Sterilization and Disinfection Methods

The device that was cleaned, just not enough

Endoscopes present a genuinely difficult reprocessing problem: they're expensive, reusable, contain long narrow internal channels that are hard to fully access, and they contact mucous membranes deep inside the body during almost every procedure. In the mid-2010s, a series of real, well-documented U.S. healthcare-associated infection investigations traced clusters of carbapenem-resistant Enterobacteriaceae (CRE) infections back to inadequately reprocessed duodenoscopes, despite staff having followed a cleaning protocol.

The failure wasn't carelessness. It was that a device contacting mucous membranes needs to reach a specific, defined level of microbial killing, high-level disinfection at minimum, and the combination of the device's complex internal geometry and the cleaning protocol in use didn't reliably achieve that level inside every channel, every time. A blood pressure cuff, which only ever touches intact skin, can be safely reused after a much lower level of cleaning. An endoscope cannot. Treating the two as if they belonged in the same category, even informally, is exactly the kind of gap that leads to outbreaks like this one.

The framework that exists specifically to prevent this confusion, deciding exactly how sterile a given device needs to be, based on what part of the body it will actually touch, is called the Spaulding Classification, and it's the organizing idea behind everything in this article.

Sterilization is the process of killing or removing all microorganisms, including highly resistant bacterial endospores. Depending on the material, sterilization can be achieved by moist heat (autoclave), dry heat (hot air oven), ethylene oxide gas, or filtration.

However, sterilization is not always attainable or practical, for example, in fresh produce processing. In those cases, rapid microbial growth is limited instead by decontamination and disinfection.

Figure: Methods of sterilization and disinfection

Disinfection is the killing of pathogenic organisms, but not necessarily all microorganisms. Pathogenic bacteria are destroyed during disinfection, but some microorganisms, including bacterial endospores, may remain viable. Disinfectant choice depends on the intended use, inanimate objects versus skin surfaces. Common disinfectants include phenol-containing compounds, quaternary ammonium compounds, alcohol, chlorine, and iodine.

Decontamination is the removal of pathogenic microorganisms from inanimate objects or surfaces so that items are safe to handle or dispose of, the least rigorous of the three terms. Simply wiping a table after a meal, for example, removes contaminating microorganisms and their potential nutrients, without claiming to kill anything specific.

The Spaulding Classification

Deciding which of these three processes a given item actually needs isn't guesswork. The Spaulding Classification, developed by Earle Spaulding and adopted by the CDC, sorts medical devices into three categories based on the infection risk of how they're used, not on how the device looks or feels:

This is exactly the framework the duodenoscope outbreaks above ran into: a semicritical device needs high-level disinfection every single time, in every channel, and any gap between that requirement and what actually happens during reprocessing becomes a real infection risk.

Factors Affecting Sterilization and Disinfection Success

Several factors determine whether a given sterilization or disinfection method actually succeeds: the microbial load and amount of organic material present, the type of organism involved (bacterial spores are far more resistant than most vegetative bacteria or enveloped viruses), the concentration and exposure time of the germicide, the physical nature of the surface, and temperature and pH.

For the full breakdown of each factor, plus the complete hierarchy of microbial resistance, see Disinfection: Methods and Uses.

Sterilization Methods

Physical Agents of Sterilization

Heat, radiation, and filtration are the most commonly used physical sterilization methods, with heat being the most widely used overall.

Incineration. Treating infectious waste by burning it to ash at 870–980°C. Incineration is the most complete method available for destroying pathogenic microorganisms, bacterial endospores, and even prions, though toxic air emissions and heavy metals in the resulting ash have limited its broader use.

Moist heat sterilization. Using autoclaves, moist heat (saturated steam under pressure) sterilizes bio-hazardous waste and heat-stable objects, the fastest and simplest physical sterilization method available. The two common sterilization parameters are 121°C for 15 minutes and 132°C for 5 minutes. Read the full mechanism, including boiling, tyndallization, and pasteurization, in Moist Heat Sterilization, or the full autoclave procedure and parts in Autoclave Sterilization.

Image: Medical Autoclave

Dry heat sterilization. Requires longer exposure (1.5–3 hours) and higher temperatures (160–180°C) than moist heat, used for glassware, oils, petrolatum, and powders. Read more in Dry-Heat Sterilization, or about the equipment itself in Hot Air Oven: Parts, Types, and Uses.

Source: Autoclave

Radiation. Ionizing radiation generates reactive species (electrons, hydroxyl and hydride radicals) that damage DNA, lipids, and protein, killing the irradiated cell. The FDA has approved radiation for sterilizing surgical supplies, disposable labware, drugs, and tissue grafts; the WHO has approved it for decontaminating foods susceptible to microbial contamination, such as fresh produce, meat, poultry, and spices. Read the full mechanism, including non-ionizing UV disinfection, in Radiation Sterilization.

Filtration. Passing an aqueous liquid through the tiny pores of a membrane filter traps microorganisms present in the liquid, rendering it sterile, without killing anything. High-efficiency particulate air (HEPA) filters serve the equivalent role for air filtration.

Read the full mechanism, including what filtration doesn't remove, in Filtration Sterilization.

Chemical Agents of Sterilization

Chemicals used to destroy all forms of life, including spores, are called chemical sterilants or biocides. Ethylene oxide and hydrogen peroxide are common examples. The same chemicals, used at lower concentration or for shorter contact time, function as disinfectants instead. Ethylene oxide (EtO) is the most common chemical sterilant for heat-sensitive objects; vapor-phase hydrogen peroxide is used to sterilize HEPA filters in biological safety cabinets and both metal and nonmetal medical instruments.

