4.2 Microbiology & Infection Control

Classifying Microorganisms

The CST exam tests four classes of microorganisms. Knowing their structure explains why some are easy to kill and others survive standard processing.

Prions are the most exam-worthy outlier: because they are misfolded proteins, they are not destroyed by routine steam sterilization. Instruments used on suspected CJD cases require special extended cycles or are discarded.

Bacterial Classification

Bacteria are sorted three ways:

• By Gram stain: Gram-positive organisms have a thick peptidoglycan wall and stain purple; gram-negative organisms have a thin wall plus an outer membrane and stain pink/red. The gram-negative outer membrane contains endotoxin (lipopolysaccharide), which drives septic shock.
• By shape: cocci (round), bacilli (rods), and spirilla/spirochetes (spiral).
• By oxygen requirement: aerobic organisms need oxygen, anaerobic organisms grow without it, and facultative anaerobes survive either way.

Spore-Formers — The Sterilization Benchmark

The genera Clostridium and Bacillus form endospores — dormant, dehydrated structures that resist heat, chemicals, and drying for years. Because spores are the hardest microbial form to kill, they are the standard against which sterilization is measured. The biological indicator used to test an autoclave contains Geobacillus stearothermophilus spores: if those spores are killed, every less-resistant microbe is too.

Asepsis, Antisepsis, Disinfection, and Sterilization

These four terms are frequently confused on the exam — distinguish them precisely:

The critical distinction: antiseptics are safe on skin; disinfectants are for surfaces and are too harsh for tissue; only sterilization eliminates spores.

Surgical-Site Infections & Transmission

A surgical-site infection (SSI) is an infection at the operative site within 30 days (or up to 90 days with an implant). The most common SSI source is the patient's own resident flora — for skin incisions, that means Staphylococcus aureus and Staphylococcus epidermidis (a normal skin commensal).

Modes of Transmission

• Contact (direct or indirect — most common in surgery)
• Droplet (large respiratory droplets, e.g., influenza)
• Airborne (tiny nuclei that stay suspended — tuberculosis, measles)
• Vehicle (contaminated solutions or instruments)
• Vector (insects — rare in the OR)

Key Healthcare-Associated Pathogens

• MRSA (methicillin-resistant Staphylococcus aureus) — a gram-positive coccus resistant to many antibiotics; a leading cause of SSIs.
• Clostridioides difficile (C. diff) — a spore-forming anaerobe causing severe diarrhea/colitis. Because spores resist alcohol, soap-and-water hand washing (not alcohol rub) is required after C. diff contact.
• VRE, Pseudomonas aeruginosa, and Acinetobacter are other resistant healthcare-associated organisms.

The single most effective measure to prevent transmission of all of these is hand hygiene. Surgical Care Improvement and CDC guidelines also emphasize prophylactic antibiotics within 60 minutes before incision, normothermia, and glucose control.

Normal Flora, Wound Classification, and the Chain of Infection

Normal flora (the microbial population that normally lives on skin and mucous membranes) is usually harmless in its native site but becomes a pathogen when introduced into a sterile space — exactly what a surgical incision creates. This is why the skin commensals Staphylococcus epidermidis and Cutibacterium acnes are common SSI culprits.

Surgical Wound Classification

The CDC classifies wounds by their expected microbial load, which predicts infection risk:

Knowing the wound class helps the team decide on antibiotics and whether to close the wound primarily.

The Chain of Infection

Infection requires six links: an infectious agent, a reservoir (where it lives), a portal of exit, a mode of transmission, a portal of entry, and a susceptible host. Sterile technique works by breaking the chain — sterilization removes the agent, asepsis blocks transmission, and the sterile barrier closes the portal of entry. If any single link is broken, infection cannot occur, which is the underlying logic of every infection-control practice in the OR.

Standard Precautions

The surgical technologist treats all blood and body fluids as potentially infectious under Standard Precautions. This means wearing gloves, gown, mask, and eye protection during any case with splash or aerosol risk, and using engineering controls such as a neutral zone ("hands-free" technique) for passing sharps to prevent sharps injuries and bloodborne pathogen exposure.