Microbiology Foundations

Understand the microorganisms relevant to healthcare, their structures, reproduction, transmission, and the principles of controlling microbial spread.

Types of Microorganisms

Bacteria, viruses, fungi, parasites, and prions

Microorganisms are classified by their cellular structure and reproduction. Bacteria are prokaryotic (no nucleus). Viruses are acellular (not truly alive, require a host cell to replicate). Fungi are eukaryotic (have a nucleus). These structural differences determine how each type is targeted by antimicrobial agents. Understanding these distinctions is essential because treatment strategies differ fundamentally between organism types.

The Five Categories of Microorganisms

Normal Flora & Colonization

The body's resident microbial communities

The human body hosts trillions of microorganisms collectively called the normal flora (microbiome). Resident microorganisms colonize body surfaces (skin, gut, respiratory tract, urogenital tract) without causing disease under normal conditions. They provide protective functions including competitive exclusion of pathogens, vitamin synthesis, and immune system development. These organisms are not contaminants, they are essential partners in health.

Protective Functions

Competitive exclusion: Normal flora occupy binding sites and consume nutrients, preventing pathogens from establishing. Immune training: Constant exposure to commensal organisms trains the immune system. Vitamin production: Gut bacteria synthesize vitamin K and some B vitamins.

When Flora Becomes Harmful

Opportunistic infection: Normal flora can cause infection if displaced to a sterile site (e.g., E. coli from gut to urinary tract → UTI). Antibiotic disruption: Broad-spectrum antibiotics kill normal flora, allowing resistant organisms like C. difficile to overgrow.

Colonization vs Infection

Colonization means microorganisms are present and multiplying but not causing tissue damage or immune response. Infection means microorganisms are invading tissue, multiplying, and causing damage with an immune response (signs: redness, swelling, heat, pain, loss of function). A patient can be colonized with MRSA on their skin without being infected, but if that MRSA enters a wound, infection develops.

Microbial Transmission

How microorganisms spread between hosts

Understanding transmission routes is the foundation of infection prevention. Strategies to interrupt the chain of infection target the weakest links: hand hygiene (breaking the mode of transmission), PPE (protecting the portal of entry), and isolation precautions (containing the reservoir). Each route requires different prevention strategies.

Contact Transmission

Direct contact: Person-to-person physical transfer (touching, bodily fluids). Indirect contact: Transfer via contaminated objects (fomites), doorknobs, stethoscopes, bed rails. This is the most common transmission route in healthcare settings. Prevention: Hand hygiene is the single most effective measure.

Droplet Transmission

Large respiratory droplets (>5 μm) generated by coughing, sneezing, or talking. They travel short distances (typically <1 meter) and fall quickly due to gravity. Examples: influenza, pertussis. Prevention: Surgical mask within close proximity.

Airborne Transmission

Tiny droplet nuclei (<5 μm) that remain suspended in air for extended periods and can travel long distances through air currents. Examples: tuberculosis, measles, varicella. Prevention: N95 respirator, negative-pressure room, airborne precautions.

Sterilization, Disinfection & Asepsis

Levels of microbial control

Microbial control exists on a spectrum from sterilization (the complete destruction or removal of ALL forms of microbial life, including bacterial endospores, achieved through autoclaving, ethylene oxide gas, or ionizing radiation) to basic cleaning. Understanding this hierarchy prevents both under-treatment (infection risk) and over-treatment (unnecessary cost/damage).

Sterilization (Highest Level)

Destroys ALL microorganisms including spores. Methods: autoclaving (121°C, 15 psi, 15+ min), chemical sterilants (glutaraldehyde), ethylene oxide gas. Used for: surgical instruments, implants, items entering sterile body cavities.

High-Level Disinfection

Destroys all microorganisms EXCEPT high numbers of bacterial spores. Used for semi-critical items that contact mucous membranes or non-intact skin (endoscopes, respiratory therapy equipment).

Intermediate & Low-Level Disinfection

Intermediate: Kills vegetative bacteria, most viruses and fungi, mycobacteria. Low: Kills most bacteria and some viruses/fungi. Used for non-critical items (blood pressure cuffs, stethoscopes, environmental surfaces).

Antisepsis

Application of antimicrobial agents to LIVING tissue (skin, mucous membranes). Antiseptics are safe for skin; disinfectants are NOT. Example: chlorhexidine for surgical hand scrub, povidone-iodine for skin prep. Never use a disinfectant as an antiseptic.

Critical Distinction

Antiseptics are for living tissue. Disinfectants are for inanimate objects. Using a disinfectant on skin causes chemical burns. Using an antiseptic on a surgical instrument does not achieve sterilization. Matching the right agent to the right surface is a safety fundamental.

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