Topic 8 Microbiology

Pathogenicity

ability of an MO to overcome hosts defences and cause a disease

Virulence

measure of pathogenicity of an MO, higher virulence = greater ability to cause a disease

Portals of entry

pathway that MO take to enter the body and cause disease, MOs can cause disease through only one portal or many

Portals of entry - mucous membranes

respiratory, MOs breathed in (influenzavirus, Bordetella pertussis - whooping cough, Streptococcus pneumoniae, Mycobacterium tuberculosis), gastrointestinal (shigella, salmonella, s. typhi), genitourinary tract (STIs)

Portals of entry - skin

usually impermeable to MOs, conjuctiva of eyes is more susceptible, hair follicles, sweat glands, some MOs can penetrate such as dermatophytic fungi and hookworms

Portals of entry - parenteral route

MO bypasses the skin, typically though an open wound

Measuring virulence

ID50 value, infectious dose is sufficient to infect half of the tested population, evaluated experimentally, can change

Measuring toxicity

measured as an LD50 value, lethal dose is sufficient to kill half of the tested population, tested on mice

Toxins tested in mice - lowest to highest dose

botulinum toxin, shiga toxin, staphyloccol enterotoxin

Bacterial damage - use of host cell nutrients

MOs uses metabolites before host cell can, analogous to a tapeworm, iron siderophores

Bacterial damage - direct damage

MOs grow so large in number that the host cells burst or rupture, intracellular MOs such as viruses and some bacteria (Chlamydia, Rickettsia), or protozoans (Plasmodium), produces enzymes that degrade plasma membrane - entry

Bacterial damage - toxins

poisonous substances produced by bacteria (or other MOs) that affect the host, toxigenicity

Exotoxins

mostly gram positive, synthesized during bacterial metabolism, secreted during growth or upon cell lysis, highly toxic, associated diseases often due to toxin, not the bacteria

Exotoxin examples

Botulism - Clostridium botulinum

Staphylococal food poisoning - Staphylococcus

Endotoxins

part of outer membrane of gram negative cells, lipid A of lipopolysaccharide, released when bacteria die, cell wall components break down, toxin is released, can be triggered by antibiotics - more bacteria die, more toxins