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. 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. Bacteria infect hosts differently than viruses do. Unlike viruses, bacteria can replicate and divide on their own without entry into a host cell. Still, in order to grow and divide, bacteria require certain nutrients from their environment. These nutrients can often be provided by host tissues, and that is why some bacteria need a host for survival. Once a bacterium recognizes the host cell receptors or nutrient-rich surroundings, it will colonize the cell surface. Bacteria have various mechanisms for colonizing host tissues. For example, biofilm production allows bacteria to adhere to the host tissue surface, and it provides a protective environment ideal for bacterial growth. Some bacteria, such as spirochetes, are capable of proliferating the host cell or tissues. This then allows the bacterium to surrounded itself in a nutrient-rich environment that protects it from immune responses and other stressors. In order 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. Even if a virus is able to bind to a host cell and transfer its genetic material through the cell membrane, the cell may not contain the necessary polymerases and enzymes necessary for viral replication to occur and for pathogenesis to continue. Many pathogens also contain important virulence factors within their genomes. In particular, pathogenic bacteria are capable of translating virulence genes located within their plasmids into different virulence factors in order to aid the bacterium in pathogenesis. Many different types of virulence factors exist within pathogens, including: adherence factors, invasion factors, capsules, siderophores, endotoxins, and exotoxins. All of these virulence factors either aid directly in host colonization or in host cell and tissue damage. Host organisms are equipped with a variety of different defense mechanisms used to protect the host from pathogenic infection. Humans in particular possess multiple lines of defense that affect pathogenesis from beginning to end. In order for a virus or bacterium to display tropism for a specific host, it must first have the means to break through the host organism's line of defense. 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. An anatomical barrier is any physical or chemical barrier that helps prevent entry of microorganisms into body. This includes the skin, sweat, mucus layer, saliva, tears, endothelial lining, and natural human microbiota. The epidermis of the skin provides a physical barrier against pathogens, but it can easily be compromised by insect bites, animal bites, scratches or other minor skin trauma. Sweat, saliva, and tears are all chemical barriers that contain enzymes, such as lysozymes, that can kill bacteria and viruses. 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. A human's microbiota, the other microorganisms living within and on the body, compete with pathogenic organisms and play a large role in pathogenic control. 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. Without this lining, viruses and bacteria could easily infect vital human organs such as the brain, lungs, and placenta.