Bacteria and archaea are microbes consisting of only a single cell. They are similar in many aspects, including sizes, shapes, reproduction, and habitats. Both are much simpler than eukaryotic cells and are considered to be early forms of life on Earth. However, they also have so many differences that eventually separate them into two biological Domains. This article will show you what bacteria and archaea are and outline the similarities and differences between the two. 
[In this image] Tree of living organisms showing the origins of eukaryotes and prokaryotes. The three-domain system is a biological classification introduced by Carl Woese in 1990 that divides life forms into Archaea, Bacteria, and Eukarya domains.
This article covers
Definition: What are prokaryotes
Prokaryotes (pro-KAR-ee-ot-es) are unicellular organisms that don’t have a membrane-bound nucleus and other organelles. Both bacteria and archaea are prokaryotes.
Learn what are the differences between Eukaryotes and Prokaryotes below.
Definition: What are bacteria?
Bacteria (singular: Bacterium) are single-celled microbes with cell walls. Bacteria are found almost everywhere on Earth. Some live in or on other organisms, including plants and animals. The human body is full of bacteria; many of these bacterial cells are found living in the gut and are important for our health. However, some bacteria are pathogenic organisms that can cause illness and diseases.
[In this figure] Diagram showing the cell structure of a bacterium.
Learn more about bacteria by watching the video below.
Definition: What are archaea?
Archaea are another group of single-celled microbes. They were originally classified as ancient bacteria but later received their own Domain. Archaea are so different from the bacteria that they must have had a long, independent evolutionary history since close to the dawn of life.
Some archaea live in environmental conditions considered too extreme for all known life to survive. Some archaea survive high temperatures, often above 100 °C (212 °F), as found in geysers, hydrothermal vents, and oil wells. Others live in icy habitats, highly saline, acidic, or alkaline water. This unique capability gives them a name – extremophiles.
Learn more about archaea by reading the article below.
What do bacteria and archaea have in common?
1. Both bacteria and archaea are prokaryotes.
Bacterial and archaeal cells do not have a membrane-bound nucleus. Their genetic materials (DNA) are within the cytoplasm. They also lack other membrane-bound cell organelles, including mitochondria, chloroplasts, endoplasmic reticulum (ER), and Golgi apparatus. For this reason, all the biochemical reactions, as well as transcription/translation happen directly in the cytoplasm. Bacterial and archaeal cells can not grow big also due to this limitation.
2. Both bacteria and archaea are single cellular organisms.
Each bacterial and archaeal cell is an independent organism. They can’t form organized tissues or organs as multicellular organisms. For this reason, each bacterial and archaeal cell has to do everything a living organism needs – feeding and reproduction.
3. Both bacteria and archaea like to form a community.
Although bacteria and archaea are single cells, they tend to grow together into a high-density community of cells, called biofilm. Living in a biofilm gives individual cells many advantages. For example, the matrix of biofilm protects the cells with high resistance from the penetration of antibiotics.
4. Bacteria and archaea have similar sizes and shapes.
Bacterial and archaeal cells are pretty tiny, only 1-1.5 µm in length. Although there are some exceptions, some bacteria can grow up to 750 µm, while Nanoarchaeota could be as small as 0.4 µm.
Bacteria and archaea are both found as rods, cocci, spirals, plates, or coiled. They both have hair-like appendages such as flagella and pili on their cell surfaces. For this reason, it is challenged to distinguish bacteria and archaea by just looking at their morphology.
5. Both bacteria and archaea can be found everywhere on Earth.
Both bacteria and archaea are very successful in terms of their adaptability. You can find them everywhere. The ocean has the most abundant amounts of bacteria and archaea. They both have members who can survive under extreme conditions, including hot springs, deep-sea vents, saline pools, or icy oceans. We call these organisms – extremophiles. Bacteria and archaea are also abundant in our bodies.
6. Both bacteria and archaea can use diverse energy sources.
When the ancestor life forms evolved, Earth’s early environment may not be as friendly as it is today. Bacteria and archaea had to diversify their energy sources in order to expand their territory. In both groups, there are phototrophs that obtain energy from sunlight, chemotrophs that rely on energy from certain chemical reactions, and organotrophs which live by breaking down organic materials.
