Systemic Bacteriology II

Key definitions

  • Virulence: the capacity of a microbe to cause damage to its host
  • Pathogen: a harmful organism that produces a pathology
  • Commensal: an organism that is part of the normal flora
    • Often a mutualistic relationship
    • Endogenous
  • Opportunistic pathogen: an organism that causes infection when opportunity/change in natural immunity arises
  • Coagulase test: use to differentiate between Staph. aureus and coagulase negative Staph. (including Staph epidermis)
  • Haemolysis: used to differentiate between Streptococci

Sepsis

  • Small blood vessels become ‘leaky’ and loose fluid into tissues - decreased blood volume so heart has to work harder
  • Poor tissue perfusion means blood supply to less essential organs is shut down
  • Blood clotting system activated - uses all clotting factors, increasing risk of haemorrhage
  • Patients with Gram-negative sepsis get very unwell very quickly due to endotoxin released when Gram-negative bacteria die

Fever

  1. Antigen/LPS interacts with macrophages
  1. Macrophages release cytokines into bloodstream
  1. Cytokines travel to anterior hypothalamus
  1. Prostaglandin E released - increases body’s thermal set point
  1. Body perceives it as cold - starts to ‘shiver’
  1. FEVER

Key gram-negative pathogens

Gram-negative cocci

  • Neisseria meningitidis: most common cause of bacterial meningitis
  • Neisseria gonorrhoeae: gonorrhea

Gram-negative cocci associated with GI tract

  • Gut commensal coliforms
    • Many are part of normal bowl flora
    • Any coliform that gets into a normally sterile environment can cause infection e.g. UTI
  • Gentamicin is the first line antibiotic for the treatment of infections caused by coliforms

Key gram-positive pathogens

Streptococcus spp.

  • Identified by haemolysis (ONLY streptococcus)
notion image
  • GAS: numerous associated diseases e.g. Scarlet fever, impetigo, β-haemolytic
  • Streptococcus pneumoniae: pneumonia, ⍺-haemolytic

Staphylococcus epidermis

  • Nosocomial - associated with foreign devices e.g. cathethers
  • Coagulase negative

Staphylococcus aureus

  • Nosocomial and community
  • Most common cause of skin, soft tissue and wound infection
  • Sepsis
  • Coagulase positive (appears golden)
  • Clump forming - looks like grapes

Enterococcus

  • Enteric infections
  • Part of normal bowel flora, can cause problems if they get into a normally sterile area
  • 𝛾-haemolytic

Clostridium spp.

  • Part of normal bowel flora
  • Produce endotoxins that can cause severe tissue damage
  • Clostridium difficile: causes antibiotic-associated diarrhoea, especially in the elderly
    • Proliferates in the absence of normal flora