be obtained from a commercial vendor. Optochin disks are often called “P disks” and many commercial versions are labeled with a capital “P”. If a commercial source of P disks is not available, a 1:4000 solution of ethylhydrocupreine hydrochloride can be applied to sterile 6 mm filter paper disks. |
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# Using an inoculating loop, streak two or three suspect colonies of a pure culture to be tested on '''5% sheep blood agar''' plate |
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# Place a P disk within the streaked area of the plate |
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# Incubate the blood agar plate at 35-37°C with ~5% CO2 (or in a candle-jar) for 18 to 24 hours. ''(Culture do not grow well in ambient air, and larger zones of inhibition occur)'' |
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'''Note:''' Two different isolates can be tested on the same plate, (by streaking onto one half of blood agar plate) but care must be taken to ensure that the cultures do not overlap. |
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# Observe the growth on the blood agar plate near the optochin disk and measure the zone of inhibition, if applicable. |
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== Interpretation == |
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<ol> |
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<li><p>Using a 6 mm, 5 µg disk, a zone of inhibition of 14 mm or greater indicates sensitivity and allows for presumptive identification of pneumococci.</p></li> |
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<li><p>Measure zones of inhibition in millimeters, including diameter of disk. In the case of an isolate completely resistant to optochin, the diameter of the disk (6 mm) should be recorded.</p> |
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<p>[https://i1.wp.com/microbeonline.com/wp-content/uploads/2013/05/use-of-biochemical-tests.gif [[File:https://i1.wp.com/microbeonline.com/wp-content/uploads/2013/05/use-of-biochemical-tests.gif?resize=450%2C562|Identification chart for Gram Positive cocci Source: sigmaaldrich.com]]]</p> |
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<p>Identification chart for Gram Positive cocci. Source: sigmaaldrich.com</p></li></ol> |
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== Expected Results == |
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# '''Positive:''' Zone of inhibition is 14 mm or greater in diameter with 6 mm disk |
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# '''Equivocal''': Any zone of inhibition less than 14 mm is questionable for pneumococci; the strain is identified as a pneumococcus only if it is [https://microbeonline.com/bile-solubility-test-principle-procedure-expected-result-and-quality-control/ bile soluble]. |
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'''Note:''' A smaller zone of inhibition (< 14 mm) or no zone of inhibition indicates that the bile solubility test is required. It is important to remember that pneumococci are sometimes optochin-resistant. |
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[https://i0.wp.com/microbeonline.com/wp-content/uploads/2013/05/optochin-sensitivity-test.jpg?ssl=1 [[File:https://i0.wp.com/microbeonline.com/wp-content/uploads/2013/05/optochin-sensitivity-test.jpg?resize=662%2C1024&ssl=1|img]]] |
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Optochin Sensitivity test for ''S.'' pneumoniae |
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== Quality Control == |
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Each new lot of optochin disks should be tested with positive and negative controls. |
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# '''Positive:''' Growth of ''S. pneumoniae'' strain ATCC 49619 is inhibited by optochin |
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# '''Negative:''' Growth of ''S. mitis'' strain ATCC 49456 is not inhibited by optochin. |
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Latest revision as of 12:52, 29 October 2019
- A test applied to alpha-hemolytic streptococci in order to presumptively identify Streptococcus pneumoniae, which is susceptible to optochin (although not all strains are susceptible)
- Apply an optochin or P disc to an SBA plate with freshly-streaked inoculum, then incubate at 35-37°C with 5% CO2 for 18 to 24 hours
- Measure the zone of inhibition
- Using a standard 6 mm disc, a zone of 14 mm or more indicates susceptibility and suggests Streptococcus pneumoniae
- Any zone of inhibition is uninterpretable, and should be followed by a bile solubility test
