Definitions

• Bacteraemia = “the presence of bacteria in the blood”
• Septicaemia = “a systemic disease caused by the spread of microorganisms and their toxins via the circulating blood”
• Sepsis = life threatening organ dysfunction causes by dysregulated host response to infection
• Blood Stream Infection (BSI) – includes bacteraemia, also includes fungaemia/ viraemia /parasitaemia.

Sepsis is an old medical term which comes from the Greek 'sepsin' meaning 'to make putrid'

Therefore most correct to use bacteraemia +/- sepsis. BSI also correct.

How does bacteraemia occur (immunology)

Initial entry to host: Body surface e.g. skin, mucous membranes, lung, gut. Or any defect in these such as trauma, viscous perforation.

1. Traverse epithelium and surface membrane
2. Enter tissue
   1. Antimicrobial substances e.g. antibodies, complement
   2. Local macrophages (histiocytes)
   3. Physical tissue barriers, e.g. hydrated gel matrix. Virulence factors aid in entry
3. Lymphatic system
   1. Conveys microorganisms to lymph nodes with large numbers of phagocytic and immunologic defences
4. Invade blood
   1. The route to achieving this varies. Some invasive organisms due so in an antagonistic approach; disabling/destroying immune response. Others in an immune evasion approach; usually surviving intracellular
      1. Examples intracellular pathogens Listeria Brucella; also EBV or Rubella. Malaria
   2. Transient bacteraemia common e.g. when tooth brushing or defecating; but usually filtered out of blood especially in spleen/liver by macrophages
      1. Certain organisms adapted to multiple in these cells within the “reticuloendothelial system” e.g. Salmonella typhi, Leishmania donovani, Yellow fever
   3. Travel to other organs
      1. Organism specific; unclear why
   4. Leave the body

Diagnosis

When to take blood cultures

Anyone who might be bacteraemic (Not just those who are pyrexial)