Pathogen – Types, Relationship between Pathogen and Host
Pathogen is defined as organism causing disease to its host. Pathogens are taxonomically diverse and comprise of unicellular and multicellular eukaryotes as well as bacteria and viruses. Though a small fraction of bacteria and viruses affect our health severely, majority of them are said to be beneficial and have no negative effect. Specifically a human pathogen is about one in a billion microbial species. Number of bacteria and viruses occupy almost every environment and their number is staggering on earth.
Types:
Four most common pathogen types are:-
1) Bacteria
2) Virus
3) Fungi
4) Parasites
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Bacteria:
Bacteria are microscopic, single celled organisms. They are present in every environment. Most bacteria serve a useful purpose to both plants and animals. They are also used for medical and industrial purposes, while other bacteria are harmful in nature.
Bacteria are diverse with variety of features and shapes with an ability to survive in extreme environmental conditions including the human body. Bacteria that cause infections are called pathogenic bacteria. Example: Citrobacterkoseri,Pseudomonas aeruginosaetc.
Human body is prone to bacterial infections when it is compromised by a virus. This disease state normally enables harmless bacteria to become highly pathogenic. Some of the diseases caused by bacteria are urinary tract infections(UTI’s), tuberculosis, E.coli infections.
Bacterial infections are treated by antibiotics . Some bacterial strains are said to be resistant to antibiotics making them difficult to treat. This can happen due to overuse of antibiotics but also naturally.
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Virus:
Virus is a highly infectious microbe which consist of a segment of nucleic acid(DNA or RNA) and is surrounded by a protein coat. A virus insect cells and uses the cell components to make copies of itself it cannot replicate alone. Viruses kills the host in the process of damaging the host cells. Examples of virus includes AIDS, covid-19, smallpox, etc.
Sometimes the viruses remain in dormant stage before multiplying again. This appears to be the recovery of a person with viral infections but gets sick again.
Antibiotics do not kill viruses so they are ineffective as a viral infection treatment.
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Fungi:
Fungi can be either single celled or complexed multicellular organism. They can be found anywhere in the environment and also on human skin.
The complex cell structure of fungi contains a nucleus and other components which are protected by a membrane and a thick cell wall making difficult to kill. Fungal strains are proved to be dangerous ex:Candida aurus.
Some of the diseases caused by fungus are ringworm, thrush, etc.
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Parasites:
Parasite are single celled Eukaryotes that can live inside or on the host and gets in food at the expense of the host. They are called “one celled animals” due to characteristics like lack of cell wall, predation, motility.
Types of Parasites:
▪︎protozoa: They are single celled organisms that can multiply in your body.
▪︎helminths: These are large multicellular organisms that live outside or inside the body. They are also called as worms.
▪︎ectoparasites: These are multicellular organisms that live on the body and feed on your skin. Ex: Ticks and insects.
Relationship between pathogen and host:
Pathogens are classified into two types obligate and facultative depending upon life cycle is attached to the host.
■ Obligate pathogens:
Obligate pathogens require a host to fulfil their life cycle. Some obligate pathogens requires multiple host to complete their life cycle. A vertebrate is often the definite host that supports adult form of pathogen and a vector(Intermediate host) is an arthropod.
The shift between vertebrate and invertebrate host is found in bacteria (lyme disease), virus(Zika virus), protozoa ( malaria).
■ Facultative pathogens:
These are the pathogens that can reproduce inside or outside (ex : on the surface) the host.
Pathogenicity and Virulence :
Pathogenicity is the ability to cause the disease by a microbial agent. On this basis pathogens are characterized into 3 groups. Primary pathogens are the major cause of disease in patients .(Salmonella sppis isolated from faeces that causes diarrheal disease.) Patients with compromised host defencesystems may be infected with opportunistic pathogens. Lastly the non pathogenic bacteria like acidophilus rarely cause human infections. Depending on the adaptability, effect of radiation therapy on resistance mechanisms they can be termed as pathogenic.Ex Serratiamarcescens , a common soil bacteria is said to cause Urinary tract infections and pneumonia in immune compromised host .Virulence is the degree to which an organism is pathogenic. When highly virulent pathogens are introduced in body cause a disease state leading to multi organ failure in healthy individuals. Less virulent pathogens not always lead severe conditions but more likely cause symptoms of diseases.Pathogen infects the host through various ways. One of the most common way is through damaging the cells or tissues during replication. This generally happens due to production of toxins that allows pathogens to exit the cells in which it replicated and reach new tissues. Immune system responds with an excessive immune response and kills both infected and uninfected cells and damages the host tissue. Example for this includes liver cancer in Hepatitis B.
Sometimes the host immune reaction pathogen by making it easier for the pathogen to transmit to uninfected hosts. Vibrio cholera triggers a inflammatory response in gut mucosa which leads to watery diarrhoea and ensure its release in the environment and infect for other hosts.
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