microbiology 2nd mbbs
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modified AFB stain Cryptosporidium H2SO4 concentration 1% immunocompromised diarrhea microbiology
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Cryptosporidium oocyst modified acid fast stain stool pink red
Here is a complete answer for both parts of this question:
Slide 1 — Modified AFB Stained Stool Smear
(a) Identification of the Protozoan Parasite
The parasite is Cryptosporidium parvum (oocysts).
Key features seen in the image:
- Small, round to oval pink/red oocysts (~4–6 µm in diameter) against a blue background
- Oocysts stain pink-red with modified AFB because they are variably acid-fast (weakly acid-fast)
- Seen in the stool of an immunocompromised patient (e.g., HIV/AIDS) with chronic diarrhea — classic clinical setting
Why Cryptosporidium?
| Feature | Cryptosporidium parvum |
|---|---|
| Size of oocyst | 4–6 µm (small, round) |
| Staining | Pink-red on modified AFB |
| Clinical setting | Immunocompromised (HIV/AIDS), chronic watery diarrhea |
| Morphology | Round, uniform; may show internal sporozoites in thick-walled forms |
| Transmission | Fecal-oral, waterborne |
Differential note: Other coccidian parasites also stain with modified AFB:
- Cyclospora cayetanensis — larger oocysts (8–10 µm), wrinkled/non-uniform staining
- Cystoisospora (Isospora) belli — large elongated oocysts (20–30 µm), easily differentiated
Given the small round uniformly pink oocysts (~4–6 µm) in an immunocompromised patient with chronic diarrhea, Cryptosporidium parvum is the definitive identification.
(b) Concentration of H₂SO₄ Used in Modified AFB
1% sulfuric acid (H₂SO₄) is used as the decolorizer in modified AFB (modified Ziehl-Neelsen / modified Kinyoun stain).
Comparison of decolorizers:
| Stain | Decolorizer | Concentration |
|---|---|---|
| Standard ZN (for Mycobacteria) | Acid alcohol (HCl + alcohol) | 3% HCl in 95% ethanol |
| Modified AFB (for Cryptosporidium, Cyclospora, Isospora) | Sulfuric acid (H₂SO₄) | 1% H₂SO₄ |
Why modified? Regular acid-alcohol (3% HCl) used in standard ZN is too strong and decolorizes the weakly acid-fast coccidian oocysts entirely. The milder 1% H₂SO₄ is used instead to preserve their pink staining.
Staining procedure (Modified Kinyoun Cold Method):
- Fix stool smear, flood with Kinyoun's carbol fuchsin (5 min, no heating)
- Rinse briefly with 50% ethanol
- Decolorize with 1% H₂SO₄ for ~2 minutes
- Counterstain with methylene blue
- Examine: acid-fast oocysts appear pink-red against blue background
Summary:
- (a) Cryptosporidium parvum oocysts
- (b) 1% H₂SO₄ (sulfuric acid) is used as the decolorizer in modified AFB stain
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