What is a microorganism?

A microorganism, also called a microbe, is a living organism that is microscopic in seize and invisible to the naked eye. Bacteria, fungi, yeast, protozoa, algae and viruses are all examples of microorganisms.


Bacteria, a prerequisite for life

Bacteria are one kind of microorganisms that are exceptionally important for human health. They were the first form of life to “populate” the Earth some four billion years ago, and still today they dictate the conditions for our existence. Throughout evolution bacteria and humans have co-existed. Lactobacillus reuteri is one important bacteria that has developed in symbiosis with us.

Read more about the evolution of Lactobacillus reuteri

Bacteria can be found everywhere, for example in soil, water, snow and even in volcanos. Some bacteria live in and on other organisms like animals and humans. Directly after birth the newborn baby is almost sterile but very soon bacteria start colonizing every part of the body – the skin, the mouth and most importantly the gastrointestinal tract. The number and diversity of bacteria continue to increase until the age of three, when it resembles the composition in an adult and more or less remains the same throughout life.

Earlier it has been widely cited in literature that humans carry ten times as many bacteria cells as body cells. However, new findings suggest that the number of bacterial cells are actually the same as the number of human cells. These bacteria are called the human microbiota and they are extremely essential for human health. The microbiota consists of some 100 trillion bacteria and is often referred to as an organ of its own right.


We can’t live without our bacteria 

Historically, all we knew about bacteria was that they were causing infections and disease, that they were pathogens. Today we know that bacteria are generally beneficial and together with other microorganisms vital for our health. Just as we cannot live without air and water, we cannot live without our bacteria.

One key function is to support the immune system and protect us from disease. About 80 percent of the immune system is located in the gut and the good bacteria are our first line of defense.

When our microbiota is diverse and in balance, bacteria cover us in an invisible body armor, that keeps environmental insults out. In addition to that, the bacteria perform a number of other important functions in the body and help us stay healthy.

Our bacteria:

  • are our first line of defense against pathogenic microorganisms that may cause disease
  • support and educate our immune system
  • help with digestion and absorption of nutrients
  • produce certain vitamins, like vitamin B and K
  • may even effect our brain and thereby our mood

We live in an age of cleanliness. We shower every day, use alco gel, and household cleaners that remove all bacteria, because we have learned that bacteria are dangerous. However, only a few bacteria are bad bacteria, most are actually good for us. We have some 100 trillion bacteria in and on our bodies, most of them are located in our gastrointestinal tract.

One way of categorizing bacteria is into commensals, useful bacteria, healthy bacteria and harmful bacteria.


Commensal bacteria

Some bacteria in our bodies are commensals, meaning that they benefit from living in our gastrointestinal tract but are not giving anything back. They are neither beneficial nor harmful. One example is non-pathogenic E. coli.


Useful bacteria

Long before refrigerators and freezers, people used “fermentation” to prolong the shelf-life of food. Fermentation is a process where lactic acid bacteria are used to preserve food or give flavor and texture to the product. Examples of fermented foods are yoghurt, sauerkraut, and pickled vegetables. Common lactic acid bacteria used for fermentation are Lactobacillus acidophilus, Lactobacillus bulgaricus and Streptococcus thermophilus.


Healthy bacteria

Truly healthy bacteria are known as probiotics. Probiotics are live bacteria that are proven to benefit our health by restoring the bacteria balance in the body and thereby have a positive effect on several health conditions. They are usually consumed as food supplements.

Read more about the definition of probiotics

A probiotic needs to be defined on strain level. One strain of Lactobacillus rhamnosus may differ from another strain of Lactobacillus rhamnosus like a Chihuahua from a Rhodesian Ridgeback.

Read more about strain specificity

Examples of true probiotics are different strains of Lactobacillus reuteri, like L. reuteri Protectis, L. reuteri Gastrus and L. reuteri Prodentis. L. reuteri is one of few bacteria that has co-evolved with humans since the beginning of time. The fact that a bacterium has remained faithful to the same host for a very long time, throughout the evolution, indicates that it lives in a mutualistic relationship with its host. L. reuteri benefits from living in the human body and the human body benefits from the presence of L. reuteri.

Read more about the evolution of Lactobacillus reuteri


Harmful bacteria, pathogens

A small percentage of all bacteria on earth are pathogenic, meaning they may cause health problems and disease. Food poisoning (E. coli and Salmonella), sepsis (S. aureus) and pneumonia (S. pneumoniae) are examples of conditions and diseases caused by pathogens.

It is important to remember that bacteria of the same species but of different strains can behave completely different. Some species, like E. coli, harbor strains that are extremely pathogenic, like EHEC and ETEC, causing severe diarrhea. On the other hand, some strains of E. coli are commensals and they are one of the most common bacteria we carry in our gastrointestinal tract.