15.2 How Pathogens Cause Disease

Identifying and Defining Pathogens

Koch's Postulates and Molecular Adaptations

Koch's postulates are the classic criteria for proving that a specific microbe causes a specific disease. They were developed in the late 1800s and still form the logical foundation for linking pathogens to diseases.

The four postulates follow a clear sequence:

1. The microorganism must be found in every case of the disease but absent in healthy individuals.
2. The microorganism must be isolated from the diseased host and grown in pure culture.
3. The pure culture, when inoculated into a healthy, susceptible host, must reproduce the disease.
4. The same microorganism must be re-isolated from the experimentally infected host and confirmed to be identical to the original causative agent.

These postulates work well for many bacterial diseases, but they have limitations. Some pathogens can't be grown in pure culture (like Treponema pallidum, the cause of syphilis), and ethical concerns prevent deliberately infecting humans. Modern molecular adaptations address these gaps:

• Nucleic acid-based identification uses PCR and sequencing to detect pathogen DNA/RNA directly in clinical samples, even when the organism can't be cultured.
• Virulence gene analysis identifies specific genes encoding toxins or other virulence factors, linking them to disease.
• Animal models and cell culture systems allow researchers to demonstrate pathogenicity without human subjects.

Together, classical postulates and molecular tools guide the development of diagnostic tests, treatments, and vaccines.

Pathogenicity and Virulence Concepts

Pathogenicity is the ability of a microorganism to cause disease in a host. Not all microbes are pathogenic; most are harmless or even beneficial.

Virulence refers to the degree of pathogenicity. Two pathogens can both cause pneumonia, but one might produce far more severe disease. Virulence is determined by factors like toxin production, invasiveness (ability to spread through tissues), and resistance to host defenses.

Two quantitative measures help compare how dangerous different pathogens are:

• Infectious dose (ID): the minimum number of microorganisms needed to establish an infection. Shigella, for example, has a very low infectious dose (as few as 10 organisms), while Vibrio cholerae requires around $10^8$ organisms. The host's immune status also affects this number.
• Lethal dose (LD): the number of organisms needed to kill the host. This is often expressed as the $LD_{50}$, the dose that kills 50% of a test population. Similarly, the $ID_{50}$ is the dose that infects 50% of a test population.

Primary vs. Opportunistic Pathogens

Primary pathogens can cause disease in otherwise healthy, immunocompetent hosts. Their virulence factors are potent enough to overcome normal defenses. Examples include the measles virus, Mycobacterium tuberculosis, and Vibrio cholerae.

Opportunistic pathogens typically cause disease only when the host's defenses are compromised. This can happen due to immunosuppression (HIV/AIDS, chemotherapy), disruption of normal microbiota (antibiotic use), or breaches in physical barriers (burns, surgical wounds). Many opportunists are part of the normal microbiota or are common in the environment:

• Pseudomonas aeruginosa is widespread in soil and water and frequently infects burn patients or those on ventilators.
• Candida albicans is a normal resident of the mouth and GI tract but can cause thrush or systemic candidiasis when immune defenses drop.
• Pneumocystis jirovecii causes severe pneumonia almost exclusively in immunocompromised individuals, particularly AIDS patients.

Pathogenesis and Transmission of Microbial Infections

Stages of Microbial Pathogenesis

Infection doesn't happen all at once. It follows a predictable sequence, and understanding each stage helps explain where interventions (antibiotics, vaccines, hygiene) can break the chain.

