Microbes and Disease

Microbes and Disease • Commensals • Pathogens - no harm to host (no disease) - harm the host (disease) What is the difference ? • Virulence / pathogenicity - possess virulence attributes What about opportunistic pathogens? • Host factors “Pathogenicity, or ability to cause disease is relative.” Virulence factors 1) Attachment 2) Survive host defences [opportunistic pathogens] • These factors define “infectivity” - the ability to establish in a host, but don’t define degree of host damage. 3. Ability to damage host • Infectious versus pathogenic (virulent) - common cold versus HIV Pathogenicity at an individual cell level or at a host level. • Microbial attachment • Microbial adhesins and complementary host receptors Microbial cell • Specificity of organism and host tissue • Good targets for immunisation • Antagonism and normal flora Surviving Host defenses • Surviving mechanical defences - attachment and E. coli • Surviving chemical defences - cyst form and Giardia, pH and Helicobacter • Surviving cellular defences (phagocytes) - avoid being recognised as foreign - survive inside the phagocyte Damage to the host • Spread - local - systemic - endocarditis - tuberculosis - very important and potent means of host damage • Direct damage • Host mediated • Toxins Bacterial toxins: exotoxins • • • • • Diverse in nature and action - organism specific Usually proteins May act locally or systemically May be produced in vitro or in vivo Good targets for immunisation • • • • Botulism Tetanus Diphtheria Cholera toxin is - home canning / flaccid paralysis - 700 000 deaths per year - controlled by vaccination with toxoid - disease is entirely toxin mediated but a poor target for vaccination. Bacterial toxins: endotoxins • A toxin of Gram negative bacteria - is actually a component of the cell wall. • Lipopolysaccharide • The toxin activates a series of HOST inflammatory pathways. • Septic shock - intravascular coagulation - bleeding - loss of blood volume and pressure - meningococcal infection. Summary Commensals VIRULENCE FACTORS 1) Attach to host 2) Survive host defenses 3) Damage the host Opportunistic pathogens • Direct damage •Host mediated Primary pathogens •Toxin mediated