Endogenous and exogenous infection
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Endogenous infection
Definition: Endogenous infections are caused by organisms that are part of the host’s own normal flora. These microbes are typically harmless under normal conditions but can become pathogenic if host defences are compromised or if they translocate to normally sterile body sites. These infections are particularly relevant in immunocompromised patients or following invasive procedures.
Clinical examples of endogenous infection:
| Infection | Notes |
|---|---|
| Aspiration pneumonia: | Caused by the inhalation of oropharyngeal or gastric flora (including anaerobes and Gram-negative bacilli) into the lower respiratory tract. Risk factors include impaired swallowing, reduced consciousness, and gastro-oesophageal reflux |
| Neutropenic sepsis: | In neutropenic patients (such as those undergoing chemotherapy), gut flora (particularly Gram-negative enteric bacilli or enterococci) can translocate across mucosal barriers and enter the bloodstream due to impaired immune surveillance |
| Cannulation: | Insertion of cannulas or catheters can breach the skin barrier, allowing normal skin flora such as Staphylococcus epidermidis or Staphylococcus aureus to invade and cause local or systemic infection (e.g., catheter-associated bloodstream infection) |
Exogenous Infection
Definition: Exogenous infections are caused by organisms that are acquired from sources external to the host, such as the environment, other humans, animals, or contaminated objects. These infections typically occur when the pathogen is introduced into the body under conditions favourable to colonisation or invasion.
Clinical examples of exogenous infection:
| Infection | Notes |
|---|---|
| Legionella: | Legionella pneumophila is found in natural and man-made water systems. Infection (Legionnaires' disease) often occurs through inhalation of contaminated aerosols from sources such as air conditioning systems, cooling towers, or poorly maintained water systems in buildings. Risk is increased by stagnant warm water and poor system hygiene |
| Clostridium: | Clostridium difficile and Clostridium perfringens are spore-forming bacteria. C. difficile infections often arise after antibiotic disruption of normal gut flora and exposure to spores in hospital environments. C. perfringens may cause gas gangrene when its spores enter wounds under anaerobic conditions, such as in traumatic or surgical injuries |
| Salmonella: | Typically acquired via ingestion of contaminated food or water. Risk is highest with undercooked poultry, raw eggs, and poor kitchen hygiene. Contaminated pet reptiles or animal contact can also be sources. Outbreaks are often linked to lapses in food safety during food preparation or mass catering events |