Microbial Pathogenesis and Host Interactions

Pathogen

A microorganism that causes disease.

Pathogenesis

The development of a disease.

Pathogenicity

The ability of a microorganism to cause disease.

Virulence Factor

A pathogen characteristic determining the extent and severity of disease.

Etiology

The study of the cause of a disease.

Infection

A condition where a pathogen becomes established in the host, causing disease.

Disease

An abnormal state in which the body deviates from normal physiological functioning.

Mutualism

A symbiotic relationship where both species benefit.

Commensalism

A symbiotic relationship where one species benefits and the other is unaffected.

Parasitism

A symbiotic relationship where one species benefits by harming the other.

Transient Microbiota

Microorganisms temporarily present on or in a host before disappearing.

Normal Microbiota

Microorganisms that permanently colonize the host without causing disease under normal conditions.

Primary Pathogen

A microbe that can infect a host regardless of resident microbiota or immunity.

Opportunistic Pathogen

A microbe causing disease only when host defenses are compromised.

Microbial Antagonism

Competition between normal microbiota and pathogens for resources like nutrients and oxygen.

Koch's Postulates (Step 1)

The suspected agent must be absent in healthy but present in diseased organisms.

Koch's Postulates (Step 2)

The causative agent must be isolated and grown in pure culture.

Koch's Postulates (Step 3)

The cultured agent must cause the same disease in a healthy, susceptible host.

Koch's Postulates (Step 4)

The same causative agent must be reisolated from the inoculated host.

Four Stages of Pathogenesis

1. Exposure, 2. Adhesion, 3. Invasion, 4. Infection.

Portals of Entry

Anatomic sites for pathogen entry: skin, mucous membranes, and parenteral route.

Parenteral Route

Direct deposition of pathogens into tissues beneath skin or mucous membranes.

ID50

The number of pathogen cells or virions required to infect 50% of a sample.

LD50

The dose of a toxin or pathogen required to kill 50% of a sample.

Adhesins (Ligands)

Pathogen molecules that bind specifically to surface receptors on host cells.

Urease (H. pylori)

An exoenzyme that neutralizes stomach acid to liquefy gastric mucin gel.

Collagenase

An exoenzyme that degrades collagen in connective tissue to allow dissemination.

Hyaluronidase

An exoenzyme that breaks down hyaluronan to let pathogens penetrate deeper.

Local vs. Systemic Infection

Local is confined to a small area; systemic is disseminated body-wide.

Primary vs. Secondary Infection

Primary is the initial infection; secondary is a follow-up opportunistic infection.

Toxigenicity

The capacity of a microorganism to produce toxins.

Toxemia

The presence of toxins in the host's bloodstream.

Endotoxin (Lipid A)

The Lipid A portion of Gram-negative LPS released upon cell lysis.

Exotoxins

Toxic, soluble proteins secreted mostly by Gram-positive bacteria to damage host cells.

A-B Toxins

Two-subunit toxins: B binds receptors, while A delivers the active enzyme.

Diphtheria Toxin Mechanism

The A subunit ADP-ribosylates EF-2, halting protein synthesis and killing cells.

Superantigens

Exotoxins triggering an excessive cytokine storm, leading to fever, shock, and death.

Genotoxins

Exotoxins that damage host cell DNA, causing mutations or cancer.

Antigenic Drift

Gradual genetic changes in a pathogen's surface antigens to evade antibodies.

Antigenic Shift

Major, sudden genetic changes in surface antigens caused by viral reassortment.