Microbiology Foundations for Sterilization

Why Microbiology Matters to the Surgical Technologist

Every instrument, drape, and implant that enters or contacts a patient's body carries the potential to introduce microorganisms. Understanding what those microorganisms are, how they behave, and how resistant they are to sterilization and disinfection is foundational knowledge for every surgical technologist. Patient safety depends on selecting the right level of treatment for every item—every time.

The Major Classes of Microorganisms

Bacteria are single-celled prokaryotes. Most vegetative (actively growing) bacteria are relatively easy to kill with standard disinfectants and sterilization methods. However, certain gram-positive bacteria—notably Clostridioides difficile and Bacillus species—produce endospores, dormant structures with thick protein coats that are dramatically more resistant to heat, chemicals, and radiation than vegetative cells.

Viruses vary widely in their resistance. Enveloped viruses (influenza, HIV, HBV, HCV) have a lipid outer membrane that is easily disrupted by many disinfectants; they are among the easiest pathogens to inactivate. Non-enveloped viruses (norovirus, adenovirus, poliovirus) lack this membrane and require higher-level disinfectants or sterilization to reliably destroy them.

Fungi, including Candida and Aspergillus species, are eukaryotic organisms. Vegetative fungal cells are generally susceptible to intermediate- and high-level disinfectants. Fungal spores are more resistant than vegetative cells but still far less resistant than bacterial endospores.

Bacterial spores represent the benchmark of microbial resistance in sterilization science. The spore's multilayered coat, low water content, and unique chemistry protect its DNA and metabolic machinery. Sterilization processes are validated against spore-forming organisms (such as Geobacillus stearothermophilus for steam and Bacillus atrophaeus for dry heat and ethylene oxide) precisely because destroying spores demonstrates the process is capable of destroying everything else.

Prions are misfolded proteins—they contain no nucleic acid at all. Because conventional sterilization targets DNA, RNA, or cell membranes, prions are uniquely resistant to all standard methods: steam sterilization at typical parameters, most chemical disinfectants, ultraviolet radiation, and ionizing radiation. They are the most resistant agents encountered in surgical practice.

The Spaulding Classification System

Developed by Earle H. Spaulding and adopted by the CDC, this system categorizes patient-care items by the infection risk they pose and specifies the minimum level of treatment required.

Critical items penetrate sterile tissue or the vascular system (surgical instruments, implants, needles). They must be sterilized—all forms of microbial life, including spores, must be eliminated. No exceptions.

Semicritical items contact intact mucous membranes or non-intact skin but do not penetrate sterile tissue (flexible endoscopes, laryngoscope blades, respiratory therapy equipment). They require, at minimum, high-level disinfection (HLD), which destroys all microorganisms except large numbers of bacterial spores.

Non-critical items contact only intact skin (blood pressure cuffs, ECG leads, bed rails). They require low- to intermediate-level disinfection, which addresses vegetative bacteria, most viruses, and fungi, but not necessarily spores.

Misclassifying an item—for example, treating a critical item with only intermediate-level disinfection—creates a direct patient safety risk and is a scope-of-practice violation.

Resistance Hierarchy: Least to Most Resistant

From easiest to hardest to eliminate:

1. Enveloped viruses (HIV, HBV, HCV, influenza)

2. Vegetative bacteria (including MRSA)

3. Fungi (vegetative forms)

4. Non-enveloped viruses (norovirus, adenovirus)

5. Mycobacteria (Mycobacterium tuberculosis)

6. Bacterial spores (C. difficile, Bacillus spp.)

7. Prions

MRSA is a vegetative bacterium. Despite its clinical danger due to antibiotic resistance, it is no more resistant to sterilization or disinfection than other non-spore-forming bacteria; standard sterilization easily destroys it. Its threat is in living patients, not on properly processed instruments.

Prions: Why Standard Steam Is Not Enough

Prions cause transmissible spongiform encephalopathies (TSEs), including Creutzfeldt-Jakob disease (CJD). They bind tightly to metal instrument surfaces and resist the protein-denaturing temperatures used in standard steam sterilization cycles. Current guidance from organizations such as the CDC and AAMI recommends enhanced reprocessing protocols for instruments that have contacted high-risk neurological tissue in known or suspected prion cases. These enhanced measures may include extended steam sterilization at elevated temperature and pressure, or instrument quarantine and destruction, depending on facility policy and the specific clinical situation. Surgical technologists must recognize when prion precautions apply and escalate immediately to the sterile processing team and surgeon.

Safety Principles for the OR

• Always verify the sterilization indicator on every package before opening it onto the sterile field. A passed external indicator does not confirm sterility; internal chemical indicators and biological indicators (spore tests) provide stronger evidence.
• Never downgrade the treatment level of a critical item due to time pressure. If a sterilized item is unavailable, the procedure must wait or use an alternative sterile item—patient safety is non-negotiable.
• Follow your facility's written policy for prion-risk instruments; do not improvise.
• Report any breach of sterilization technique immediately through your facility's quality and safety reporting system.