Sterilization Methods

Definitions

Sterilization: kills ALL microorganisms + spores. ≥10⁶ log CFU reduction. Agent = sterilant.

Disinfection: kills most pathogens, spores may survive. ≥10³ log CFU reduction. Agent = disinfectant. On body surface = antiseptic; process = asepsis.

Cleaning/decontamination: reduces microbial load to safe level. ≥1 log CFU reduction. Soap/detergent, mechanical.

CSSD flow: Decontamination area → Packaging area → Sterilization area → Sterile storage area.

Spaulding’s Classification (1971)

Factors Affecting Efficacy

Organism load — more bioburden → more contact time needed.

Resistance hierarchy (decreasing): Prions > bacterial spores > coccidian oocysts > mycobacteria > non-enveloped viruses > fungi > vegetative bacteria > enveloped viruses.

Concentration — optimal only; too high corrodes, too low under-kills.

Contact time — MOST crucial factor.

Temperature, stability (hypochlorite unstable, prepare fresh daily), pH, humidity (critical for ETO).

Organic matter interferes with hypochlorite/QAC (overcome by cleaning first or ↑time/concentration); phenolics/glutaraldehyde unaffected.

Biofilm — blocks agent penetration.

Ideal agent: broad spectrum, fast, unaffected by organic matter, non-toxic, material-compatible, odorless, economical, eco-friendly.

Steam Sterilizer (Autoclave)

Principle: pressure cooker — ↑pressure → ↑boiling point. Moist heat → coagulation/denaturation of proteins.

Components: pressure chamber (cylinder + steam jacket), lid (discharge tap, pressure gauge, safety valve), electrical heater.

Phases: Conditioning (air displaced by steam, pressure to 15 psi) → Exposure/holding (~15 min) → Exhaust (cooling).

Standard condition: 121°C, 15 min, 15 psi. Range: 121-135°C, 3-18 min.

Uses: heat/moisture-resistant critical/semicritical items, culture media, BMW treatment.

Avoid: waterproof materials (oil/grease), dry powder. Separate autoclave for BMW.

Advantages: cheap, fast, nontoxic, no residue. Disadvantages: damages acrylics/PVC, corrodes some metals, harms electronics.

Flash sterilization: 134°C, 3-10 min, UNWRAPPED. Emergency only. Not for porous/cannulated/implants. Recontamination risk (wet, unwrapped).

Sterilization control: BI = Geobacillus stearothermophilus spores (killed in 12 min at 121°C). Chemical: autoclave tape, Bowie-Dick test, internal pack indicator. Physical: digital displays.

Ethylene Oxide (ETO) Sterilizer

Mechanism: alkylation of proteins/DNA/RNA. Broad spectrum incl. spores.

Stages: Preconditioning (vacuum, set temp/pressure/humidity) → Sterilization (4 parameters: gas conc., temp, humidity, exposure time; 700mg/L + 40-80% RH → 4-5hr@38°C or 1hr@55°C) → Aeration/degassing (8-12hr, removes toxic residue).

Uses: heat/moisture-sensitive critical items — heart-lung machine parts, sutures, catheters, stents, electronics, multi-lumen tubing.

Advantages: large chamber, heat-sensitive OK, high penetration, non-corrosive. Disadvantages: flammable, irritant, explosive, CARCINOGENIC, long cycle (12-14hr), costly.

BI: Bacillus atrophaeus.

Plasma Sterilization (Sterrad)

Steps: Vacuum → H2O2 injected (vaporized to 6mg/L, diffuses 50min @ 37-44°C) → Electrical field → gas plasma → H2O2 breaks to OH-/HO2- free radicals (microbicidal) → excess gas removed (byproducts nontoxic — no aeration needed).

Cycle: 75 min (older), 24-52 min (newer).

Uses: heat/humidity-sensitive but need faster than ETO — arthroscope, microsurgical/vascular instruments, laparoscope.

Limitations: items must be DRY before loading, no linen/paper/cellulose/liquid, poor lumen penetration, small chamber (no bulk).

BI: Bacillus (Geobacillus) stearothermophilus.

Dry Heat Sterilizer (Hot Air Oven)

For: moisture-damage-prone or moisture-impenetrable materials — glassware, powders, petroleum products, sharp instruments.

