Microbiology: Host-Parasite Relationship

Bacterial Pathogenicity

• Microorganisms (viruses, bacteria, fungi, and protozoans) are closely associated with human hosts, but not all host-microbe associations are pathogenic.

Host-Parasite Relationship

• One species living in or on the body of another, called a "Symbiotic association".
• Can be harmless or harmful, usually on body surfaces (skin, mucosa).
• 3 categories of associations: Commensalism, Mutualism, and Parasitism.

Commensalism

• Microorganism uses a host's body for food and shelter, without causing harm.
• Normally harmless, but can be harmful if environmental conditions change (e.g., E. coli in urinary tract).
• Opportunistic pathogens.
• Commensals can benefit the host, e.g., producing metabolites used by the host.

Mutualism

• Benefit both the host and the microorganism.
• Prevent colonization of pathogens on mucosal surfaces.
• Examples: Bacteria in ruminants' intestines that digest cellulose.

Parasitism

• Harmful to the host, beneficial to the microorganism.
• Examples: Rabies virus.

Infection and Pathogenicity

• Infection: growth and multiplication of a microbe in or on the body, with or without disease.
• Pathogenicity: The capacity of a bacterium to initiate disease.
• Pathogenesis: Mechanism of infection and disease development.

Frequency of Clinically Apparent Disease

• In some infections, all or majority infected will have clinical disease (e.g., Rabies, smallpox).
• In others, majority may have mild or asymptomatic infection, and only a minority will have clinical disease (e.g., Polio).

Pathogenicity Requirements

• Transmissibility (ability to move from one host or reservoir to another host).
• Survival (in the new host).
• Infectivity (ability to breach the new host defenses).
• Virulence (capacity of the pathogen to harm the host).

Types of Bacterial Pathogens

• Primary pathogens: Capable of establishing infection and causing disease in previously healthy individuals with intact immunological defenses.
• Opportunistic pathogens: Rarely causes diseases in previously healthy individuals, but causes diseases with impaired immunological defenses.

Characteristics of Pathogenic Bacteria

• Establishment of infection (source, route of entry, transmission).
• Adherence to host cells.
• Invasion of host cells and tissue.
• Evasion of the host immune system.
• Toxigenicity.

Source of Infection

• Patients (clinical disease or subclinical infections, infected carriers).
• Normal commensal flora.
• Inanimate objects (non-living things).
• Environmental (water, food, air, etc.).

Entry into the Human Body

• Most frequent portals of entry: Mucous membranes, skin.
• Routes: Ingestion, inhalation, trauma, needles, catheters, arthropod bites, sexual transmission.

Transmission of Infection

• Direct contact: From one person to another person.
• Indirect contact: Via contaminated food and water, air-borne particles, animals, and insects.

Pathological Mechanisms of Bacterial Infections

• Bacteria-mediated Pathogenesis.
• Host-mediated Pathogenesis.
• Bacterial virulence factors.