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Pathogen Routes of Entry
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Pathogen Routes of Entry

Common routes of pathogen entry:
Skin
  • Typically blocked physically, by the stratified epithelium of the epidermis, and chemically, by antimicrobial defensins and fatty acids.
– When these barriers fail, microorganisms from the surface of the skin, such as the bacteria Staphylococcus aureus, gain entry. – Other organisms take a more aggressive approach: for example, the larvae of the parasitic flatworm Schistosoma release enzymes to destroy keratinocyte bonds, thus actively creating their own means to entry.
Respiratory Tract
  • Protected by mucus and cilia that clear harmful materials, and by phagocytic alveolar macrophages in the lungs.
  • However, some microbes have adaptations to override these protections:
– The influenza virus uses hemagglutinins to bind respiratory epithelial cells. – The bacteria Mycobacterium tuberculosis survives the phagolysosomes of the alveolar macrophages.
Gastrointestinal Tract
  • Epithelial lining of the gastrointestinal tract provides multiple sources of protection: the gastric glands produce a viscous layer of mucus, acids, and enzymes to trap and disable pathogens.
  • The normal gut flora prevents pathogenic microorganisms from colonizing.
  • However, when antibiotics kill the microbial flora, Clostridium difficile can flourish, leading to diarrhea.
  • Poliovirus is an example of a microorganism that gains entry by "high jacking" normal host physiology: it appears that gut lymphoid tissue, which naturally pulls antigens into the circulation, may likewise take up poliovirus and facilitate its spread.
Urogenital Tract
  • The vagina is protected by microbes that maintain an acidic pH.
– However, when these microbes eradicated by antibiotics, the yeast Candida albicans can flourish.
  • The urinary tracts of both males and females are protected by thick layers of epithelia, as we can see in the histology sample; furthermore, urination "flushes" microbes from the urethra.
– However, the bacteria Escherichia coli has special adhesion molecules that allow it to adhere to the urothelium; attachment is especially likely if urination is obstructed.

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