The 4 types of pathogens (and their characteristics)

These are the different elements that, when entering our organism, can generate diseases.

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

However, infectious diseases are still one of the most frequent causes of death in the world today, and their study has never ceased. Thanks to these lines of research, we know that these conditions are caused by the infection of the patient by some type of pathogenic agent.

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

What is a pathogenic agent?

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

Traditionally it was considered to be any invasive organism, although it is now separated into two terms: pathogens, which includes acellular microorganisms, prokaryotes and fungi; 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 and biology.

Types of pathogens

Microorganisms are the major cause of diseases in humans.. Pathogens are adapted to live inside other organisms (host), since by themselves they cannot cover all their needs, such as feeding or reproduction. As a result, they induce damage to host cells, which triggers disease.

The way to classify the types of pathogens depends on the taxonomic category to which they belong, i.e. whether they are bacteria or viruses, for example.i.e. whether it is a bacterium or a virus, for example. In this case we will name these types of pathogens from the simplest to the most complex (at the structural level).

1. Prions

This strange type of pathogen is basically a protein. It does not even have genetic material, but it has a great capacity to damage the organism; It causes transmissible spongiform encephalopathy (TSE), a neurodegenerative neurodegenerative disease.It causes transmissible spongiform encephalopathy (TSE), a lethal 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 the "prp" (PRoteína Prionica). The curious thing is that it is a protein that is specific to our cells, mainly present in neurons. and the gene that produces it is in the mammalian genome, so it originates in this group of vertebrates.

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, to gain resistance to proteases (enzymes that degrade certain proteins) and to accumulate amyloid bodies, which causes neuronal death, degenerating into disease.

Prions are linked to conditions such as kuru disease (due to human cannibalism), Creutzfeldt-Jakob disease (genetic) or bovine spongiform disease, commonly referred to as "mad cow disease".

2. Viruses

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

Viruses cause a large number of infections in humans, and act in very different ways. They can cause temporary (e.g. influenza virus), chronic (e.g. chronic hepatitis B virus) or latent (e.g. herpes virus) infections.. The latter case refers to pathogens that enter the host and generate a disease, but upon recovery from this, the infectious agent is not completely eliminated from the body and goes unnoticed, becoming active periodically, causing a new disease. In some cases, they can degenerate into Cancer with the insertion of the genetic material into the cell's chromosome, as is the case with human papillomavirus (HPV) and cervical cancer.

3. Bacteria

The next type of pathogen is bacteria, although not all of them act in this way, since this is a biological category.Bacteria are a very diverse biological category, encompassing a whole kingdom of prokaryotic cells. Prokaryotic cells differ from eukaryotic cells (the ones we have) in that they do not have a nucleus inside to house their genetic material (DNA), do not have membranous organelles (cell machinery) and have a cell wall that protects them (with some exceptions).

Many criteria are used to classify bacteria, but mainly by cell wall composition (gram staining), structural form (Bacillus, coccus or spirochete) and interaction with oxygen (aerobic or anaerobic).

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

Like viruses, there are bacteria that are obligate intracellular pathogens, because they do not have their own mechanisms for obtaining ATP, the cell's energy. An example of this is Chlamydia.

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

Finally, we have extracellular pathogensThey are found inside the organism, but never enter the interior of the cells. An example of this group is Streptococcus.

Although we are not aware of it, we are surrounded by microorganisms, and millions of bacteria live in our skin, mouth or digestive tract. Whether we get the disease is sometimes no more than the product of a combination of factors, such as the initial amount of the pathogen or the state of the immune system of the host, our organism. In the case of infectious bacteria, their damage can be caused by their own action on cells or by the effect of the toxins they secrete, which sometimes cause tissue destruction.

4. Fungi

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

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

In the second case would be the mycotic infectionwhich is when it acts inside the host, either on the mucous membranes or in organs (e.g., Candida).

What about parasites?

Although nowadays they are not usually included among the types of pathogenic agents, in the past they were. Let us look at their different categories.

Protozoa are unicellular eukaryotic microorganisms.. Like bacteria, this category encompasses different ways of living, including both extracellular and intracellular parasitic organism. The Plasmodiumwhich causes the disease Malaria, is currently the most lethal protozoan, wreaking havoc in developing countries.

Another group of parasites are helminths, which are worms, i.e. eukaryotic multicellular organisms.. As before, they are both free-living and parasitic, and normally have a very complex life cycle, with phases of sexual reproduction (union of sexual cells or gametes) and asexual reproduction (identical copies). Examples are intestinal tapeworms, Ascaris (intestinal nematode) or Trichinella (nematode causing trichinellosis).

Finally, there are the ectoparasites. These are arthropods, namely insects (such as the louse) and arachnids (the mite) that attach themselves to the host's skin or burrow into it over a long period of time. They do not usually cause great harm. The greatest danger from arthropods is when they act as vectors, in other words, when they carry within them a pathogen (such as Borrellia bacteria and the tick in Lyme disease) or parasites (Plasmodium and the mosquito in Malaria) and transfer it with their bite.

Bibliographic references:

• Kumar V., Abbas A. and Aster J. (2013). "Robbins, human pathology" (9th ed.) Elsevier Saunder Publishing House.
• Cacace V. (2011). "Biology of Prions". https://arxiv.org/ftp/arxiv/papers/1106/1106.3533.pdf.
• Iracheta M. A. (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 Filamentous Fungal and Yeast Species." https://seimc.org/contenidos/documentoscientificos/eimc/seimc_eimc_v31n05p328a341.pdf.