Bacteria and archaea
Under good laboratory conditions one E. coli cell can become two in about 20 minutes.
Three common shapes
Bacteria are often described by shape. Round ones are cocci (one is a coccus). Rod-shaped ones are bacilli. Curved ones go by several names: a comma-shaped curve is a vibrio, a rigid spiral is a spirillum, and a flexible corkscrew is a spirochete. Shape alone does not tell you the species, but it is one of the first things a microbiologist notes under the microscope.
What is inside a bacterium
A bacterial cell has no nucleus. Its DNA sits in a region called the nucleoid, and most bacteria have a single circular chromosome, though there are exceptions with linear ones. The cytoplasm is full of ribosomes that build proteins. Around it is a plasma membrane, and around that most bacteria have a cell wall containing peptidoglycan, a mesh of sugars and short protein chains. Some bacteria also have a slimy capsule outside the wall, whip-like flagella for swimming, and short hair-like pili for sticking to surfaces.
Binary fission
Most bacteria reproduce by binary fission, a simple division into two. First the cell grows and makes more of everything inside it. Copying of the chromosome starts at one spot, the origin of replication, and runs in both directions around the circle until the two copies are complete. A protein called FtsZ then gathers into a ring, the Z ring, around the middle of the cell. More proteins join it, a new wall and membrane grow inward as a dividing septum, and the cell pinches into two daughter cells, each with a full copy of the genome. Reproduction in prokaryotes is always asexual, although bacteria do swap genes by other routes.
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Binary fission is not the only way. Some cyanobacteria build up many nucleoids along a filament and then break apart into many new cells at once, which is called fragmentation. Other bacteria grow a narrow extension from one end whose tip swells into a smaller cell, a bud. The giant bacterium Epulopiscium grows several daughter cells inside the parent, which then breaks open and releases them.
Bacteria matter far beyond the laboratory. Cyanobacteria carry out the same oxygen-releasing photosynthesis as plants and algae. Rhizobia living in the root nodules of peas and beans turn nitrogen gas from the air into ammonia that the plant can build into proteins. Countless others break down dead material in soil and water. A few cause disease, which is why the next units look at how fast they grow and how scientists learned to recognize them.
Archaea
Archaea look like bacteria under a microscope, single cells without a nucleus, but they have a separate evolutionary history and differ in their genes, chemistry, cell walls and membranes. Their walls do not contain peptidoglycan; some methanogenic archaea have a similar substance called pseudopeptidoglycan instead. Archaea live in almost every habitat, including places that are very hot, very cold, very acidic or very basic. The coloured rings of Morning Glory pool, a hot spring in Yellowstone National Park, come from different microbial communities living at different water temperatures.
| Bacteria | Archaea | |
|---|---|---|
| Nucleus | No | No |
| Peptidoglycan | Most have it | No |
| Typical size | About 1 µm | Similar |
| Extreme habitats | Some | Many |
- Shapes: cocci (round), bacilli (rods), curved forms.
- No nucleus; DNA in the nucleoid; peptidoglycan wall in most bacteria.
- Binary fission: DNA copied, FtsZ ring, septum, two cells.
- Archaea: no peptidoglycan, many in extreme habitats.
| Coccus | Round bacterium. |
| Bacillus | Rod-shaped bacterium. |
| Nucleoid | Region of a prokaryotic cell holding the DNA. |
| Peptidoglycan | Mesh of sugars and short peptides in most bacterial walls. |
| Binary fission | Division of one cell into two equal cells. |
| FtsZ | Protein that forms the Z ring where the cell divides. |
- A rod-shaped bacterium is called…
- a bacillus
- a coccus
- a spirillum
- a vibrio
- Where is the DNA of a bacterium?
- In the nucleoid
- In a nucleus
- In the cell wall
- In the flagellum
- What substance is in the cell walls of most bacteria but not archaea?
- Peptidoglycan
- Cellulose
- Chitin
- Keratin
- Which protein forms the ring where a dividing bacterium pinches in two?
- FtsZ
- RuBisCO
- Hemoglobin
- Actin from muscle
- Put the steps of binary fission in order.
- A cell divides every 20 minutes. How many generations is 3 hours?
- Starting from one cell, how many cells are there after those 9 generations?
- Cyanobacteria that break a filament into many new cells at once reproduce by…
- fragmentation
- budding
- binary fission
- mitosis
- Where are archaea found?
