Host tropism

Researchers can classify pathogenic organisms by the range of species and cell types that they exhibit host tropism for.

For example, biofilm production allows bacteria to adhere to the host tissue surface, and it provides a protective environment ideal for bacterial growth.

For viruses to replicate within a host cell and for bacteria to carry out the metabolic processes needed to grow and divide, they must first take in necessary nutrients and transcription factors from their surroundings.

The first line of defense, known as the innate immune system, is meant to prevent initial pathogenic entry and establishment.

The innate immune system is only broadly specific to pathogens and includes: anatomical barriers, inflammation, phagocytosis, and nonspecific inhibitors.

This includes the skin, sweat, mucus layer, saliva, tears, endothelial lining, and natural human microbiota.

The mucus layer lines the nasopharynx and serves as a physical barrier that encases foreign pathogens and carries them back out of the body through snot and phlegm.

Lastly, a semi-permeable membrane known as the blood-brain-barrier is a lining of endothelial cells separating the blood from the tissues and organs.

[6] Without this lining, viruses and bacteria could easily infect vital human organs such as the brain, lungs, and placenta.

[6][7] Coagulation of blood (clotting) also occurs in an inflamed area, providing a physical barrier against pathogenic infection.

[8] These changes ultimately create unfavorable living conditions for the pathogen (i.e. pH changes, decrease in ATP, and changes in cellular metabolism) and prevent further replication and growth.

[12] EBV enters the body through oral transfer of saliva, and it is thought to infect more than 90% of the world's adult population.

[12] The Zika virus is a mosquito-borne arbovirus in the genus Flavivirus that exhibits tropism for the human maternal decidua, the fetal placenta, and the umbilical cord.

[16] The cell envelope glycoconjugates surrounding M. tuberculosis allow the bacteria to infect human lung tissue while providing an intrinsic resistance to pharmaceuticals.

[17] However, since the macrophages are unable to completely kill M. tuberculosis, granulomas are formed within the lungs, providing an ideal environment for continued bacterial colonization.

[19] Enteropathogenic and enterohaemorrhagic Escherichia coli (EPEC and EHEC respectively) exhibit tropism for human gut epithelial cells, leading to food poisoning and digestive problems.