The 4 types of pathogens (and their characteristics)

There is no doubt that one of the great advances of humanity has been medicine. Thanks to this science, our longevity has been significantly lengthened in a matter of decades.

However, infectious diseases remain today one of the most frequent causes of death in the world, and their study has never stopped. Thanks to these lines of research, we know that these conditions originate from the infection of the patient by some type of pathogen.

To understand this process a little more, here we will see a summary of the most important types of pathogens.

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What is a pathogen?

We understand as pathogen or infectious agent any microorganism that infects other organisms, causing damage and injury.

Traditionally, all invasive organisms were considered, although now they are separated into two terms: pathogens, which includes acellular, prokaryotic and fungal microorganisms; and parasites, for the rest of eukaryotes (protozoa, helminths and ectoparasites) that generate parasitic diseases.

Thus, pathogens are studied by fields of science such as Medicine or Biology.

Types of pathogens

Microorganisms are the major cause of disease in humans. Pathogens are adapted to live inside other organisms (host), since by themselves they cannot cover all their needs, such as feeding or reproducing. By this fact, they induce damage to the host's cells, which triggers the disease.

The way to classify the types of pathogens depends on the taxonomic category to which they belong., that is, if it is a bacterium or a virus, for example. In this case we will be naming these types of pathogens from the simplest to the most complex (at a structural level).

1. Prions

This strange type of pathogen is fundamentally a protein. It does not even have genetic material, but it has a great capacity to damage the organism; causes transmissible spongiform encephalopathy (TSE), a fatal neurodegenerative disease for which there is no cure. Several variants are known, and there is one that affects mammals, including humans.

The protein that causes this infection in these cases is “prp” (Prionic Protein). The funny thing is that It is a protein of our cells, which is mainly present in neurons and the gene that produces it is in the genome of mammals, which is why it originates in this group of vertebrates.

In order for the normal protein (prpc) to become its pathogenic form (prpsc), it must produce a change in its protein structure. This variation causes the protein to lose its natural function and acquire the ability to self-reproduce, which gains resistance to proteases. (enzymes that break down certain proteins) and accumulate amyloid bodies, which causes the death of neurons, degenerating into the disease.

Prions are linked to conditions such as kuru diseases (from human cannibalism), Creutzfeldt-Jakob (genetics) or bovine spongiform disease, which is commonly referred to as "cow disease crazy ”.

2. Virus

The next type of pathogenic agent is made up of viruses. Acellular, it is generally a protein structure (capsid) that houses genetic material inside. They are obligate intracellular parasitic microorganisms, since they cannot reproduce on their own, and they need the machinery of a cell to multiply in number. This fact causes that by damaging the host cells, the disease is generated. Various criteria are used to classify them, based on their genetic content or their structure.

Viruses cause a large number of infections in humans, and they act in many different ways. They can cause temporary (such as the flu virus), chronic (chronic Hepatitis B virus), or latent (herpes virus) conditions.. This last case refers to pathogens that enter the host and generate a condition, but when recovering from it, the infectious agent is not totally eliminated from the body and goes unnoticed, activating periodically, originating a new condition. In some cases, they can degenerate into cancer with the insertion of genetic material into the cell's chromosome, as is the case with human papillomavirus (HPV) and cervical cancer.

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3. Bacteria

The next type of pathogen is bacteria, although not all of them act like this, since this is a very diverse biological category, encompassing an entire kingdom of prokaryotic cells. Prokaryotic cells differ from eukaryotic cells (the one we have) by not having a nucleus inside to house their genetic material (DNA), not having membranous organelles (cell machinery) and having a cell wall that protects them (with some exception).

Many criteria are used to classify bacteria, but mainly by the composition in the cell wall (gram stain), its structural form (bacillus, coconut or spirochete) and its interaction with oxygen (aerobic or anaerobe).

When acting as a pathogen, bacteria are differentiated according to the way they interact with the host.

Like viruses, there are bacteria that are mandatory intracellular pathogens, because they do not have their own mechanisms to obtain ATP, the energy of the cell. An example of this is Chlamydia.

Other bacteria have the ability to enter cells, but it is not necessary for their survival either, and can also be outside the cells; in this case, it is known as a facultative intracellular pathogen. That does not mean that it must be inside another organism, that is, it does not live in an open environment. An example of this group of pathogens is the Salmonella.

Last we have extracellular pathogens, is that they are found inside the organism, but never enter the interior of the cells. Example of this group is Streptococcus.

Although we are not aware, we are surrounded by microorganisms, and millions of bacteria live on our skin, mouth or digestive system. Sometimes we get the disease is nothing more than the product of a combination of factors, such as the initial amount of the pathogen or the state of the host's immune system, our body. In the case of infectious bacteria, their damage can be due to their own action on cells or due to the effect of the toxins they secrete, which sometimes cause tissue destruction.

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4. Mushrooms

The last type of pathogen is fungi. These are eukaryotic organisms, which, unlike prokaryotes, already have an intracellular nucleus and membranous organelles. In addition, the cells of the fungi are reinforced with a cell wall. Their cellular organization can be unicellular (yeast) or in filamentous hyphae (chains).

In the case of infectious fungi, they act in two different ways. The first are superficial infectionsIn this case, the pathogen is dermatophytes, which attack the skin, hair or nails (for example, athlete's foot).

In the second case it would be the fungal infection, which is when its action is inside the host, either on the mucous membranes or in organs (for example, Candida).

What about parasites?

Although currently they are no longer usually included within the types of pathogens, in the past they were. Let's look at its different categories.

Protozoa are eukaryotic single-celled microorganisms. Like bacteria, this category includes different ways of living, among which are the parasitic of both extracellular and intracellular organism. The Plasmodium, which causes the disease Malaria, would be the most lethal protozoan today, wreaking havoc in developing countries.

Another group of parasites are helminths, which are worms, that is, eukaryotic multicellular organisms. As before, it exists free-living as a parasitic one, and normally has a very long life cycle. complex, with phases of sexual reproduction (union of sex cells or gametes) and asexual (copies identical). Examples are the intestinal tapeworms, Ascaris (intestinal nematode) or Trichinella (the nematode that causes Trichinellosis).

Lastly, there are the ectoparasites. These are arthropods, specifically insects (such as the louse) and arachnids (the mite) that adhere to or burrow into the host's skin for a long period of time. They usually do not cause great harm. The greatest danger of arthropods is when they act as vectors, in other words, when they carry a pathogen (such as Borrellia bacteria and the tick in Lyme disease) or parasites (Plasmodium and the mosquito in Malaria) and transfers it with its bite.

Bibliographic references:

• Kumar V., Abbas A. and Aster J. (2013). "Robbins, human pathology" (9th ed.) Editorial Elsevier Saunder.
• Cacace V. (2011). "Biology of Prions". https://arxiv.org/ftp/arxiv/papers/1106/1106.3533.pdf.
• Iracheta M. TO. (2009). "Bacteria and viruses, how do we defend ourselves?" http://www.rac.es/ficheros/doc/00919.pdf.
• Permán J. and Salavert M. (2013). "Epidemiology and prevention of nosocomial infections caused by species of filamentous fungi and yeasts". https://seimc.org/contenidos/documentoscientificos/eimc/seimc_eimc_v31n05p328a341.pdf.