Question
Enumerate the methods of moist heat sterilization. Write briefly on the principle, application and sterilization control of laboratory autoclave. (3+1+2 +2)4. Write the pathogenesis, clinical features and laboratory diagnosis of Hydatid disease
Answer
Question 3 — Moist heat sterilization and the laboratory autoclave
Methods of moist heat sterilization:
- Autoclaving (steam under pressure) — the standard method for sterilizing heat/moisture-stable items; typically 121°C at 15 psi for 15–20 minutes.
- Pasteurization — moist heat at sub-boiling temperatures (e.g., 63°C for 30 minutes, or 72°C for 15 seconds — HTST), used for disinfection (not sterilization) of liquids such as milk.
- Boiling — 100°C for a defined period, achieves disinfection but does not reliably kill bacterial spores, so it is not a true sterilization method.
- Tyndallization (fractional/intermittent sterilization) — steaming at 100°C for 20 minutes on three successive days, with intervening incubation periods allowing spores to germinate into vegetative forms that are then killed on the next steaming cycle; used for heat-sensitive fluids that cannot withstand autoclaving.
Principle of the laboratory autoclave: steam under pressure raises the boiling point of water above 100°C, allowing higher, more lethal temperatures to be achieved (typically 121°C at 15 psi). Saturated steam has high latent heat, which is released on contact with the (cooler) surface of the load, rapidly transferring heat and causing coagulation and denaturation of microbial proteins, killing all forms of microbial life including bacterial spores.
Application: sterilization of surgical instruments, glassware, culture media, dressings, gowns, and other heat/moisture-stable items in hospitals and microbiology laboratories — the most widely used and most reliable method of sterilization.
Sterilization control (monitoring):
- Physical indicators — autoclave gauges/charts recording time, temperature, and pressure achieved during the cycle.
- Chemical indicators — heat-sensitive tape/strips (e.g., autoclave tape with diagonal stripes) that change colour on exposure to the required temperature, placed on the outside of each pack; more sophisticated chemical integrators placed inside packs confirm parameters were met throughout the load.
- Biological indicators — vials/strips containing Geobacillus stearothermophilus spores, the gold-standard confirmation of sterilization efficacy, since failure to kill these highly heat-resistant spores indicates a sterilization failure; run periodically (e.g., weekly) and with every load containing an implant.
Leaked question 4 content (see SOURCE GAP note above) — Hydatid disease
Pathogenesis: Hydatid disease is caused by the larval (metacestode) stage of the tapeworm Echinococcus granulosus. Humans are an accidental intermediate host, acquiring infection by ingesting eggs shed in the faeces of the definitive host (dog). Ingested eggs hatch in the small intestine, releasing oncospheres that penetrate the intestinal wall and are carried via the portal circulation to the liver (most common site) or, after bypassing the liver, to the lungs and other organs, where they develop slowly into a hydatid cyst — a fluid-filled cyst with a germinal layer producing numerous protoscolices and daughter cysts, surrounded by a host-derived fibrous pericyst.
Clinical features: often asymptomatic for years due to slow cyst growth; symptoms arise from pressure effects of the enlarging cyst on surrounding structures (right upper quadrant pain/hepatomegaly for hepatic cysts; cough, chest pain, haemoptysis for pulmonary cysts); cyst rupture can cause anaphylactic reaction (due to release of highly antigenic cyst fluid) and secondary dissemination of protoscolices to form new cysts; secondary bacterial infection of the cyst can occur, mimicking a liver abscess.
Laboratory diagnosis:
- Imaging — ultrasound/CT/MRI, showing characteristic cystic lesion(s), often with daughter cysts (“cyst within a cyst” appearance) or a “water-lily sign” (collapsed endocyst membrane in a ruptured/degenerating cyst) — the primary diagnostic modality.
- Serology — detection of anti-Echinococcus antibody (ELISA, indirect haemagglutination), supportive but not always positive (particularly with intact, non-leaking cysts).
- Casoni’s skin test — a classical delayed hypersensitivity test, historically used but now largely abandoned due to poor sensitivity/specificity.
- Eosinophilia — may be present, particularly following cyst leakage.
- Direct examination of cyst fluid (obtained cautiously, e.g., during surgery, given rupture risk) — demonstration of protoscolices/hooklets confirms the diagnosis.
- Percutaneous aspiration (fine-needle) for diagnosis is generally avoided/performed with great caution given the risk of anaphylaxis and secondary dissemination from cyst content spillage.