Disinfection Methods

Physical Methods of Disinfection

Boiling and pasteurization. Boiling at 100°C kills most vegetative bacteria and non-sporing organisms but does not reliably kill bacterial endospores, making it a disinfection method, not a sterilization one. Pasteurization uses controlled heat to reduce pathogens (including those causing tuberculosis, brucellosis, and typhoid) without sterilizing the product.

For the full comparison of boiling, tyndallization, pasteurization, and true steam sterilization, see Moist Heat Sterilization.

Non-ionizing radiation (UV). UV rays are long-wavelength, low-energy radiation that can damage microbial DNA directly, but they don't penetrate solid, opaque, or light-absorbing surfaces. This is why UV disinfection is limited to directly exposed surfaces, such as the work surface of a biological safety cabinet after use.

Image: Use of UV Light for Sterilization

For the full mechanism, including how UV was used during the COVID-19 pandemic, see Radiation Sterilization.

Chemical Methods of Disinfection and Levels of Disinfection

Chemical disinfectants are classified by how much they can kill: low-level disinfectants handle most vegetative bacteria and some fungi/viruses; intermediate-level disinfectants add mycobacteria; high-level disinfectants kill everything except large numbers of bacterial spores. Common chemical disinfectants include alcohols, phenolics, aldehydes, halogens (chlorine and chlorine compounds), hydrogen peroxide, peracetic acid, and quaternary ammonium compounds. Bleach (sodium hypochlorite) is commonly used to disinfect food-preparation surfaces.

Image source: Chemical disinfectants

For the full activity-level table, the hierarchy of microbial resistance, and why some disinfectants can't be relied on against spore-forming pathogens, see Disinfection: Methods and Uses.

Antiseptics

Antiseptics are disinfectants formulated for use on living tissue, such as skin or mucous membranes, rather than inanimate objects. They matter for personal laboratory safety and for preparing patients before invasive procedures such as blood draws or surgery.

Iodine compounds (tincture-iodine or povidone-iodine) are widely used antiseptics. A combination of 70% ethyl alcohol followed by an iodophor is the most common approach to skin disinfection before drawing blood cultures or performing surgery.

Clinical Use

Sterilization and disinfection are basic components of hospital infection control. According to the CDC, more than 99,000 deaths occur each year in the United States alone due to hospital-acquired infections, making the correct application of the framework above a matter of real clinical consequence, not just an exam topic.

Devices intended to enter normally sterile sites (surgical instruments, cardiac catheters, implants, needles) should be sterilized by steam, plasma, or ethylene oxide, per the Critical category above. Devices contacting only intact skin (blood pressure cuffs, stethoscopes) are decontaminated with an intermediate- or low-level disinfectant, per the Noncritical category.

Commonly used disinfection or sterilization methods and their clinical use:

How to Remember

The "risk elevator" analogy for the Spaulding Classification. Picture medical devices riding an elevator with three floors, based on how deep into the body they go. Ground floor: intact skin only (noncritical), a quick clean is enough. Middle floor: mucous membranes (semicritical), the device needs high-level disinfection before it's allowed back on. Top floor: sterile tissue or bloodstream (critical), nothing short of full sterilization is acceptable. The higher the floor, the less room there is for anything to survive.

Anchor for sterilization vs. disinfection vs. decontamination: sterilization means zero survivors, no exceptions, spores included. Disinfection means the dangerous ones are dead, but spores might still be there. Decontamination is the lowest bar of all: just "safe enough to handle," with no specific claim about what's still alive.

Anchor for the hook: the duodenoscope outbreaks happened because a semicritical device needs high-level disinfection reliably, every channel, every time, and a complex device's geometry made that harder to guarantee than a simple blood pressure cuff ever would be. The Spaulding Classification exists precisely so that "how sterile does this need to be" is never left to judgment call.

Key exam facts in one table

Where Students Get Confused

• Confusing high-level disinfection with sterilization. High-level disinfection kills everything except large numbers of bacterial spores. Only sterilization guarantees zero survivors, spores included. This distinction is exactly what separates the Critical and Semicritical Spaulding categories.
• Using "decontamination" and "disinfection" interchangeably. Decontamination is the broadest, least rigorous term, "safe to handle", with no specific claim about which organisms are dead. Disinfection specifically means pathogens are killed, even if some other organisms (including spores) survive.
• Assuming Spaulding classification depends on a device's physical properties. It depends entirely on what tissue the device contacts in use, not on whether it's rigid, flexible, disposable, or reusable.
• Assuming any hospital-grade method works for any device. A method appropriate for a noncritical item (like a stethoscope) is not automatically adequate for a semicritical or critical one, exactly the gap that caused real duodenoscope-related outbreaks.

References

1. Forbes, B. A., Sahm, D. F., & Weissfeld, A. S. Bailey & Scott's Diagnostic Microbiology (11th ed.).
2. Review of Medical Microbiology and Immunology (13th ed.). Lange Medical Books.
3. Mohapatra, S. (2017). Sterilization and Disinfection. In Essentials of Neuroanesthesia (pp. 929–944). https://doi.org/10.1016/B978-0-12-805299-0.00059-2
4. Madigan, M., Martinko, J., Stahl, D., & Clark, D. (2018). Brock Biology of Microorganisms (15th ed.). Pearson Education.

5. Centers for Disease Control and Prevention. (2008). Guideline for Disinfection and Sterilization in Healthcare Facilities. https://www.cdc.gov/hicpac/pdf/guidelines/Disinfection_Nov_2008.pdf