Key differences between bacteria and archaea
[In this image] Structural similarities and differences between bacterial and archaeal cells.
Although bacterial and archaeal cells are similar, there are several critical differences. These differences are substantial enough to warrant that archaea have a separate domain.
See the table below as a summary:
Table: differences and similarities between bacteria and archaeaCharacteristics Bacteria Archaea Cell type Prokaryotic Prokaryotic Cell morphology Sphere, rod, spiral Sphere, rod, spiral, plate Cell wall composition Made of peptidoglycan Made of protein; lack peptidoglycan Cell membrane composition Ester linked lipids with D-glycerol (straight chain) Ether linked lipids with L-glycerol (branch chain) Ribosome 70S ribosomes 70S ribosomes Flagella Yes Archaeal flagella, also termed archaella Pili Yes Yes Chromosomal DNA long, circular form long, circular form Plasmid Yes Yes Unique structures Capsule Cannulae and Hamus Reproduction Fission, budding, and fragmentation; forming endospores Fission, budding, and fragmentation; no endospore Pathogenicity Some are pathogens No Photosynthesis Some species can do, like cyanobacteria Some species are phototrophs, but do not perform oxygen-generating photosynthesis Glucose oxidation Glycolysis and Kreb’s cycle Do not use glycolysis or Kreb’s cycle Habitats Many habitats ranging from soil, water to inside living organisms. A small number are extremophiles. Many are extremophiles and are found in extreme environments like the deep sea, hot springs, salt brine, etc. Also abundant in the oceans. Examples Escherichia coli (E. coli)
Staphylococcus aureus
Bacillus subtilis
Pseudomonas aeruginosa
Salmonella Typhi
Lactobacillus brevisSulfolobus tokodaii Haloquadratum walsbyi Nanoarchaeum equitans Methanopyrus kandleri Pyrococcus furiosus Pyrodictium abyssi
Cell wall
Cell wall is a protective layer that surrounds cells and gives them shape and rigidity. The chemical composition of the cell walls varies between archaea and bacteria.
Bacterial cell walls contain peptidoglycan, composed of polysaccharide chains and peptides. Cell walls are thinner in Gram-negative bacteria than Gram-positive bacteria. Gram-positive bacteria also have lipoteichoic acids which anchor the cell wall to the cell membrane. Some pathogenic bacteria have an extra thick layer of sugars outside the cell wall, which is called the capsule.
Archaeal cell walls do not have peptidoglycan. The protection is provided by surface-layer proteins, called S-layer. Archaea do not have the capsule.
[In this image] Cell walls and membranes of Gram-positive or negative bacteria.
Plasma membrane
Archaeal plasma membranes are made of molecules that are different from those in all other life forms.
In most organisms (bacteria and eukaryote), cell membranes are made of molecules known as phospholipids, which have a phosphate “head” and a lipid “tail”. The lipid (fatty acid) in the membranes of other organisms have straight carbon chains. However, archaeal lipid tails are long isoprenoid chains with multiple side branches and rings. These branched chains may prevent archaeal membranes from leaking at high temperatures.
[In this image] Comparison of plasma membrane lipid between Bacteria, Eukarya, and Archaea.
Image source: openstax
Another unique feature of the archaeal membrane is that it is composed of glycerol-ether lipids, whereas bacteria and eukaryotes have membranes composed mainly of glycerol-ester lipids.
Flagella and Pili
Both bacteria and archaea have hair-like structures on their cell surface. The long one is called the flagellum (plural: flagella), while the short one is pilus (plural: pili).
Archaeal flagella are known as archaella (singular: archaellum), which operate like bacterial flagella and use to propel cells by rotation.
[In this image] A bacterium with three long flagella and many pili covered its cell. This is a 3D illustration of Escherichia coli, not a real picture.