Mechanism: oxidation of cell constituents.

Cycles: 170°C/60min, 160°C/120min, 150°C/150min.

Advantages: nontoxic, low cost, good penetration, noncorrosive. Disadvantage: high temp unsuitable for most materials.

BI: Bacillus atrophaeus (more heat-resistant than G. stearothermophilus in dry conditions).

Filtration

Removes (not kills) organisms.

Depth filters — throughout thickness. Water purifiers only, NOT hospital sterilization.

Membrane filters — surface trapping by pore size. Hospital standard.

• Bacterial filter: 0.22 μm (removes bacteria, viruses pass)
• Viral filter: smaller pore

Air filtration:

• Surgical mask/respirator — simple pore filter
• HEPA filter — 99.97% of particles ≥0.3 μm. Used: biosafety cabinets, OT airflow, isolation rooms
• ULPA filter — 99.999% of particles ≥0.12 μm

Liquid filtration: dialysis water testing; heat-labile pharma fluids (sera, vaccines, antibiotics).

BI for membrane filters: Brevundimonas diminuta, Serratia marcescens.

Radiation

Ionizing (cobalt-60 gamma, electron accelerators): breaks DNA via ionization. Used: tissue grafts, pharmaceuticals, devices. = “COLD STERILIZATION” (no heat rise). Advantages: high penetration, rapid. Disadvantages: costly, damages polyethylene. BI: Bacillus pumilus.

Non-ionizing:

• Infrared — alternative for heat-resistant instruments
• UV — does NOT achieve true sterilization (spores survive) = intermediate-level disinfectant only

Incineration

BMW treatment (non-plastic infectious waste). 870-1200°C → ash+flue gas+heat. NEVER incinerate PVC/halogenated plastics — generates carcinogenic furans.

Microwave

2450 MHz radio-frequency → water molecule friction → heat. Small: contact lens, dental instruments, dentures, self-catheterization catheters. Large: plastic BMW disposal. Must check product compatibility.

Other Less Common Methods

Low-temp steam formaldehyde, beta-propiolactone vapor, gaseous chlorine dioxide, vaporized peracetic acid, vaporized H2O2, ozone.

Defining the Terms Precisely

Sterilization, disinfection, and cleaning are related but distinct processes, and the distinction is not academic — it determines which method is safe to use on which item.

Sterilization destroys or removes every living microorganism, including the most resistant bacterial spores, achieving at least a 10⁶ log reduction in colony-forming units. The agents that achieve this are called sterilants.

Disinfection destroys most pathogenic organisms but may leave spores intact, achieving roughly a 10³ log reduction, and is normally applied only to inanimate objects rather than body surfaces. When a disinfectant is formulated to be safe on skin or mucosa, that specific use is called an antiseptic, and the process itself is termed asepsis.

Cleaning (decontamination) simply reduces the microbial load to a level considered safe to handle without protective equipment — at least a 1 log reduction — achieved mechanically with soap and detergent to remove organic debris, without necessarily killing anything.

Most hospital sterilization for surgical instruments and other critical items runs through the Central Sterile Supply Department (CSSD), organized as four unidirectional zones separated by physical barriers: decontamination area (manual or automated cleaning) → packaging area (items enclosed to allow sterilant penetration while protecting against post-sterilization contamination) → sterilization area (steam, ETO, or plasma sterilizer) → sterile storage area (with an issue counter supplying OTs and wards).

Spaulding’s Classification — Matching the Method to the Risk

Earle Spaulding’s 1971 classification remains the working framework for deciding how aggressively an item needs to be reprocessed, based on the risk of infection its intended use carries:

What Determines Whether a Sterilant Actually Works

Several factors govern efficacy in practice, and all of them are exam-relevant because they explain why the same agent can succeed on one item and fail on another:

