Host tropism

Host tropism is the infection specificity of certain pathogens to particular hosts and host tissues. This type of tropism explains why most pathogens are only capable of infecting a limited range of host organisms.

Researchers can classify pathogenic organisms by the range of species and cell types that they exhibit host tropism for. For instance, pathogens that are able to infect a wide range of hosts and tissues are said to be amphotropic. Ecotropic pathogens, on the other hand, are only capable of infecting a narrow range of hosts and host tissue. Knowledge of a pathogen's host specificity allows professionals in the research and medical industries to model pathogenesis and develop vaccines, medication, and preventative measures to fight against infection. Methods such as cell engineering, direct engineering and assisted evolution of host-adapted pathogens, and genome-wide genetic screens are currently being used by researchers to better understand the host range of a variety of different pathogenic organisms.

A pathogen will display tropism for a specific host if it can interact with the host cells in a way that supports pathogenic growth and infection. Various factors affect the ability of a pathogen to infect a particular cell, including: the structure of the cell's surface receptors; the availability of transcription factors that can identify pathogenic DNA or RNA; the ability of the cells and tissue to support viral or bacterial replication; and the presence of physical or chemical barriers within the cells and throughout the surrounding tissue.

Pathogens frequently enter or adhere to host cells or tissues before causing infection. In order for this connection to occur, the pathogen needs to recognize and then bind to the cell's surface. Viruses, for example, must often bind to specific cell surface receptors to enter a cell. Many viral membranes contain virion surface proteins that are specific to particular host cell surface receptors. If a host cell expresses the complementary surface receptor for the virus, then the virus can attach and enter the cell. If a cell does not express these receptors, then the virus cannot normally infect it. Therefore, if the virus cannot bind to the cell, then it will not display tropism for that host.

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