Bacterial Pathogenicity

• Microbial Pathogenicity

  Why microbes cause disease and the mechanisms they use to become pathogens
• Microorganisms


  • Agents of infectious diseases
  • Ubiquitous in nature and in/on human body
  • Most microorganisms harmless for humans
  • Some can cause disease
  • Divided into bacteria, fungi, viruses, prions, parasites
• Pathogenesis of Infection


  1. Microbes find a new host and start to multiply (colonisation)

  2. Balance can develop between colonised microbes and humans (normal flora)

  3. Microbe causes disease (infection)

  4. Source of microbe is patient's own flora (endogenous infection)

  5. Source of microbe is flora from outside the patient's body (exogenous infection)
• Koch's Postulates


  • The microbe must be present in every case of the disease
  • The microbe must be isolated from the diseased host and grown in pure culture
  • The disease must be reproduced when a pure culture is introduced into a susceptible host
  • The microbe must be recovered from an experimentally infected host
• Ways microorganisms get into body

  • Person-to-person
  • Contaminated blood or other bodily fluids
  • Touch
  • Saliva
  • Air
  • Fomites
  • Insects
  • Water
  • Food
• Pili (Fimbriae)


  • Proteinaceous with variable dimensions
  • 4-10 μm wide, 0.5-4 μm long
  • Overcome mechanical forces
  • Specificity to HOST and TISSUE
  • ATTACHMENT
• Urinary Tract Infection's (UTI's)

  • Approx. 50% woman develop a UTI at some point in lifespan
  • 7 million UTI's in USA/year
  • UPEC's account for 90% of community acquired UTI's
  • Accounts for 1-6% of consultations at clinics
  • Females under 10 and 20-40 years old commonly affected
  • Colonise from faeces or perineal regions
• Host Protection from UTI's

  • Main defence is flushing action of urine
  • Tamm-Horsfall protein helps bind specific Escherichia coli strains
  • Bacteria produce specialised adhesive structures to circumvent flushing action
  • Adheres to bladder mucosal cells
  • E. coli usually isolated
• Encapsulated Infections


  • Infections include meningitis, pneumonia, otitis media and sinusitus
  • Commonly caused by Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis and group B Streptococcus
  • Morbidity generally related to immune response to the capsule
• Capsular Function


  • Mediate adhesion- helps colonise
  • Immune evasion
  • Protection from desiccation
  • Reserves of carbohydrate
  • Encapsulated bacteria give rise to smooth colonies
  • Capsule material gives rise to 'capsular antigens'
• Toxins


  • Endotoxin
  • Lipopolysaccharide (LPS): Endotoxin
  • Lethal Bacterial Proteins
• Lethal toxicity of various toxins compared to other substances
• Endo- versus Exo-toxins


  • Endotoxin: Lipopolysaccharide (mw = 10kDa), part of outer membrane, not denatured by boiling, relatively low potency, low specificity, no enzymatic activity, pyrogenic
  • Exotoxin: Protein (mw = 50-1000kDa), extracellular and diffusible, usually denatured by boiling, relatively high potency, high specificity, usually enzymatic activity, occasionally pyrogenic