Methods of Sterilization

Definition

Sterilization is a process that destroys or eliminates all forms of microbial life, including bacterial spores (the most resistant form). It is carried out in healthcare facilities by physical or chemical methods. When chemicals are used to destroy all microbial life, they are called chemical sterilants.

Key distinction: Sterilization is an absolute process — an item is either sterile or it is not. Terms like "partially sterile" are incorrect. Sterilization differs from disinfection, which reduces (but may not eliminate) microbial load. (Disinfection and Sterilization in Healthcare Facilities, p. 9)

Classification of Sterilization Methods

A. Physical Methods

1. Heat Sterilization

Most medical and surgical devices are heat-stable and undergo heat-based sterilization.

Type	Method	Temperature	Duration	Mechanism
Moist Heat (Autoclave)	Steam under pressure	121°C (15 psi) or 132–134°C (30 psi)	15–30 min	Denaturation of proteins by steam
Dry Heat	Hot air oven	160–170°C	1–2 hours	Oxidation of cellular components
Boiling	Boiling water	100°C	20 min	Kills vegetative forms (NOT spores — not true sterilization)
Incineration	Burning	>1000°C	—	Combustion of all microbial forms

Autoclave (Steam under pressure) is the gold standard for heat-stable items (surgical instruments, drapes, glassware).
Dry heat is used for items damaged by moisture (powders, oils, glass syringes).

2. Radiation Sterilization

Type	Application	Mechanism
Ionizing radiation (Gamma rays, Electron beam, X-rays)	Industrial sterilization of single-use medical devices (syringes, gloves, sutures)	DNA strand breakage
Non-ionizing radiation (UV light)	Surface/air sterilization in labs, OTs	Formation of thymine dimers in DNA

Gamma irradiation is widely used for factory sterilization of pre-packaged disposables.
UV is limited to surface sterilization due to poor penetration.

3. Filtration

Used to sterilize heat-sensitive liquids and gases (IV fluids, vaccines, air in laminar flow hoods).
Common filters: Millipore filters (0.22 µm pore size) — removes bacteria and fungi but NOT viruses.
HEPA filters are used for air sterilization in operation theaters and biosafety cabinets.

B. Chemical Methods

1. Gaseous / Low-Temperature Sterilization

Used for heat- and moisture-sensitive devices (endoscopes, plastics, electronic equipment).

Agent	Method	Notes
Ethylene Oxide (EtO)	Gas exposure at 37–55°C	Used since the 1950s; alkylates DNA/proteins; items require aeration post-sterilization (toxic residues)
Hydrogen Peroxide Gas Plasma	Low-temperature plasma	Rapid, no toxic residue; used for heat-sensitive devices
Formaldehyde Gas	Low-pressure steam + formaldehyde	Less commonly used; toxic
Ozone	Gaseous ozone	Oxidative mechanism; emerging technology

(Disinfection and Sterilization in Healthcare Facilities, p. 59)

2. Liquid Chemical Sterilants (High-Level Disinfection/Sterilization)

Used for critical items that cannot withstand heat or gas sterilization.

Agent	Examples	Mechanism
Glutaraldehyde	Cidex (2%)	Alkylation of proteins and nucleic acids
Peracetic Acid	Steris system	Oxidative destruction of cellular components
Ortho-phthalaldehyde (OPA)	Cidex OPA	Reacts with amino acids; superior mycobactericidal activity
Hydrogen Peroxide (6–7.5%)	—	Oxidative damage to membranes, enzymes, DNA

Summary Table

Method	Mechanism	Best Used For	Limitations
Steam autoclave	Protein denaturation	Metal instruments, drapes	Cannot use for heat-sensitive items
Dry heat	Oxidation	Powders, oils, glassware	Long exposure time, high temp
EtO gas	Alkylation	Plastics, electronics, endoscopes	Toxic, requires aeration
H₂O₂ gas plasma	Oxidation	Heat/moisture-sensitive devices	Cannot be used for lumened devices >1m
Gamma radiation	DNA strand breaks	Pre-packaged disposables	Expensive, industrial use
Filtration	Physical removal	Liquids, gases	Does not remove viruses
Peracetic acid	Oxidation	Endoscopes	Single-use; no long-term storage
Glutaraldehyde	Alkylation	Endoscopes, lensed instruments	Toxic fumes, long exposure (10 hrs)

Key principle: The choice of sterilization method depends on the nature of the item (heat-stable vs. heat-sensitive), the material composition, and the intended use (critical vs. semi-critical vs. non-critical device).

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