Pathogen

In biology, a pathogen (Greek: πάθος pathos "suffering", "passion" and -γενής -genēs "producer of") in the oldest and broadest sense, is anything that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ.

The term pathogen came into use in the 1880s.[1][2] Typically, the term is used to describe an infectious microorganism or agent, such as a virus, bacterium, protozoan, prion, viroid, or fungus.[3][4] Small animals, such as certain kinds of worms and insect larvae, can also produce disease. However, these animals are usually, in common parlance, referred to as parasites rather than pathogens. The scientific study of microscopic organisms, including microscopic pathogenic organisms, is called microbiology, while the study of disease that may include these pathogens is called pathology. Parasitology, meanwhile, is the scientific study of parasites and the organisms that host them.

There are several pathways through which pathogens can invade a host. The principal pathways have different episodic time frames, but soil has the longest or most persistent potential for harboring a pathogen. Diseases in humans that are caused by infectious agents are known as pathogenic diseases, though not all diseases are caused by pathogens. Some diseases, such as Huntington's disease, are caused by inheritance of abnormal genes.

Pathogenicity

Pathogenicity is the potential disease-causing capacity of pathogens. Pathogenicity is related to virulence in meaning, but some authorities have come to distinguish it as a qualitative term, whereas the latter is quantitative. By this standard, an organism may be said to be pathogenic or non-pathogenic in a particular context, but not "more pathogenic" than another. Such comparisons are described instead in terms of relative virulence. Pathogenicity is also distinct from the transmissibility of the virus, which quantifies the risk of infection.[5]

A pathogen may be described in terms of its ability to produce toxins, enter tissue, colonize, hijack nutrients, and its ability to immunosuppress the host.

Context-dependent pathogenicity

It is common to speak of an entire species of bacteria as pathogenic when it is identified as the cause of a disease (cf. Koch's postulates). However, the modern view is that pathogenicity depends on the microbial ecosystem as a whole. A bacterium may participate in opportunistic infections in immunocompromised hosts, acquire virulence factors by plasmid infection, become transferred to a different site within the host, or respond to changes in the overall numbers of other bacteria present. For example, infection of mesenteric lymph glands of mice with Yersinia can clear the way for continuing infection of these sites by Lactobacillus, possibly by a mechanism of "immunological scarring".[6]

Related concepts

Virulence

Virulence (the tendency of a pathogen to cause damage to a host's fitness) evolves when that pathogen can spread from a diseased host, despite that host being very debilitated. Horizontal transmission occurs between hosts of the same species, in contrast to vertical transmission, which tends to evolve symbiosis (after a period of high morbidity and mortality in the population) by linking the pathogen's evolutionary success to the evolutionary success of the host organism.

Evolutionary medicine has found that under horizontal transmission, the host population might never develop tolerance to the pathogen.[citation needed]

Transmission

Transmission of pathogens occurs through many different routes, including airborne, direct or indirect contact, sexual contact, through blood, breast milk, or other body fluids, and through the fecal-oral route.

Types of pathogens

Prions

Prions are misfolded proteins that can transfer their misfolded state to other normally folded proteins of the same type. They do not contain any DNA or RNA and cannot replicate other than to convert already existing normal proteins to the misfolded state. These abnormally folded proteins are found characteristically in some diseases such as scrapie, bovine spongiform encephalopathy (mad cow disease) and Creutzfeldt–Jakob disease.[7]

Viruses

Viruses are small particles, typically between 20 and 300 nanometers in length,[8] containing RNA or DNA. Viruses require a host cell to replicate. Some of the diseases that are caused by viral pathogens include smallpox, influenza, mumps, measles, chickenpox, ebola, HIV, and rubella.

Pathogenic viruses are diseases mainly of the families of: Adenoviridae, Picornaviridae, Herpesviridae, Hepadnaviridae, Flaviviridae, Retroviridae, Orthomyxoviridae, Paramyxoviridae, Papovaviridae, Polyomavirus, Rhabdoviridae, and Togaviridae. HIV is a notable member of the retroviridae family which affected 37.9 million people across the world in 2018.[9]

Bacteria

The vast majority of bacteria, which can range between 0.15 to 700 μM in length,[10] are harmless or beneficial to humans. However, a relatively small list of pathogenic bacteria can cause infectious diseases. Pathogenic bacteria have several ways that they can cause disease. They can either directly affect the cells of their host, produce endotoxins that damage the cells of their host, or cause a strong enough immune response that the host cells are damaged.

One of the bacterial diseases with the highest disease burden is tuberculosis, caused by the bacterium Mycobacterium tuberculosis, which killed 1.5 million people in 2013, mostly in sub-Saharan Africa.[11] Pathogenic bacteria contribute to other globally significant diseases, such as pneumonia, which can be caused by bacteria such as Streptococcus and Pseudomonas, and foodborne illnesses, which can be caused by bacteria such as Shigella, Campylobacter, and Salmonella. Pathogenic bacteria also cause infections such as tetanus, typhoid fever, diphtheria, syphilis, and leprosy.

Fungi

Fungi are eukaryotic organisms that can function as pathogens. There are approximately 300 known fungi that are pathogenic to humans[12] including Candida albicans, which is the most common cause of thrush, and Cryptococcus neoformans, which can cause a severe form of meningitis. The typical fungal spore size is <4.7 μm micrometers in length, but some spores may be larger.[13]

Algae

Algae are single-celled plants that are generally non-pathogenic although pathogenic varieties do exist. Protothecosis is a disease found in dogs, cats, cattle, and humans caused by a type of green alga known as prototheca that lacks chlorophyll.[14]

Other parasites

Some eukaryotic organisms, including a number of protozoa and helminths, are human parasites.