• Organism load — a higher bioburden needs a longer contact time to achieve the same log reduction.
• Nature of the organism — resistance varies enormously by microbial category. The decreasing order of resistance to sterilization/disinfection agents is: prions > bacterial spores > coccidian oocysts > mycobacteria > non-enveloped viruses > fungi > vegetative bacteria > enveloped viruses. This hierarchy is the reason spores specifically are used as biological indicators — if the most resistant organism is killed, everything less resistant is too.
• Concentration — must sit at the validated optimum; too high can corrode material, too low under-kills.
• Contact time — the single most crucial factor; insufficient exposure time simply does not achieve killing, regardless of how good the agent is otherwise.
• Temperature, stability, pH, humidity — each agent has its own sensitivities (hypochlorite is unstable and must be freshly prepared daily; ethylene oxide activity depends heavily on relative humidity).
• Organic matter (blood, pus, stool) — interferes with many agents (hypochlorite, QAC) unless the item is mechanically cleaned first or exposure is increased; a few agents (phenolics, glutaraldehyde) retain activity regardless.
• Biofilm — physically blocks the sterilant/disinfectant from reaching organisms embedded within it.

An ideal sterilant/disinfectant should be broad-spectrum, fast-acting, resistant to interference by organic matter, non-toxic, material-compatible, odourless or pleasant, economical, and environmentally safe — no single agent hits every criterion, which is why different sterilants dominate different niches.

Sterilants

Steam Sterilizer (Autoclave)

The autoclave is the workhorse of hospital sterilization, working on the same principle as a pressure cooker: raising the pressure inside a closed vessel raises the temperature at which water boils, and moist heat at that elevated temperature kills microorganisms by irreversible coagulation and denaturation of enzymes and structural proteins.

The device is a pressure chamber (a cylinder with a steam jacket), a lid carrying a discharge tap, pressure gauge, and safety valve, and an electrical heater. A cycle runs through three phases: conditioning (air is displaced by steam until the set pressure, typically 15 psi, is reached), exposure/holding (the actual sterilizing period, about 15 minutes at the standard setting), and exhaust (cooling back to atmospheric pressure). The most commonly used condition is 121°C for 15 minutes at 15 psi, though cycle duration can range 3–18 minutes across the 121–135°C range depending on the temperature chosen.

Steam sterilization is used for all heat- and moisture-resistant critical/semi-critical items — surgical instruments, drapes, linens — plus culture media preparation and biomedical waste treatment, but must never be used on waterproof materials (oil, grease) or dry powders, and separate autoclaves should be dedicated to biomedical waste. Its advantages are low cost, speed, and leaving no toxic residue; its disadvantages are potential damage to heat-sensitive plastics, corrosion of some metals, and moisture damage to electronics.

Flash sterilization is an emergency-only modification — 134°C for 3–10 minutes, instruments left unwrapped — reserved for situations like an instrument being contaminated mid-surgery and needing immediate replacement. It is unsuitable for porous, cannulated, or implanted items, and because instruments emerge wet and unwrapped, they carry a real recontamination risk.

Sterilization control uses Geobacillus stearothermophilus spores as the biological indicator (killed within 12 minutes at 121°C), alongside chemical indicators (autoclave tape, the Bowie-Dick test for air removal/steam penetration, internal pack indicators) and physical indicators (digital time/temperature/pressure displays).

Ethylene Oxide (ETO) Sterilizer

ETO is a gaseous chemical sterilant with genuinely broad microbicidal action, including against spores — it works by alkylating cell proteins, DNA, and RNA. A cycle has three stages: preconditioning (vacuum creation, then setting temperature/pressure/humidity), sterilization (gas exposure — governed by four parameters: gas concentration, temperature, relative humidity, exposure time; at 700 mg/L and 40–80% relative humidity, sterilization takes 4–5 hours at 38°C or 1 hour at 55°C), and aeration/degassing (8–12 hours, essential because ETO residue is toxic).

ETO is reserved for critical (and some semi-critical) items too heat- or moisture-sensitive for steam — heart-lung machine components, sutures, catheters, stents, electronic devices, multi-lumen tubing. Its advantages are large chamber capacity, suitability for heat-sensitive items, high penetration power, and non-corrosiveness; its disadvantages are that it is flammable, irritant, explosive, and carcinogenic, plus a long cycle (12–14 hours) and high cost. Bacillus atrophaeus spores are the biological indicator.

