Microbiology Foundations for Aseptic Practice

Why Microbiology Matters in the OR

Surgical site infections (SSIs) are among the most serious complications a patient can experience after an operation. As a surgical technologist (ST), you are one of the primary defenders of the sterile field. To protect that field effectively, you must first understand what you are defending against: the microorganisms that cause infection, how they travel, and why certain control measures work against some but not others.

Types of Microorganisms Relevant to Surgical Practice

Bacteria

Bacteria are single-celled prokaryotic organisms — they have no true nucleus. They reproduce rapidly by binary fission and are classified by shape (cocci, bacilli, spirilla) and by their response to Gram staining (Gram-positive or Gram-negative). This distinction matters in the OR because it predicts which organisms are likely present on skin, in the gut, or in the environment, and how resistant they may be to certain disinfectants.

Common SSI-causing bacteria include Staphylococcus aureus (including methicillin-resistant strains, MRSA), Streptococcus species, Pseudomonas aeruginosa, and Escherichia coli. Bacteria are effectively destroyed by standard steam sterilization (autoclaving), chemical sterilants, and most high-level disinfectants when contact time and concentration are correct.

Fungi

Fungi are eukaryotic organisms — they have a true nucleus — and include yeasts and molds. Candida species can cause SSIs, particularly in immunocompromised patients or in procedures involving the abdomen. Fungal spores are more resistant to environmental stressors than vegetative bacteria but are generally eliminated by standard sterilization methods used in surgical settings.

Viruses

Viruses are not true cells; they are obligate intracellular parasites that require a host cell to replicate. They are far smaller than bacteria. In the surgical setting, bloodborne viruses — including human immunodeficiency virus (HIV), hepatitis B virus (HBV), and hepatitis C virus (HCV) — pose significant occupational and patient-safety risks. Transmission routes include contact with infected blood or body fluids through percutaneous injury (needlestick or sharps injury) or mucous membrane exposure. Standard precautions and careful sharps handling are the primary protective strategies. Most enveloped viruses are susceptible to alcohol-based agents and standard sterilization; non-enveloped viruses are more resistant to some disinfectants.

Prions

Prions are misfolded proteins — they contain no DNA or RNA. They cause fatal neurodegenerative diseases such as Creutzfeldt-Jakob disease (CJD). Prions are extraordinarily resistant to conventional sterilization, including standard steam autoclaving, most chemical disinfectants, and radiation. Instruments used on patients with known or suspected prion disease require special decontamination protocols established by facility policy and public health guidelines. The ST must be aware of prion risk and follow institutional protocol precisely.

Transmission Routes and SSI Risk

Understanding how microorganisms travel from a source to a susceptible host is the foundation of aseptic technique. The chain of infection has six links:

1. Infectious agent — the pathogen itself

2. Reservoir — where the organism lives (patient skin, OR personnel, environment, instruments)

3. Portal of exit — how it leaves the reservoir (droplet, shedding, bleeding)

4. Mode of transmission — contact (direct or indirect), droplet, or airborne

5. Portal of entry — how it enters the new host (surgical incision, mucous membrane)

6. Susceptible host — the surgical patient, whose immune defenses may be compromised

The ST can break this chain at multiple links: performing a surgical scrub, maintaining sterile technique, confining movement near the sterile field, and ensuring instruments are properly sterilized before use.

Sterilization Effectiveness by Organism Type

Not all sterilization methods are equally effective against all microorganisms. The Spaulding classification guides instrument processing, and the hierarchy of microbial resistance (from most to least resistant) is generally: prions → bacterial spores → mycobacteria → non-enveloped viruses → fungi → vegetative bacteria → enveloped viruses. Steam sterilization (autoclave) is effective against bacteria, fungi, and most viruses but requires augmented protocols for prions. Ethylene oxide (ETO) gas sterilization covers heat-sensitive items and is broadly effective. Hydrogen peroxide plasma systems are also used for heat-sensitive devices.

Safety Points for the Surgical Technologist

• Never assume an instrument is sterile because it looks clean. Visual cleanliness is not sterility.
• Break the chain of infection by maintaining a strict sterile conscience: if you see or suspect a breach of sterile technique, speak up immediately, regardless of who committed it.
• Standard precautions apply to every patient, every case. Treat all blood and body fluids as potentially infectious.
• Prion-risk cases require you to follow your facility's specific written protocol. When in doubt, ask your supervisor before the case begins, not after.
• Report any sharps injury immediately according to facility policy to initiate post-exposure evaluation.

A firm grasp of these microbiology fundamentals gives you the scientific reasoning behind every aseptic practice you will perform throughout your career.