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Laboratory diagnosis

Lab Diagnosis Of S Pneumoniae -Optochin Disk Test-Quellung Test

Positive quellung test under microscope

Laboratory Diagnostic Tests. (Laboratory diagnosis Of Streptococcus pneumoniae
Blood, sputum, pus or CSF.
1. Microscopy:
Gram stain smear of rusty sputum is seen under a microscope. Gram-positive diplococci, neutrophils, and red blood cells Can be identified. Capsule can be demonstrated by "Negrosin staining or Indian Ink."
2. Culture:
Specimens are cultured on blood agar and chocolate agar, It causes alpha hemolysis on blood agar. It forms small round colonies, at first dome-shaped, later develop a central plateau with an elevated rim in chocolate agar.
3. Quellung's Test.
It is the capsule swelling test. Fresh emulsified sputum is mixed with specific anti-capsular polysaccharide antibodies. If capsule swells markedly it means test is positive.

4. Optochin Disk Test.
This test differentiates between alpha hemolytic pneumococci and alpha hemolytic streptococcus viridans. The optochin disk is laid down on an agar plate surface which is inoculated with unknown organisms. Pneumococci are very sensitive to it and so will fail to grow in its proximity whereas viridans are insensitive to it and will grow adjacent to implanted disks.
5. Animal Inoculation Test.
Sputum is intraperitoneally injected into laboratory mice. mice die within 18 - 48 hours because it is very sensitive to S. pneumonia. Heart blood gives a pure culture of pneumococci which can be used for confirmation.

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