Microbial Mechanisms of Pathogenesis PDF

Chapter 15 Microbial Mechanisms of Pathogenesis  Microbial Mechanisms of Pathogenesis To cause disease, a pathogen must: 1. Gain access to the host (portal of entry) 2. Adhere to host tissues 3. Penetrate or evade host defenses 4. Damage host tissues ...

Chapter 15 Microbial Mechanisms of Pathogenesis  Microbial Mechanisms of Pathogenesis To cause disease, a pathogen must: 1. Gain access to the host (portal of entry) 2. Adhere to host tissues 3. Penetrate or evade host defenses 4. Damage host tissues  1. Gain Access to Host by “Portals of Entry” Mucous membranes – Respiratory tract - GI tract – Genitourinary tract - Conjunctiva (eye) Skin – 1st defense against disease – Access by openings in skin – Bore into shin (hookworm) Parenteral route – “direct deposit” in the skin – Puncture, injections, bites, surgery, etc Preferred portal: prerequisite to disease (Table 15.1, pg. 425)  Numbers of Microbes Most of the time need large numbers of organisms to cause disease Infectious dose (50%) – ID50: # of organisms necessary for infection – B. anthracis example (pg. 420) Lethal dose (50%) – LD50: # necessary for death – Measures the potency of a toxin: Botulinum toxin: 0.03ng/kg Shiga toxin: 250ng/kg Staph. enterotoxin: 1350 ng/kg  Botox Botulinum toxin paralyzes muscles (prevents the activation of neurons in the neuromuscular junction) https://www.youtube.com/watch?v=VsJKPdOuzZw  2. Adherence Fig. 15.1a Attaching to the host surface Adhesins: surface molecule on microbe (glyco/lipoproteins) Receptor: surface molecule on host (sugars) Biofilms: communities of microbes (dental plaque, algae on pools, scum on shower doors) that work together for Bacterial Adherence Fig. 15.1  Can you name a biofilm? Biofilms  3. Penetrate Host Defenses Must penetrate into the host to cause disease by: a. Capsule (glycocalyx): prevents phagocytosis (S. pneumoniae, K. pneumonia, B. anthrasis) b. Cell wall components: M protein (GAS), fimbriae (E. coli, N. gonorrhoeae), waxes c. Exoenzymes: coagulase (clot blood), kinase (fibrin), hyaluronidase (connective tissue), collagenase, IgA protease d. Antigenic variation: alter surface antigens (flu virus, Trypanosomes) Penetration into the Host Fig. 15.2  Microbial Mechanisms of Pathogenesis To cause disease, a pathogen must: 1. Gain access to the host (portal of entry) 2. Adhere to host tissues 3. Penetrate or evade host defenses 4. **Damage host tissues**  4. Damage to Host Cells 1. Using host nutrients, mainly iron o Siderophores (iron scavenger proteins) 2. Direct damage at site of invasion o Rupture of cells after invasion and spread to other tissues 3. Produce toxins (Fig. 15.4, pg. 431) a. Exotoxins (G+) b. Endotoxins (LPS Lipid A) 4. Induce immune hypersensitivity Exotoxins and Endotoxins Fig. 15.4  a. Exotoxins Poisonous soluble proteins that cause disease (220 total, 40% cause disease) Work by inhibiting cellular functions; very specific to the tissue of infection The host produces antitoxins to provide immunity to exotoxins 3 groups: (fig. 15.5) – A-B toxins - Membrane- disrupting toxins A-B Toxins  Examples of Exotoxins Table 15.2, pg. 427 Diphtheria toxin Erythrogenic toxin Botulinum toxin Tetanus toxin Vibrio enterotoxin Staphylococcal enterotoxin KNOW the major cellular effect of each toxin *********  b. Endotoxin (Lipid A) Fig. 15.6 Released from the G- bacteria as they are lysed Stimulate MF to release cytokines in [high]= TOXIC Septic shock: 3 cases per 1000/year; 1/3  Endotoxin Chpt. 4 Gram negative Part of LPS – Lipid A = endotoxin – O antigen (polysaccharide)  b. Endotoxin (Lipid A) Fig. 15.6 Released from the G- bacteria as they are lysed Stimulate MF to release cytokines in [high]= TOXIC Septic shock: 3 cases per 1000/year; 1/3 GREAT Study Tool!  Other Bacterial Causes of Pathogenicity Plasmids – R factors – Virulent plasmids Bacteriophage= lysogeny – Lysogenic conversion (only pathogenic if infected by a lysogenic virus)  Pathogenicity of Other Microbes 1. Virus: intracellular parasite that causes cytopathic Thereeffects is no good virus for humans… 2. Fungi: not well studied, several toxins that cause infections 3. Protozoa: their presence alone signals infection 4. Helminths: “ “ “ “ “ 5. Algae: produce neurotoxins  Portal of Exit In general, the portal of exit is the same as the portal of entry 3 common portals: – Respiratory - Gastrointestinal tract – Genitourinary tract GREAT Study Tool! Fig. 15.9