5.1 Human Pathogens

Assessment and Identification of Unknown Pathogens

  • Human Microflora Context:

    • The human body contains 10×10\times more bacterial cells than its own eukaryotic cells.

    • Commensal microflora (common or normal flora) inhabits the skin, body cavities, and intestines without causing sickness or disease.

    • Normal microflora plays an active role in helping the host control and combat invading pathogens.

  • Handling Unknown Laboratory Samples:

    • Universal Precautions: Any unknown biological sample must be handled under the assumption that it is pathogenic and hazardous.

    • Personal Protective Equipment (PPE): Laboratory personnel must equip full PPE before handling and analyzing unknown samples.

  • Diagnostic Identification Methods:

    • Pathogen identification relies on a combination of visual observation and differential testing.

    • Observation via Microscopy: Evaluates cellular morphology and motility.

    • Staining Procedures: Determines structural characteristics such as Gram stain status.

    • Chemical Reactions: Detects specific metabolic enzymes produced by the pathogen.

    • Selective and Differential Media Plating: Isolates the pathogen and analyzes specific growth characteristics.

Pathogenicity and Virulence

  • Pathogen Definition: A microorganism capable of causing disease in a host.

  • Pathogenicity:

    • Refers to the qualitative ability of a microbe to cause disease.

    • Functions as a binary (yes or no) classification determining whether a microbe is a pathogen.

  • Virulence:

    • Refers to the relative ease with which a pathogen infects its host.

    • Exists across a wide continuum ranging from less virulent to highly virulent.

The Five Steps of Infection: Access and Adhesion

For an infection to take place, a pathogen must gain access to host tissues and disrupt normal physiology. Infection proceeds through five distinct steps:

  • Step 1: Portal of Entry:

    • The portal of entry is typically dictated by the pathogen's mode of transmission.

    • Mucous Membranes: Primary entry points include the eyes, gastrointestinal (GI) tract, respiratory tract, urinary system, and reproductive system.

    • Transmission Examples: Fecal-oral transmission utilizes the GI tract; airborne transmission utilizes the respiratory tract.

    • Parenteral Entry: A portal of entry that bypasses the GI tract. Typically involves direct injection (such as via a needle) or direct introduction into the host's bloodstream.

  • Step 2: Adhesion to Host Tissues:

    • Pathogens must locate and bind to specific cellular targets in the host to colonize and multiply; failure to attach prevents colonization.

    • Adhesion Factors (Adhesins): Surface molecules on the pathogen (frequently proteins) that act as ligands binding directly to specific target receptors on host cells.

    • Receptor Specificity: If the host lacks the complementary receptor, the pathogen cannot establish infection.

    • Genetic Mutations: Mutations in the pathogen's DNA that alter the protein structure of its adhesion factors can prevent receptor binding, resulting in a total loss of virulence.

The Five Steps of Infection: Invasion, Evasion, and Transmission

  • Step 3: Invasion of Host Tissues:

    • Microorganisms must secure host nutrients to successfully replicate and multiply.

    • Surface Residence: Remaining on the mucosal surface of host tissues.

    • Intracellular Pathogenicity: Entering and residing directly inside host cells, effectively hiding from immune detection.

    • Enzymatic Invasion: Producing specialized enzymes that enable the pathogen to pass through host cells or penetrate between cells to access deeper tissues.

  • Step 4: Evasion of Host Immune Defenses:

    • Pathogens must avoid destruction by host immune defenses to replicate effectively.

    • Pathogen adaptations for evasion fall into two broad tactical approaches: hiding from the immune system or actively undermining immune responses.

  • Step 5: Transmission to a New Host:

    • An infectious pathogen must successfully transfer or move from one host or reservoir to another.

Mechanisms of Host Immune Evasion

  • Strategies for Hiding from the Immune System:

    • Antigen Masking: The pathogen conceals or cloaks its host-recognized antigens so the immune system fails to detect its presence.

    • Antigen Mimicry: The pathogen produces antigens that structurally resemble host molecules, causing the immune system to recognize the pathogen as host tissue.

    • Antigen Variation: The pathogen dynamically alters its surface antigens over time ("changing costumes"), rendering previously mounted host immune responses ineffective.

  • Strategies for Undermining the Immune System:

    • Decreasing Immune Function: Actively dampening or suppressing host immune pathways.

    • Avoiding Phagocytosis: Evading engulfment and destruction by immune phagocytes, which also prevents these cells from presenting antigens and alerting other immune components.

Transmission Pathways

  • Direct Contact Transmission: Requires direct interaction between the pathogen source and the host.

    • Person-to-Person: Skin-to-skin contact, or exposure to infected body fluids or excrement.

    • Animal Interaction: Pathogen transfer via animal scratches or bites.

    • Environmental Contact: Direct pickup from environmental reservoirs, such as inhaling or ingesting contaminated water, inhaling soil dust, or suffering puncture wounds from contaminated soil.

    • Vertical Transmission: Transmission from mother to fetus or infant across the placenta, during vaginal delivery, or through breastfeeding.

  • Indirect Contact Transmission: Pathogen transfer without direct physical contact between the source and the new host.

    • Respiratory Aerosols: Fine suspended particles carrying bacteria or viruses across distances.

    • Vehicle Transmission: Transmission via contact with contaminated inanimate objects (fomites), such as contaminated needles.

    • Biological Vectors: Vectors (insects or animals) in which the pathogen carries out a portion of its biological life cycle prior to host transmission.

    • Mechanical Vectors: Vectors (insects or animals) that passively transport ("hitchhike") the pathogen to a host without being infected themselves.

Healthcare Transmission Precautions

  • Standard Transmission Precautions:

    • Universal measures applied to all patient interactions in healthcare settings.

    • Hand Hygiene: Thorough handwashing with warm soap and water or use of alcohol-based hand sanitizer before and after patient contact.

    • Protective Gloves: Worn whenever exposure to wounds, body fluids, or mucous membranes is anticipated.

    • Barrier Protection: Use of face shields or barrier gowns during procedures with splash risks.

    • Disinfection: Routine decontamination of clinical equipment and surfaces.

  • Contact Precautions:

    • Applied alongside standard precautions when contact transmission risks are present.

    • Limit patient movement and transport outside the room.

    • Enforce hand hygiene immediately before and after patient care.

    • Mandatory use of protective gloves and gowns.

    • Use dedicated single-patient equipment to eliminate cross-contamination between patients.

  • Droplet Precautions:

    • Targeted at large, heavy respiratory mucus droplets that quickly fall to surfaces and do not travel far through the air.

    • Limit patient transport outside the room.

    • Healthcare workers must wear protective masks at all times within the patient environment.

  • Airborne Precautions:

    • Targeted at small aerosol particles that remain suspended in air currents and travel long distances.

    • Healthcare workers must wear a snug-fitting N95 respirator mask designed to filter fine airborne particles.

    • Patients are isolated in Airborne Isolation Rooms (AIR)—specially pressurized rooms designed to prevent contaminated air from escaping into surrounding medical facility areas.