Lecture 14: Microbial mechanisms of pathogenicity

Pathogenicity

The ability of an organism to cause disease

True pathogens

capable of causing disease in healthy persons

Opportunistic pathogens

Cause disease when the host’s defenses are compromised

Virulence

The degree of pathogenicity of an organism(inversely proportional to ID50)

Portals of entry

characteristic route a microbe follows to enter the tissues of the body

Exogenous agents

originate from source outside the body

Endogenous agents

already exist on or in the body normal microbiota)

Types of portals of entry

Skin, gastrointestinal tract, respiratory tract, urogenital tract, transplacental.

Infectious does (ID)

Minimum number of microbes required for infection to proceed

Adhesion

Microbes gain a stable foothold at the portal of entry, dependent on binding between specific molecules on the host and pathogen

Exoenzymes

Bacteria produce extracellular enzymes that dissolve barriers and penetrate through or between cells to invade underlying tissues.

Toxins

Toxins secreted by bacteria damage target cells, which die and begin to slough off

Blocked phagocytic response

Encapsulated bacteria can escape phagocytosis and continue to grow and cause further infections

Toxicoses

Adverse effects of toxins

Toxemias

when the toxin is spread by the blood from the site of infection

Intoxications

Caused by ingestion of toxins

Endotoxin

not secreted, but released after the host cell is damaged

Exotoxin

secreted by a living bacterial cell into the infected tissue

Localized infection

microbes enter the body and remain confined to a specific tissue

Systemic infection

Infection spreads to several sites and tissue fluids usually in the bloodstream

Focal Infection

When an infectious agent breaks loose from a local infection and is carried to other tissues

Mixed infection

Several microbes grow simultaneously at the infection site

Primary infection

initial infection

Secondary infection

Subsequent infection by a different microbe

Acute infection

comes on rapidly, severe but short-lived effects

Chronic infections

Progress and persist over the long period of time

Epidemiology

Study of the frequency and distribution of disease and other health-related factors in a defined human population