Germs: What They Are, Characteristics, and Types

Not all microorganisms are germs, but all germs are included in a group of microorganisms. Although viruses and bacteria are the best known, there are other types too.

Written and verified by el biólogo Samuel Antonio Sánchez Amador.

Last update: 19 March, 2021

According to the Global Burden of Disease Study (GBD), 95% of the world’s population has some type of disease. Just to highlight one, lower respiratory infections and diarrheal diseases caused by germs are a major cause of global morbidity and mortality.

The concept “germ” surrounds us everywhere, because simply in and on the human body there are about 48 billion bacteria. Due to their presence throughout the globe, bacteria are the second largest contributor to terrestrial biomass —after plants— forming 15% of the total amount.

So, some microorganisms are good, some are neutral, and some are pathogens. Beyond the field of ecology, would you be able to say what germs are? This term, increasingly in disuse, refers to a small fraction of the living things that inhabit the Earth: those that are microscopic and pathogenic.

What are germs?

According to the US National Cancer Institute, a germ is a bacterium, virus, or other microbe that can cause infection and disease. These microscopic beings are characterized by their potential pathogenicity. This is a unique attribute of each genus and species, which allows the microorganism in question to cause damage in susceptible hosts.

Rather than “germ”, the correct term is always a biological pathogen. As indicated by the amBientech portal, this term refers to any biological entity capable of producing an infectious disease in a host – be it human, plant, or other animal – that is predisposed to do so.

This living being that’s predisposed to infection in a biological way is called a host. A living being can become a host for a germ based on the following parameters:

1. Genetic factors, age, physiological disorders, and concomitant diseases: All these parameters can cause depression in the immune system, which facilitates the entry and the establishment of germs.
2. Place of residence and environmental characteristics: Depending on where a living being lives, it will be much more likely to become a host for one or more germs. Animals that a pathogen can infect are one thing, and those at a human’s disposal are quite another.
3. Behavior and lifestyle: Parameters such as exercise, personal hygiene, diet, interpersonal contacts, the consumption of toxins, and other factors can either favor or hinder a germ from reaching a human host.

It’s almost impossible to understand a germ or biological pathogen without its host counterpart.

The relationship between the two is a kind of “evolutionary dance.” The affected person develops and perfects their immune system to avoid infection, while the pathogen mutates and changes to circumvent the new biological barriers.

The 6 types of germs

Now that we have introduced the term, and how it relates to the host, we’re ready to explore the 6 types of germs. These are bacteria, viruses, fungi, protozoa, viruses, viroids, and prions. Let’s have a look.

1. Bacteria

As the National Human Genome Research Institute indicates, bacteria are prokaryotic organisms — their cellular DNA is free in the cytoplasm — unicellular and cosmopolitan. They are found in almost all parts of the Earth and have essential ecosystem tasks, such as recycling organic matter and producing oxygen.

Beyond their contagious meaning, many bacteria are human being’s “guests” – they do neither good nor bad, nor do they relate to us.

According to the Scientific Culture Notebook, there is such a tremendous amount of bacteria on Earth that all of them together add up to 70 gigatons of carbon, 15% of the total organic matter in the world.

Here are some of the typical characteristics of almost all bacteria:

1. Like eukaryotic cells, bacteria have a cytoplasm, a cell membrane, ribosomes – in charge of synthesizing proteins – and bacterial DNA. In this case, the DNA is free in the cytoplasm and isn’t delimited by a nuclear membrane. Bacteria can also have capsules, not present in eukaryotes.
2. Bacteria are small, and so they have to be viewed under a microscope. Its length is between 0.5 and 5 micrometers. Despite their minuscule size, they have very varied shapes, such as cocci, bacilli, and spirilla.
3. Some bacteria use body twists, cilia, and flagella to move through their surroundings.

Not all bacteria are pathogenic, but some of them are responsible for very serious diseases in humans.

Some bacteria known for their infectious capacity are those of the genus Salmonella and Shigella and the species Campylobacter jejuni, Clostridium botulinum, Staphylococcus aureus
, and Vibrio cholerae.

2. Viruses

As indicated by the Kids Health portal, viruses are even smaller than bacteria (from 10 to 300 nanometers) and aren’t even considered to be fully living beings.

They don’t have a real cellular structure, since they lack cytoplasm, ribosomes and other organelles. Most viruses are composed of free DNA or RNA and a capsid or capsomere, made from proteins.

Since they don’t have cells, they aren’t capable of reproducing; and since they cannot reproduce, they aren’t considered living beings. Rather than microorganisms, they’re called infectious agents, since they cannot live without their host or remain in the environment for a long time.