- Almost everywhere, including hot springs and very acidic places
- Only in the human gut
- Only in the deep sea
- Only on plant leaves
- Prokaryotic reproduction is always…
- asexual
- sexual
- by spores only
- by meiosis
Sources for this course (15)
- OpenStax Microbiology 1.1 What Our Ancestors Knew (fermented drink from rice, honey and fruit in Neolithic China by 7000 BC, Saccharomyces cerevisiae in bread, Leeuwenhoek 1675 "animalcules" in rain water, Golden Age of Microbiology 1857 to 1914; openstax.org/books/microbiology/pages/1-1-what-our-ancestors-knew)
- 1.3 Types of Microorganisms (about 100 µm to see with the eye, animal cell about 10 µm, bacteria about 1 µm, viruses about 100 nm, three domains, coccus/bacillus/spirillum, archaea without peptidoglycan and in extreme places such as Morning Glory pool, protists, algae, diatoms 2 to 200 µm, protozoa with cilia, flagella or pseudopods, fungi with chitin walls, more than 1000 yeast species, molds and penicillin, helminths, prions about one-tenth of a virus; .../1-3-types-of-microorganisms)
- 3.1 Spontaneous Generation (Aristotle, van Helmont's mice recipe of 3 weeks, Redi 1668 with six jars, Needham 1745, Spallanzani, Pasteur 1858 gun-cotton filter, swan-neck flasks, Alhumbert Prize 1862, 1864 lecture; .../3-1-spontaneous-generation)
- 3.2 Foundations of Modern Cell Theory (Semmelweis 1847, 10 to 20 percent vs 1 percent, chlorinated lime; Snow; Pasteur 1856 fermentation; Lister 1867 carbolic acid; Koch's postulates; .../3-2-foundations-of-modern-cell-theory)
- 6.1 Viruses (acellular, obligate intracellular parasites, DNA or RNA never both, capsid, envelope, spikes, helical/polyhedral/complex, 20 nm to 900 nm, Ivanovski 1892 with the Chamberland filter of 1884, pore 0.1 µm, Beijerinck 1898 and virus = poison, Stanley 1935, host range, bacteriophages, FDA 2006 phage spray on lunch meats; .../6-1-viruses)
- 9.1 How Microbes Grow (binary fission, FtsZ Z ring, generation time, E. coli 20 minutes, human generation 25 years, M. tuberculosis 15 to 20 hours, M. leprae 14 days, 2^48 = 281,474,976,710,656 after 24 h at 30 minutes, Nn = N0 2^n, lag/log/stationary/death, budding, fragmentation, Epulopiscium; .../9-1-how-microbes-grow)
- Royal Society: Philosophical Transactions 12:821-831, published 25 March 1677, Leeuwenhoek's Dutch letter of 9 October 1676 "concerning little animals" (doi.org/10.1098/rstl.1677.0003)
- Royal Society archive EL/L1/22 (letter to Oldenburg dated at Delft, 18 pages, partial translation by Oldenburg)
- Royal Society blog "Celebrating Leeuwenhoek", 2023 (first letter via de Graaf, Phil. Trans. 19 May 1673; 190 letters in fifty years; died 26 August 1723 aged 90)
- N. Lane 2015, Phil. Trans. R. Soc. B 370:20140344 (upwards of 1,000,000 creatures in a drop of pepper-water; letter read aloud over several sessions)
- Lens on Leeuwenhoek, letter L-040 (Oldenburg printed less than half; Hooke succeeded later in 1677)
- CDC MMWR 53(34):783, 2004, 150th Anniversary of John Snow and the Pump Handle (31 August 1854, handle removed 8 September 1854, Vibrio cholerae isolated 1883; cdc.gov/mmwr/preview/mmwrhtml/mm5334a1.htm)
- CDC History of Smallpox and Smallpox timeline (Jenner 1796; 33rd World Health Assembly declared the world free of smallpox on 8 May 1980; cdc.gov/smallpox/about/history.html)
- American Chemical Society, Discovery and Development of Penicillin landmark (Fleming 1928 at St Mary's Hospital London, paper June 1929, Florey and Chain at Oxford from 1939; acs.org/education/whatischemistry/landmarks/flemingpenicillin.html)
- Fleming 1929, Br J Exp Pathol 10:226. Sender, Fuchs and Milo 2016, PLoS Biol 14:e1002533 (3.8 × 10^13 bacteria and 3.0 × 10^13 human cells in a 70 kg reference man, ratio about 1.3, bacteria about 0.2 kg).
Checked: 2026-10-17