Microbial Mechanisms of Pathogenicity: Virulence

Microbial Mechanisms of Pathogenicity

Mechanisms of Pathogenicity
• Pathogenicity: • Virulence:

Mechanisms of Pathogenicity

Figure 15.9

Portals of Entry
• Mucous membranes • Skin • Parenteral route

Numbers of Invading Microbes
• ID50: Infectious dose for 50% of the test population (animals)
• Dose of pathogen that will infect half of the test animals

• LD50: Lethal dose (of a toxin) for 50% of the test population (animals)
• Dose of toxin that will kill half of the test animals

Adherence
• Adhesins/ligands bind to receptors on host cells
• Glycocalyx: Streptococcus mutans • Fimbriae: Escherichia coli • M protein: Streptococcus pyogenes

• Form biofilms

How bacterial pathogens penetrate host defense
• • • • • • Capsules Cell wall components Enzymes Antigenic variation Penetration into the host cell cytoskeleton; invasin Intracellular growth

Capsules
• Prevent phagocytosis
 Streptococcus pneumoniae  Haemophilus influenzae  Bacillus anthracis

Cell Wall Components
• M protein resists phagocytosis and improves adherence
 Streptococcus pyogenes

• Fimbriae & Opa protein attachment to host cells.
 Neisseria gonorrhoeae

• Mycolic acid (waxy lipid) resists digestion
 Mycobacterium tuberculosis

Enzymes
• • • • • Coagulase: Coagulates fibrinogen Kinases: Digest fibrin clots Hyaluronidase: Hydrolyzes hyaluronic acid Collagenase: Hydrolyzes collagen IgA proteases: Destroy IgA antibodies

Enzymes

Antigenic Variation
• Alter surface proteins

Penetration into the Host Cell Cytoskeleton
• Invasins
• Salmonella alters host actin to enter a host cell

• Use actin to move from one cell to the next
• Listeria

Figure 15.2

How bacterial pathogens damage host cells
• • • • • Siderophores Direct damage Toxins Lysogenic conversion Cytopathic effect

Using the Host’s Nutrients: Siderophores
• Use host’s iron

Figure 15.3

Direct Damage
• Disrupt host cell function • Produce waste products