Bacteria are diverse and extremely abundant in the large intestine. Many of these are helpful by digesting cellulose or producing vitamins that we absorb. But other bacteria can be harmful.

Some Archaea are also present in the large intestine, mainly producing methane. Cells of Archaea appear similar to bacteria, but enough fundamental differences exist to be classified as an entirely different Domain.

Why this is important…In the last few decades, there has been a shift in thinking about bacteria, from “all bad” to “mostly useful.” This is especially true with it comes to the bacterial flora of our intestines. These bacteria can influence our digestive and overall health. A working knowledge of bacteria is crucial to understand this digestive partnership as well as many diseases that affect humans.

Foundations

Recall that the 1.8 million species known on earth are classified into three Domains – Eukarya, Bacteria and Archaea. The last two are simple organisms with no nucleus or organelles (prokaryotes). However, this doesn’t mean that they are simple in all ways. While they may be small, they have a big impact on our lives.

Details

The details about bacteria were covered in Chapter 2g. This section is more specific to Archaea and the process of binary fission.

Bacteria

Bacteria play an important role in the human digestive system. Most people have around 500 different species of bacteria living in their digestive tract. Most of these are helpful. However, some species, like Clostridioides difficile (also known as C. diff.) or certain varieties of Escherichia coli (E. coli), can be extremely harmful if their populations get out of control.

Stop & Think #14: If bacteria were not expelled frequently through bowel movements, how might this affect the large intestine?

Most bacteria live in the large intestine, feeding off of the organic matter that was not absorbed by the stomach or small intestine. Over half of the dry weight of feces is actually bacteria, which means they are reproducing at an astounding rate! This is because binary fission (the way bacteria and archaea produce new cells) can happen very quickly, allowing the population size to double every 20 minutes!

Stop & Think #15: If the stomach and small intestine fail to do their job properly, nutrients that are normally digested and absorbed make their way to the large intestine. What effect might this have on the population of Archaea and Bacteria? What symptoms might that cause?

Archaea

Archaea are similar to bacteria in many ways, but share some biochemical features with eukaryotes. Like eukaryotes, archaea wrap their DNA around proteins called histones. Unlike Bacteria or Eukarya, the Archaea use different types of phospholipids to make cell membranes, have a slightly different way they use their genetic code for proteins, and use some different amino acids than the standard 20 used by most other organisms. A few other key features of Archaea:

Cell walls are made of unique proteins, similar to bacteria but not the same
Flagella, which help archaea swim, are made of unique proteins but rotate like bacteria flagella
Asexual reproduction by binary fission is their only way to reproduce
Energy sources vary greatly – a few are photosynthetic, some get energy from minerals, and most absorb organic molecules

Two examples of archaea cells from electron microscope. Both are rods that look like chains of sausages.

FIGURE 5-18. Archaea examples. Left: halobacteria is a photosynthetic archaean. Right: a methanogen that produces methane and lives in high-temperature environments. ("Halobacteria " by US Government is in the Public Domain, CC0 || "Methanothermobacter thermautotrophicus " by Andreas Klingl is licensed under CC BY 4.0)

In the large intestine, methanogens are the main type of archaea we know about. Methanogens get their energy from hydrogen gas, which is produced by some bacteria. After stripping energy from hydrogen, they combine it with carbon dioxide to produce methane – a flammable gas. How intestinal Archaea influence our health has not been studied very well. Stay tuned for updates as new research reveals more.

Binary Fission

Binary fission is a simple form of cell division. It is simpler than mitosis because bacteria and archaea are simpler, lacking organelles, a nucleus, or multiple chromosomes. All they have is one circular ring of DNA. The steps of binary fission, therefore, are fairly simple (Figure 5-19):

DNA is replicated, making two copies of genetic instructions
Cell elongates moving the DNA (chromosomes are attached to the cell wall)
A new cell membrane and cell wall form, making two cells

Stop & Think #16: Compare and contrast mitosis and binary fission.

Since bacteria and archaea are single-celled organisms, dividing into two cells doubles the population size. Just like in mitosis, the daughter cells are clones of the original. In many species, the cells remain linked together and appear as a chain of cells, but each cell operates independently.

Process of binary fission in stages.

FIGURE 5-19. Binary fission. Bacteria and archaea divide using this method. ("Binary fission" by Ecoddington14 is licensed under CC BY-SA 3.0)

Learning objectives

Describe the identifying features of Archaea and Bacteria.
Describe the process of binary fission.
Explain how the rapid cell division of bacteria impact human health.

Stop & Think Questions Explained

14. If bacteria remained in the intestines, it would start to fill up the intestines and block food movement. They may also irritate the lining of the large intestine causing indigestion or diarrhea. There would probably be many other unpleasant side effects, as well.

15. With extra food, bacteria and archaea would reproduce. They would also eat more and this could produce additional gas and bloating.

16. Both duplicate DNA and duplicate cells, producing two identical cells. However, besides being faster, binary fission does not use microtubules, does not separate multiple chromosomes, and does not have to coordinate cell division with other cells.