Treatment and health care

Bacteria are usually treated with antibiotics while viruses are treated with antiviral compounds. Eukaryotic pathogens are typically not susceptible to antibiotics and thus need specific drugs. Infection with many pathogens can be prevented by immunization. A small amount of pathogens are used in vaccines to make immunity stay alert and strengthen defense on the insides to prepare for a larger quantity of the virus ever getting inside. Hygiene is critical for the prevention of infection by pathogens.

Sexual interactions

Many pathogens are capable of sexual interaction. Among pathogenic bacteria sexual interaction occurs between cells of the same species by the process of natural genetic transformation. Transformation involves the transfer of DNA from a donor cell to a recipient cell and the integration of the donor DNA into the recipient genome by recombination. Examples of bacterial pathogens capable of natural transformation are Helicobacter pylori, Haemophilus influenzae, Legionella pneumophila, Neisseria gonorrhoeae and Streptococcus pneumoniae.[15]

Eukaryotic pathogens are often capable of sexual interaction by a process involving meiosis and syngamy. Meiosis involves the intimate pairing of homologous chromosomes and recombination between them. Examples of eukaryotic pathogens capable of sex include the protozoan parasites Plasmodium falciparum, Toxoplasma gondii, Trypanosoma brucei, Giardia intestinalis, and the fungi Aspergillus fumigatus, Candida albicans and Cryptococcus neoformans.[15]

Viruses may also undergo sexual interaction when two or more viral genomes enter the same host cell. This process involves pairing of homologous genomes and recombination between them by a process referred to as multiplicity reactivation. Examples of viruses that undergo this process are herpes simplex virus, human immunodeficiency virus, and vaccinia virus.[15]

The sexual processes in bacteria, microbial eukaryotes and viruses all involve recombination between homologous genomes that appears to facilitate the repair of genome damages inflicted on the genome of the pathogens by the defenses of their respective target hosts.

See also

References

  1. ^ "Pathogen". Dictionary.com Unabridged. Random House. Retrieved August 17, 2013.
  2. ^ Casadevall, Arturo; Pirofski, Liise-anne (11 December 2014). "Ditch the term pathogen". Comment. Nature (paper). 516 (7530): 165–6. doi:10.1038/516165a. PMID 25503219.
  3. ^ Alberts B; Johnson A; Lewis J; et al. (2002). "Introduction to Pathogens". Molecular Biology of the Cell (4th ed.). Garland Science. p. 1. Retrieved 26 April 2016.
  4. ^ "MetaPathogen – about various types of pathogenic organisms". Retrieved 15 January 2015.
  5. ^ "1.2. Definitions: pathogenicity vs virulence; incidence vs prevalence". COLOSS.
  6. ^ Carl Nathan (2015-10-09). "From transient infection to chronic disease". Science. 350 (6257): 161. doi:10.1126/science.aad4141. PMID 26450196.
  7. ^ "The prion diseases" Stanley B. Prusiner, Scientific American
  8. ^ Viral Special Pathogens Branch | [26] Moved | CDC Archived May 6, 2009, at the Wayback Machine
  9. ^ July 31, Content Source: HIV govDate last updated:; 2019 (2019-07-31). "Global Statistics". HIV.gov. Retrieved 2019-10-04.CS1 maint: extra punctuation (link)
  10. ^ Weiser, Jeffrey N. (February 2013). "The Battle with the Host over Microbial Size". Current opinion in microbiology. 16 (1): 59–62. doi:10.1016/j.mib.2013.01.001. ISSN 1369-5274. PMC 3622179. PMID 23395472.
  11. ^ Zumla, Alimuddin; Petersen, Eskild; Nyirenda, Thomas; Chakaya, Jeremiah (2015-03-01). "Tackling the Tuberculosis Epidemic in sub-Saharan Africa – unique opportunities arising from the second European Developing Countries Clinical Trials Partnership (EDCTP) programme 2015-2024". International Journal of Infectious Diseases. Special Issue: Commemorating World Tuberculosis Day 2015. 32: 46–49. doi:10.1016/j.ijid.2014.12.039. ISSN 1201-9712.
  12. ^ "Stop neglecting fungi". Nature Microbiology. 2 (8): 1–2. 2017-07-25. doi:10.1038/nmicrobiol.2017.120. ISSN 2058-5276.
  13. ^ Yamamoto, Naomichi; Bibby, Kyle; Qian, Jing; Hospodsky, Denina; Rismani-Yazdi, Hamid; Nazaroff, William W; Peccia, Jordan (October 2012). "Particle-size distributions and seasonal diversity of allergenic and pathogenic fungi in outdoor air". The ISME Journal. 6 (10): 1801–1811. doi:10.1038/ismej.2012.30. ISSN 1751-7362. PMC 3446800. PMID 22476354.
  14. ^ Satoh, Kazuo; Ooe, Kenji; Nagayama, Hirotoshi; Makimura, Koichi (2010). "Prototheca cutis sp. nov., a newly discovered pathogen of protothecosis isolated from inflamed human skin". International Journal of Systematic and Evolutionary Microbiology,. 60 (5): 1236–1240. doi:10.1099/ijs.0.016402-0. ISSN 1466-5026.CS1 maint: extra punctuation (link)
  15. ^ a b c Bernstein, Harris; Bernstein, Carol; Michod, Richard E. (2018). "Sex in microbial pathogens". Infection, Genetics and Evolution. 57: 8–25. doi:10.1016/j.meegid.2017.10.024. PMID 29111273.

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