At what minimum temperature and exposure time does dry heat sterilisation achieve sterilisation of bacterial spores in a hot air oven?
A. 80°C for 2 hours
B. 160°C for 1 hour
C. 100°C for 30 minutes
D. 121°C for 15 minutes
Explanation
Dry Heat Sterilisation Parameters
Key Point
Dry heat sterilisation in a hot air oven requires 160°C for 1 hour (or 170°C for 1 hour, or 180°C for 30 minutes) to reliably kill bacterial spores, which are the most heat-resistant microbial forms.
Mechanism of Dry Heat Sterilisation
Dry heat kills microorganisms by:
1.
Protein denaturation — high temperature denatures and coagulates microbial proteins
2.
Oxidation — causes oxidative damage to cellular components
3.
Dehydration — removes water from microbial cells
Bacterial spores (e.g., Bacillus, Clostridium) are extremely resistant due to their:
Thick peptidoglycan layers
Low water content
Protective dipicolinic acid-calcium complexes
Standard Dry Heat Sterilisation Schedules
Table
Temperature
Time
Notes
160°C
1 hour
Standard schedule
170°C
1 hour
Faster alternative
180°C
30 minutes
Rapid schedule
150°C
2.5 hours
Extended time for lower temp
Mnemonic
160-1-60 = 160°C for 1 hour (remember: 160 is the minimum, 1 hour is the minimum time)
Why NOT the Other Options?
121°C for 15 minutes: This is the standard for moist heat (autoclaving), not dry heat. Moist heat is more efficient because water conducts heat better than air.
100°C for 30 minutes: This is boiling water temperature — insufficient for spore sterilisation even with extended time.
80°C for 2 hours: Far too low; would only disinfect, not sterilise.
High-YieldNEET PG
Dry heat requires much higher temperatures and longer times than moist heat because air is a poor heat conductor. This is why autoclaving (121°C, 15 min) is preferred in clinical practice over dry heat ovens.
Clinical Pearl
Dry heat is used for:
Glassware and metal instruments (no corrosion)
Oils and powders (moisture-sensitive)
Items that cannot tolerate moisture
But it is slower and less efficient than autoclaving, so it is reserved for specific applications.
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