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Three Life Forms That Dwell Inside of You

bacterium illustration

According to the latest scientific research, there are about 100 trillion microorganisms living in your intestinal tract. That is (according to most estimates) at least twice greater than the number of human cells in your body. To put 100 trillion in perspective, a clock ticking off 100 trillion SECONDS would need a battery strong enough to last for over three million years!

In other words, much of what you consider “You” is not really you at all. To be fair, those 100 trillion cells only weigh a few pounds and they are much smaller than anything you’ve (literally) ever seen.

Sounds unbelievable, doesn’t it? Microbiologists and molecular physicists, however, are used to dealing with the vastness of the world within. And because these issues may have critical implications for health, it is time the rest of us begin learning about gut flora and how they affect our lives.

Pinworms in a human
Pinworm cross sections (via Ed Uthman, MD)

What types of microorganisms are present in the human gut?

According to the biological classification system most widely used today, cellular life forms may be divided into three recognizably different domains: archaea, bacteria, and eukaryote.

Gut microorganisms include all three of these life forms, though about 90% of the gut flora consists of bacteria (and more than half of the solid mass of fecal material is composed of bacteria).

Further analysis reveals about 500 different species of bacteria in the gut, with 90% of those belonging to one of perhaps 40 most abundant species. These figures, of course, are estimates and assume a person of average age and size. The primary take-away is that most of those 100 trillion cells of gut flora are bacteria and there are only about 40 kinds of bacteria making up the bulk. Remember, though, just because the other life forms in your intestinal tract are less numerous — that does not mean they are less important.

Let’s look at each domain and their characteristics


Archaeans are single-celled microorganisms. They typically measure less than one micron (one millionth of a meter) across. Once thought to inhabit only the harshest of environments, scientists now say archaeans are abundant in the ocean, especially in plankton.

  • Archaea are thought to play a significant role in the digestion of foods, especially in the colon.
  • There are no identified pathogens in the archaean domain.
  • Once thought to be bacteria, scientists have discovered many similarities between archaeans and humans.
  • Archaea are most often thought of as the inhabitants of environmental extremes – acidic run-offs from mines, for instance, or in salt marshes. Scientists were surprised to find them in the soil and in the human body.

Eubacteria (i.e. “bacteria”)

Like archaeans, bacteria do not have a nucleus. They tend to be several times larger than archaeans, and are thought to be the most abundant form of life on Earth. Three-hundredths of an ounce of fresh water from a mountain stream would likely contain a million bacterial cells.

Some bacteria are pathogenic. These include cholera, tuberculosis, and anthrax. Some bacteria are predatory – they kill and consume other microorganisms. One fascinating area of research is centered on determining whether microbiological predators can be enlisted to treat infections that have become impervious to modern antibiotics. Don’t pass by this idea too quickly: a colony of biological assassins could one day save your life!

Most of the time, friendly bacteria form allegiances that pathogens at bay. When the immune system is weakened, however, disease-causing organisms can take over and make you ill. Problems can also occur when antibiotics are used. Friendly bacteria are killed along with the pathogens. In the rush to repopulate lost ground, territory that was once protected by friendly life forms can be taken over by the harmful microbes, leading to an infection.

  • Some bacteria (photoautotrophs) are solar-powered.
  • The three domains describe cellular life forms. Viruses are not (currently) considered to be alive, since they are not cellular. There is an ongoing dispute, within science, to say exactly what viruses are.
  • Bacteria are found in every ecological role. They can be consumers, producers, decomposers, predators … you name it; bacteria can do it.


The cells of the eukaryotes do contain a nucleus, and they tend to be ten times larger than the cells of bacteria. Kingdoms within the domain include fungi, plants, and animals. Because most of the gut flora community consists of bacteria, it is common to hear ALL gut flora referred to as “bacteria.” The common and troublesome Candida albicans, for instance, is a eukaryote, not a bacterium. While both archaeans and bacteria are asexual – reproducing primarily though division – many eukaryotes are capable of sexual reproduction. Although Candida albicans is the most widely known of the pathogens in this domain, Giardia and Plasmodium (malaria), among others, are of great concern.

  • “Eukaryote” literally means “true nut” in Greek. The name refers to the fact that this is the only domain with a nucleus.
  • The nucleus of eukarya contains the DNA
  • Eukaryotes are generally much more complex than members of the other domains.

The human biome — an ongoing study

Discoveries concerning the numbers and importance of microorganisms to the human biome have prompted many scientists to look at our gut flora as a separate organ. Moreover, many say there is a fourth domain — that of the virus. Science isn’t quite sure what to do with viruses. Are they biological chemicals? Are they the precursors of life? The debate is, at times, intense — especially with the recent discovery of the “Pandoravirus.” At one micron across, it is the largest virus yet found.

Because the technology required to study the microscopic universe has only recently become available, new developments and fresh observations are being recorded by researchers daily. Don’t ever think the age of scientific discovery is over — it may be just getting ready to begin!

Dr. Haley and the Stockton Research Library staff are monitoring science news and journal articles in order to report new theories as they emerge. If you have yet to register for the Stockton Aloe Research Library newsletter, do so right now.

Lack of knowledge can keep you suffering needlessly.

Perhaps no area of research has shown more promise for health than that of the gut flora and the microbiological community that lives within you.

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