Plasma Sterilization

Plasma sterilizers (commercial brand: Sterrad) create a gaseous state of ions, photons, free electrons, and free radicals. The cycle: vacuum creation → injection of hydrogen peroxide solution, vaporized to 6 mg/L, diffusing through the chamber at 37–44°C for about 50 minutes → an electrical field applied to generate gas plasma, breaking H₂O₂ into microbicidal hydroxyl and hydroperoxyl free radicals → removal of excess gas (the by-products, water vapour and oxygen, are non-toxic, so no aeration step is needed, unlike ETO). Total cycle time is 75 minutes on older units, down to 24 minutes on newer ones.

It is used for materials that can’t tolerate steam’s heat/humidity but need faster turnaround than ETO — arthroscopes, microsurgical and vascular instruments, laparoscopes. Limitations: items must be dry before loading, it cannot process linen/paper/cellulose/liquids, it penetrates long narrow lumens poorly, and its small chamber rules out bulk loads. Bacillus stearothermophilus (i.e. Geobacillus stearothermophilus) spores are the biological indicator.

Dry Heat Sterilizer (Hot Air Oven)

Used for materials moist heat would damage or that moist heat cannot penetrate — glassware, powders, petroleum products, sharp instruments. It works by oxidation of cell constituents, at common cycles of 170°C/60 min, 160°C/120 min, or 150°C/150 min — all substantially longer exposure than steam sterilization, since dry heat transfers energy less efficiently than moist heat. Advantages: non-toxic, low operating cost, good penetration, non-corrosive to metals. Disadvantage: the high temperatures required are unsuitable for most materials. Bacillus atrophaeus spores are again the biological indicator (chosen because they are more heat-resistant than G. stearothermophilus under dry conditions).

Filtration

Filtration removes organisms physically rather than killing them, which is exactly why some authors dispute calling it “sterilization” at all — CDC nonetheless classifies it as such when it achieves the required removal. Depth filters trap particles throughout their thickness (used for drinking water purification, not hospital-grade sterilization); membrane filters trap everything larger than their pore size on the surface, and are the hospital standard — bacterial filters (0.22 μm pore) remove bacteria while letting viruses through; viral filters use an even smaller pore.

Air filtration uses surgical masks/respirators (simple pore-size filtration) and, at the high end, HEPA filters (removing 99.97% of particles ≥0.3 μm, used in biosafety cabinets, OT airflow systems, and isolation rooms) and ULPA filters (removing 99.999% of particles ≥0.12 μm). Liquid filtration is used for bacteriological water testing (especially dialysis water) and for sterilizing heat-labile pharmaceutical fluids — sera, vaccines, antibiotic solutions — that cannot survive any heat-based method. Brevundimonas diminuta and Serratia marcescens are used to test membrane filter efficacy.

Radiation

Ionizing radiation (cobalt-60 gamma rays, electron accelerators) sterilizes by directly breaking DNA through molecular ionization, used industrially for tissue grafts, pharmaceuticals, and medical devices. Because it works without raising temperature, it is also called cold sterilization — advantages are high penetrating power and rapid action, disadvantages are high cost and possible damage to polyethylene equipment. Bacillus pumilus is the biological indicator.

Non-ionizing radiation — infrared (an alternative for selected heat-resistant instruments) and ultraviolet (which does not achieve true sterilization — spores survive UV exposure — and is properly classed as an intermediate-level disinfectant instead).

Incineration

Used specifically for treating biomedical waste (non-plastic infectious waste), incineration burns material at 870–1200°C, converting it to ash, flue gas, and heat. Halogenated plastics (PVC) must never be incinerated, since doing so generates carcinogenic furans.

Microwave

Microwaves (2450 MHz radio-frequency waves) generate heat through water-molecule friction, used at small scale for disinfecting soft contact lenses, dental instruments, dentures, and self-catheterization urinary catheters, and at large scale for plastic biomedical waste disposal. Compatibility with the specific product must always be checked first.

Other, Less Common Methods

Low-temperature steam formaldehyde, beta-propiolactone vapour, gaseous chlorine dioxide, vaporized peracetic acid, vaporized hydrogen peroxide, and ozone round out the less frequently used sterilization methods, generally reserved for specific niche applications where the standard methods above are unsuitable.

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