1. Exposure: The host comes into contact with the pathogen through one of the portals of entry (respiratory, GI, skin, etc.).
2. Adhesion: The pathogen attaches to host cells or tissues using surface molecules called adhesins (such as pili/fimbriae or surface glycoproteins). Without adhesion, the pathogen gets flushed away by mucus, urine, or other mechanical defenses.
3. Invasion: The pathogen penetrates into host cells or deeper tissues. This can involve enzymes that break down tissue barriers (like hyaluronidase or collagenase), toxins, or specialized structures like type III secretion systems, which act like molecular syringes to inject proteins directly into host cells.
4. Colonization: The pathogen establishes itself and multiplies within the host, competing for nutrients and space.
5. Evasion of host defenses: Pathogens use diverse strategies to dodge the immune system. These include antigenic variation (changing surface proteins so antibodies no longer recognize them), producing capsules that resist phagocytosis, and secreting immunosuppressive factors.
6. Tissue damage: Harm to the host results from pathogen-produced toxins, destructive enzymes, or the host's own inflammatory and immune responses. Sometimes the immune response itself causes more damage than the pathogen.
7. Dissemination: The pathogen spreads to other body sites via the bloodstream (bacteremia/viremia) or lymphatic system, potentially causing systemic infection.
8. Shedding: The pathogen exits the host, enabling transmission to new susceptible hosts and continuing the cycle of infection.

Pathogen Entry and Exit Routes

Pathogens have specific portals of entry and portals of exit. The route of entry often determines the type of disease that develops.

Entry routes:

• Respiratory tract (inhalation): Influenza virus, Streptococcus pneumoniae, Mycobacterium tuberculosis
• Gastrointestinal tract (ingestion): Salmonella, norovirus, Vibrio cholerae
• Skin and wounds (contact or penetration): Staphylococcus aureus through cuts; Clostridium tetani through deep puncture wounds
• Urogenital tract (sexual transmission): HIV, Neisseria gonorrhoeae, Treponema pallidum
• Vertical (parent to child): Transmission during pregnancy, childbirth, or breastfeeding. Examples include Toxoplasma gondii and Zika virus.
• Zoonotic transmission: Pathogens that jump from animals to humans, such as rabies virus (animal bites) and Borrelia burgdorferi (tick vectors causing Lyme disease)

Exit routes:

• Respiratory secretions: Coughing and sneezing expel droplets containing pathogens (influenza, M. tuberculosis)
• Fecal-oral route: Pathogens shed in feces contaminate food or water (Salmonella, norovirus)
• Skin lesions or wound drainage: Discharge from infected sites (S. aureus, P. aeruginosa)
• Sexual fluids: Semen and vaginal secretions (HIV, Chlamydia trachomatis)
• Blood and body fluids: Via insect vectors (malaria parasites from mosquitoes), contaminated needles, or transfusions (hepatitis B virus)

The portal of exit often mirrors the portal of entry, which is why understanding transmission routes is central to infection control.

Microbial Strategies for Pathogenesis

Pathogens don't passively infect hosts. They actively deploy virulence factors and survival strategies that tip the balance in their favor.

Immune evasion is one of the most critical strategies. Pathogens avoid detection or destruction by the immune system through mechanisms like capsule production (which prevents phagocytosis), intracellular survival (hiding inside host cells where antibodies can't reach), and antigenic variation.

Toxins are harmful substances that damage host cells or disrupt normal physiology. They fall into two major categories:

• Exotoxins are proteins secreted by living bacteria. They're often highly specific in their targets. For example, botulinum toxin blocks neurotransmitter release, causing paralysis.
• Endotoxins are lipopolysaccharide (LPS) components of gram-negative bacterial outer membranes. They're released when the cell lyses and trigger a broad inflammatory response that can lead to fever, shock, and even death.

Biofilms are structured communities of microorganisms encased in a self-produced matrix of polysaccharides, proteins, and DNA. Bacteria in biofilms are up to 1,000 times more resistant to antibiotics than free-floating (planktonic) cells. Biofilms form on medical devices like catheters and prosthetic joints, making these infections notoriously difficult to treat.

Quorum sensing is a cell-to-cell communication system in which bacteria release and detect signaling molecules called autoinducers. When the population reaches a threshold density, gene expression shifts to activate virulence factor production, biofilm formation, or toxin release. This allows bacteria to coordinate an attack only when their numbers are large enough to overwhelm host defenses.

Reservoirs are the environments or organisms where pathogens persist between infections. Reservoirs can be human (carriers who shed pathogen without symptoms), animal (zoonotic reservoirs like bats for Ebola), or environmental (soil for Clostridium species). Identifying reservoirs is essential for outbreak prevention and public health surveillance.