Question

Heat-stable liquids: water, salt solutions, autoclavable media and moderately heat-stable plastics used in animal cell culture can be sterilised by which of the following method?

• A. Moist heat
• B. Dry heat
• C. Gamma irradiation
• D. Short UV exposure

Hint:

Moist heat sterilization (autoclaving) requires that steam directly contacts all surfaces.
Caps on liquid bottles must be kept loose during the cycle to allow pressure equalization and prevent the bottles from bursting.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Sterilization is the complete destruction or removal of all forms of microbial life, including highly resilient bacterial endospores.
The choice of sterilization method depends on the physical and chemical properties of the material being sterilized.

Step 2: Detailed Explanation:
Physical sterilization methods are broadly classified into thermal (heat-based) and non-thermal (radiation or filtration) methods.
Thermal sterilization includes:
Dry heat: This method uses hot air ovens at high temperatures (typically $160^\circ\text{C}$ to $180^\circ\text{C}$) for at least two hours. It sterilizes via protein oxidation. It is used for glassware, metal instruments, and anhydrous materials, but it melts most plastics and evaporates liquids.
Moist heat (autoclaving): This method uses saturated steam under pressure in an autoclave. The standard operating condition is $121^\circ\text{C}$ at $15\text{ psi}$ pressure for $15$ to $20$ minutes.
Moist heat sterilizes by denaturing and coagulating essential microbial proteins and enzymes.
Water molecules facilitate heat transfer, making moist heat much more efficient than dry heat. This efficiency allows sterilization at lower temperatures and shorter exposure times.
Moist heat is the preferred method for sterilizing heat-stable liquids (such as water, physiological salt solutions, and some cell culture media) because the pressurized environment prevents the liquids from boiling and evaporating.
It is also suitable for moderately heat-stable plastics (such as polypropylene) used in laboratory consumables.
Radiation methods (like gamma irradiation) are reserved for industrial sterilization of heat-sensitive, single-use plastics, while UV light is used only for surface disinfection due to its poor penetration depth.

Step 3: Final Answer:
The specified materials are sterilized using moist heat (autoclaving).