BIOMED SCIE Lecture 13 The Bacterial Pathogens

• Virulence


  The capacity of a microbe to cause disease
• Pathogenesis


  The development of disease
• Virulence and pathogenesis are used interchangeably
• Causes of disease and damage to host tissue


  • Host immune responses including inflammation
  • Toxin & enzyme production by the pathogen
• Most infections are minor
• Most minor infections are viral
• Common minor infections


  • Common cold
• Opportunistic pathogens


  Bacteria that may cause disease but are normally part of the body's normal flora
• Many bacterial infections do not always require antibiotics, e.g. food poisoning when clean water source available
• Many bacterial infections can be avoided by simple measures, e.g. avoid contaminated foods (Salmonella), avoid unprotected sex (Gonorrhoea)
• Vaccines protect against some of the most virulent bacteria, e.g. Diphtheria, Bordetella
• Organisms capable of causing disease


  • Do not always cause disease in every exposed individual
  • Virulence of the microbe
  • Inoculum (number of microbes)
  • Site of infection/port of entry
  • Host susceptibility and capacity to resist
• Portals of Entry


  • Ingestion (Listeria, Salmonella)
  • Inhalation (TB, Bordetella)
  • Penetration (tetanus, S. aureus wound infection, Hep B needlestick)
  • Sexual (Chlamydia, Gonorrhoea)
• Overview of Bacterial Virulence


  • Adherence
  • Invasion
  • Toxin Production
  • Inflammation
  • Antibiotic Resistance
  • Enzyme Production
• Virulence factors that promote colonisation


  • Fimbriae
  • Pili
  • Capsules
  • Slime layers
  • Biofilm
• Invasins


  Proteins that activate the host cell's cytoskeletal machinery enabling bacterial entry into the cell by phagocytosis
• Siderophores


  Iron chelators (compounds capable of binding iron) produced by bacteria to obtain iron for growth
• Iron sources used by pathogenic bacteria


  • Human transferrin
  • Lactoferrin
  • Ferritin
  • Hemin
• Bacterial enzymes that contribute to disease


  • Collagenase
  • Streptokinase
  • Lecithinase
  • Hyaluronidase
• Types of exotoxins


  • Type I toxins or Superantigens
  • Type II toxins - exotoxins that damage host cell membranes
  • Type III toxins or A-B toxins that interfere with host cell function
• Endotoxin


  Toxins which are actual components of the bacterial cell envelope
• High concentrations of endotoxins from all bacteria can have the same non-specific effect, particularly when released into the blood: Endotoxic or septic shock
• Components of endotoxin


  • Lipopolysaccharide
  • Peptidoglycan
• Bacterial Skin Diseases


  • Folliculitis
  • Stye
  • Furuncle
  • Carbuncle
  • Impetigo
  • Necrotizing fasciitis
  • Burns
• Bacterial Respiratory Tract Infections


  • Ear, sinus infections (Haemophilus influenzae, Streptococcus pneumoniae)
  • Throat/URT infections (Streptococcus pyogenes)
  • Lungs/LRT infections (Streptococcus pneumoniae, Mycobacterium tuberculosis, Pseudomonas aeruginosa, Bordetella pertussis)
• Bacterial Tissue Infections


  • Brain (Neisseria meningitidis)
  • Heart (Oral streptococci and Staphylococci)
• Infective Endocarditis


  Inflammation of the endocardium
• Causes of Infective Endocarditis


  • Subacute bacterial (slow) - Alpha-hemolytic streptococci from mouth
  • Acute bacterial (fast) - Staphylococcus aureus
• Rheumatic Fever


  Complication following pharyngitis (sore throat) caused by Streptococcus pyogenes, involving antibody cross-reactivity and inflammation of the heart valve, polyarthritis, and subcutaneous nodules
• Tetanus


  Caused by Clostridium tetani toxin, results in trismus (lockjaw) followed by general rigidity and death by interference with mechanics of respiration
• Botulism


  Caused by Clostridium botulinum neurotoxin, results in weakness, paralysis, and respiratory failure
• Botox


  Cosmetic use of Clostridium botulinum toxin to relax facial muscles, effect lasts for approx 3 months
• Gas gangrene


  Caused by Clostridium perfringens, results in necrosis of soft tissue, treated by surgical removal of necrotic tissue and/or hyperbaric oxygen
• Helicobacter pylori


  Causes gastritis, stomach ulcers, and stomach cancer
• Salmonella typhi


  Bacteria spread throughout body in phagocytes, 1-3% of recovered patients become chronic carriers, 200,000 deaths/yr
• Salmonella is the second most important bacterial food pathogen next to Campylobacter
• Common Urinary Tract Infections


  • Urethritis
  • Cystitis
  • Pyelonephritis
• Sexually Transmitted Bacterial Infections


  • Chlamydia trachomatis
  • Treponema pallidum (Syphilis)
  • Neisseria Gonorrhoea
• Bacteria can use lots of different virulence factors - adhesins, invasins, toxins etc
• Need to take into account different sites of infection and modes of transmission when considering bacterial infections