For cell division to occur, a microorganism must have a nucleus and ribosomes, which will synthesize the necessary proteins to give rise to new structures. Since viruses lack this capacity, they have to “hijack” the cells of their host and take advantage of its mechanisms in order to replicate.

According to the CK12 portal, once inside the infected cell, viruses use their ATP, ribosomes, enzymes, and other parts of it to replicate, that is, to multiply their genetic information and give rise to the necessary proteins with which they will form capsids. When they leave the cell body, viruses often cause their death or apoptosis.

Some of the typical viruses that infect humans are the following: influenza viruses A and B, human papillomavirus (HPV, with more than 200 variants), norovirus, cytomegalovirus, and many others.

The epidemiology of viruses

Epidemiology is the branch of medicine that’s responsible for studying the distribution, frequency, and determining factors of existing diseases in a specific geographical area, especially if they are caused by viruses.

Thanks to this field of research, we know, for example, that up to 20% of the world’s population has the flu at any given time and place.

3. Protozoa

As the Mayo Clinic indicates, protozoa are single-celled beings that are slightly more “advanced” on the evolutionary scale. They behave more or less like animals, hunting and gathering other microbes to survive. These germs live in humid or aquatic environments, such as salty and fresh waters and the internal environments of living beings.

Protozoa measure 10-50 microns, so they are much larger than viruses and bacteria. Having said that, its taxonomic diversity is lower: around 30,000 total species are known worldwide.

As predators, these beings have cilia and flagella and hunt algae, bacteria, and unicellular or filamentous micro-fungi.

While not all protozoa are human pathogens, some of them are very dangerous. In addition, many use invertebrate vectors to reach us, such as mosquitoes, ticks, or leeches.

Some of the most common protozoan diseases are the following:

• Malaria: This is caused by protozoa of the genus Plasmodium, transmitted through the bite of a mosquito. Malaria is estimated to affect more than 200 million people in low-income countries, causing 400,000 deaths annually.
• Trypanosomiasis (sleeping sickness): A disease caused by protozoa of the genus Trypanosoma. This pathology moves in epidemiological waves, but luckily its global incidence has slowed down considerably.
• Leishmaniasis: This is caused by protozoa of the genus Leishmania. This disease ranges from mild forms, which affect the skin with a welt, to fatal conditions, which inflame and irreparably damage the liver and spleen.

As you have seen, many protozoan diseases are transmitted by the bite of insects and other arthropods. Therefore, its prevalence increases greatly in tropical regions, where the biodiversity index is high.

4. Fungi

The Fungi kingdom encompasses all types of fungi, from molds to yeasts, and those that form mushrooms, and well as many other types. They differ from plants in that they are heterotrophs – they obtain their energy from organic matter – and that their cell wall is composed of chitin, not cellulose.

Pathogenic fungi are the ones that cause harm to humans; most of them are microscopic in size, despite being eukaryotic. Medical mycology is responsible for studying and preventing the effects of infections of some 300 species within this kingdom.

Some of the fungi considered as germs are within the group of dermatophytes. According to the SEIMC, these are pathogens that settle on the superficial layers of the skin, hair, and nails. Some attack keratin, while others focus on more specialized structures.

Ringworm, nail infections, and candidiasis are considered types of pathologies caused by dermatophytes. In addition to these, there are other fungi that cause much more serious diseases, such as pulmonary aspergillosis. In this fatal pathology, the hyphae of the fungus grow in the lungs of the patient.

5. Viroids

We’re not going to spend much time talking about these germs, as they are rare biological pathogens and quite difficult to understand. Viroids are similar to viruses, but even simpler. These lack lipids and proteins, so they are only made up of a short cyclic strand of RNA.

They’re the lowest rung on the biological scale, as they cannot be any simpler and it’s impossible to classify them as living beings. Viroids are typical plant pathogens, as some 300 species of them have been described that affect higher plants.

6. Prions

A prion is a badly folded protein capable of transmitting its non-functional form to other proteins. These proteins cause transmissible spongiform encephalopathies, such as the extremely high-profile epidemiological outbreak of mad cow disease. It’s surprising that such a simple structure can generate pathogenicity in humans!

Germs are complex and highly varied

We hope you’ve enjoyed our look at the 6 types of germs that exist on our planet, in addition to all their characteristics and examples of diseases that these can cause.

Without a doubt, viral and bacterial agents beat the rest when it comes to infectiousness, but we mustn’t forget about fungi and protozoa.

Finally, it’s intriguing to discover how such basic structures as viroids and prions can end up killing people’s lives. They’re not even considered to be cellular entities and, because of this, finding the biological reason for their existence is a real puzzle.