Microbial Mechanisms of Pathogenicity

• Pathogenicity – the ability to cause disease by overcoming host defenses.
• Virulence – degree of pathogenicity.
• For pathogens to cause disease:

  1. Must gain access to the host
  2. Adhere to host tissues
  3. Penetrate or evade host defenses
  4. Damage host tissue
• Respiratory tract: inhalation into nose or mouth in drops of moisture and dust particles.
• Gastrointestinal tract: food and water via contaminated fingers.
• Genitourinary tract: sexually contracted diseases (STIs); other diseases require a cut or abrasion of some types.
• Conjunctiva: Acanthamoeba spp. (keratitis) and loa-loa.
• Skin - largest organ in terms of surface area and weight.
• The portals of entry for pathogens are: 1. Mucous membranes 2. Skin 3. Parenteral Route

  1. Mucous membranes
  2. Skin
  3. Parenteral Route
• LD50 – lethal dose for 50% of the inoculated hosts
• ID50 – infectious dose for 50% of the inoculated hosts
• Adherence - Also called adhesion; a necessary step in pathogenicity referring to the attachment of pathogens to host tissues at their portal of entry.
• Adhesins / ligands: pathogen surface molecules; bind specifically to complementary surface receptors on cells of host tissues.
• Pathogens penetrate tissues, causing diseases using several factors: capsules, cell wall components, enzymes, antigenic variation, penetration into the host, and biofilms.
• Capsules prevent pathogens from being phagocytized.
• Pathogens’ proteins in cell wall facilitates adherence and prevent from being phagocytized.
• Streptococcus pyogenes: produces heat-resistant and acid-resistant protein called M protein.
• Neisseria gonorrhoeae: uses fimbriae and opa (outer membrane protein) for attachment.
• Mycobacterium tuberculosis: has waxy cell wall; increases virulence by resisting digestion by phagocytes.
• Coagulase - bacterial enzyme that causes the clotting of blood plasma.
• • Kinases: destroy blood clots.
  • Hyaluronidase: destroys mucopolysaccharide that holds cells together.
  • Collagenase: hydrolyzes connective tissue collagen.
• IgA proteases - destroy IgA antibodies.
• Extracellular Polymeric Substances (EPS): inactivates or kills phagocytes; shield antigens to be not recognized by the immune system.
• Siderophores - proteins secreted by pathogens to obtain iron.
• Toxins - poisonous substances produced by microbes.
• Toxigenicity - ability to produce toxins.
• Exotoxins - produced by bacteria and released into the surrounding medium; produces symptoms of disease.
• Antitoxins - antibodies produced against exotoxins.
• A-B toxins - consist of an active component inhibiting cellular process; a binding component that attaches two portions to target cell.
• Membrane-disrupting toxins: cause cell lysis.
• Superantigens: cause release of cytokines.
• Endotoxins - a component of cell wall of gram-negative bacteria; release is stimulated by bacterial cell death, antibiotics, and antibodies; cause fever and shock.
• Limulus Amebocyte Lysate (LAL) assay is used to detect endotoxins in drugs and on medical devices.
• Plasmids may carry genes for antibiotic resistance, toxins, capsules, and fimbriae.
• Cytopathic effects (CPE) - visible signs of viral infections; include stopping mitosis, lysis, formation of inclusion bodies, etc..