Short note on eleks test from apurba shastry
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ELEKS test Apurba Shastry elbow
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Elek gel precipitation test principle procedure result interpretation microbiology diphtheria toxin
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I now have comprehensive information. Here is the detailed short note on Elek's (Gel Precipitation) Test as described in medical microbiology (as per Apurba Shastry and corroborated by other authoritative sources):
Elek's Gel Precipitation Test (Elek's Test)
Definition
Elek's test is an in vitro immunodiffusion (immunoprecipitation) test used to demonstrate the toxigenicity of Corynebacterium diphtheriae - i.e., to determine whether a given isolate produces diphtheria exotoxin.
It is also called the Elek-Ouchterlony test or Elek's gel precipitation test, and is considered the gold standard for detecting diphtheria toxin in vitro. - Medical Microbiology 9e, Toxigenicity Testing
Principle
The test is based on the principle of double immunodiffusion in gel (Ouchterlony technique). When the diphtheria toxin (antigen) produced by the test organism and the diphtheria antitoxin (antibody) incorporated in filter paper both diffuse through an agar medium and meet at the zone of equivalence, they form a visible precipitin line. If the test strain is non-toxigenic, no toxin is produced and therefore no precipitin line forms.
Medium Used
- Special serum agar (20% horse serum added to peptone agar)
- Serum enrichment is required for luxuriant growth and toxin production by C. diphtheriae
Procedure
- A strip of sterile filter paper pre-impregnated with diphtheria antitoxin (500 IU/mL; approximately 10 IU per disk in the WHO modified method) is placed in the center of the serum agar plate before it sets.
- The agar is poured over and around the strip so the strip is embedded in the agar.
- Once the agar solidifies, the test strain is streaked in a line at right angles to the antitoxin strip, 7-9 mm away from it.
- Positive control (known toxigenic strain) and negative control (known non-toxigenic strain) are also streaked in parallel lines on the same plate.
- The plate is incubated at 37°C for 24-48 hours.
- After incubation, the plate is examined using transmitted (oblique) light for precipitin lines.
- Jawetz, Melnick & Adelberg's Medical Microbiology 28e, p.203
Result Interpretation
| Finding | Interpretation |
|---|---|
| Precipitin line forms at 45° angle between growth streak and antitoxin strip | Positive - strain is toxigenic |
| Precipitin line of test strain fuses/merges with the control precipitin line (line of identity) | Confirms positive result |
| No precipitin line forms | Negative - strain is non-toxigenic |
The precipitin line represents the point where toxin diffusing from bacterial growth and antitoxin diffusing from the paper strip meet at the zone of equivalence.
Key Points
- Test organism: Corynebacterium diphtheriae (and also C. ulcerans, C. pseudotuberculosis)
- The toxin is produced only if the bacterium is lysogenized by beta-corynephage (carries the tox gene)
- This test detects toxin protein (phenotypic expression), unlike PCR which detects the tox gene (genotypic)
- A strain can carry the tox gene but be non-expressing (PCR positive, Elek negative) - these are called NTTB (Non-Toxigenic Tox-Bearing) strains
- Sensitivity can be affected by weakly toxigenic strains
Other Methods for Toxin Detection
| Method | Type |
|---|---|
| Elek's test | In vitro (gold standard) |
| Schick test | In vivo (historical) |
| PCR for tox gene | Molecular (rapid, genotypic) |
| ELISA | Immunological |
| Immunochromatographic strip assay | Rapid (highly sensitive) |
- Quick Compendium of Clinical Pathology 5e
Clinical Significance
Detection of toxigenicity is essential because:
- Only toxigenic strains cause classic diphtheria with its systemic complications (myocarditis, neuropathy, renal damage)
- Non-toxigenic strains can still cause local infection, septicemia, endocarditis, and osteomyelitis
- Results guide the decision to administer diphtheria antitoxin (which must be given early, before toxin binds to cells)
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