Sterilization and disinfection – Identify the truly bactericidal method: Which one of the following procedures reliably kills bacteria (rather than merely removing, inhibiting, or preserving them)?

Introduction / Context:
Sterilization and preservation methods differ in whether they kill microorganisms outright or merely halt their growth or remove them from a sample. This question asks you to identify the approach that is truly bactericidal, meaning it causes irreversible microbial death rather than simple inhibition, removal, or preservation.

Given Data / Assumptions:

• Bactericidal methods inactivate vital cellular targets and lead to death.
• Some methods only lower temperature or water activity to pause growth.
• Filtration can remove cells without killing them, leaving viable organisms on the filter.

Concept / Approach:
Ionizing radiation (e.g., gamma rays from cobalt-60, electron beams) is recognized as a terminal sterilization modality. It produces DNA double-strand breaks and free-radical damage to proteins and membranes. By contrast, cold-based methods preserve viability, and filtration physically separates microbes from fluids without lethality. Therefore, select the option that kills rather than stores or separates microbes.

Step-by-Step Solution:
1) Define bactericidal: a process that irreversibly destroys bacterial viability.2) Evaluate ionizing radiation: causes DNA damage and oxidative injury that cells cannot repair at sterilizing doses → bactericidal.3) Evaluate membrane filtration: 0.22 µm filters trap bacteria but do not kill; trapped cells remain viable.4) Evaluate freeze-drying and deep freezing: both are preservation techniques that reduce water activity or temperature, maintaining organisms for long-term storage, not killing them.5) Confirm the best match: ionizing radiation.

Verification / Alternative check:
Ionizing radiation is widely used to sterilize single-use medical devices, pharmaceuticals, and heat-sensitive materials, demonstrating its bactericidal (and sporicidal, with adequate dose) capacity at scale.

Why Other Options Are Wrong:

• Membrane filtration: removal without killing; viable cells remain on the membrane.
• Freeze-drying: preserves cultures; not lethal.
• Deep freezing: preserves long-term viability; not lethal.
• Refrigeration: slows growth; not bactericidal.

Common Pitfalls:
Equating “no growth” during storage with death; assuming any “processing step” must be sterilizing; overlooking that filtration is a physical separation, not a kill step.

Final Answer:
Ionizing radiation (gamma rays or high-energy electrons).