Photo source: Microbiology Society
Cannulae and Hamus
Cannulae and Hamus are two structures unique to archaea. Both are tube-like appendages that can help archaeal cells stay together and attach to a surface. Learn more about Archaea by reading the article – “Archaea – Definition, Structure, Types & Extremophile Habitats”.
[In this image] Pyrodictium abyssi, disk-shaped archaeal cells, within a web of matrix made up of hollow tubules, the cannulae (Latin: “little reeds”). These cannulae can withstand temperatures up to 135°C.
Image source: Semantic scholar
[In this image] Electron micrograph of Altiarchaeales biofilm (a mass of bacteria or archaea). Cells connect to each other by tube-like appendages called hami. (A) overview, (B) closer view.
Image source: Microbe Wiki
Archaea are closer to eukarya than bacteria
Surprisingly, scientists found that compared to bacteria, archaea in many aspects are closer to eukaryotic cells! You can see the comparison in the table below:
Table: distinct characteristics between three domainsCharacteristics Bacteria Archaea Eukarya Cell type Prokaryotic Prokaryotic Eukaryotic Nucleus No No Yes Organelles No No Yes Metabolism Bacterial Bacterial-like Eukaryotic Chromosomal DNA Circular Circular Linear Telomers No No Yes Introns No intron Some introns Most genes have introns Histones No Have proteins similar to histones Have histones DNA replication Bacterial Eukaryotic-like Eukaryotic Transcription Bacterial Eukaryotic-like Eukaryotic Transcription start site Diverse types Similar to TATA box TATA box RNA polymerase numbers One Several Several Poly A addition No Yes Yes Translation Bacterial Eukaryotic-like Eukaryotic Ribosome structure 30S, 50S 30S, 50S (but look like eukaryote) 40S, 60S First amino acid in protein synthesis Formyl-methionine Methionine Methionine Binding of ribosome SD sequence mRNA cap mRNA cap Cell wall composition Made of peptidoglycan Made of protein; lack peptidoglycan Not always present Plants: Cellulose Fungi: Chitin Cell membrane composition Ester linked lipids with D-glycerol (straight chain) Ether linked lipids with L-glycerol (branch chain) Ester linked lipids with protein (straight chain)
In brief, archaea are structurally closer to bacteria, but closer to Eukarya in molecular machinery. In several aspects related to DNA replication, transcription (DNA to mRNA), and translation (mRNA to protein), archaea are similar to eukaryotic cells. For this reason, some people believe archaea might be the common ancestor of all eukaryotic organisms.
Summary – bacteria vs. archaea
1. Bacteria and archaea are two groups of microbes that belong to Domain Bacteria and Doman Archaea, respectively.
2. However, archaea and bacteria share many similarities as well. They are prokaryotic, single-celled microorganisms with cell walls. Both form biofilms and live everywhere on Earth by adapting to use diverse sources of energy.
3. Archaea don’t have peptidoglycan in their cell walls while bacteria do.
4. Archaea have unique cell membranes with branched lipid molecules.
5. In terms of DNA replication, transcription, and translation, archaea are more similar to Eukarya, unlike bacteria.
6. Some pathogenic bacteria could cause diseases. No archaea are found to be pathogens.
Q&A: Some frequently asked questions are quickly answered here
List 5 differences between archaea and bacteria.
1. Cell wall – Archaeal cell walls don’t contain peptidoglycan.
2. Cell membrane – Archaea have unique cell membrane with branched lipid molecules.
3. rRNA sequence – Scientists use ribosomal RNA sequences to define the Domains of Bacteria and Archaea.
4. Gene, transcription, and translation – Archaeal genes and the process of transcription/translation are more similar to eukaryotic cells, unlike bacteria.
5. Habitats – More archaea can survive in extreme environments such as hot springs, deep-sea vents, and salty pools.
What came first, archaea or bacteria?
Archaea are possibly the most ancinet form of life. Since bacteria and archaea do not have distinct morphologies, the shape of fossils cannot be used to identify them as bacteria or archaea. However, the lipid composition is very different between bacteria and archaea. Therefore, the chemical fossils (metasedimentary rocks), the oldest known traces of these isoprene lipids which belongs to archaea have been found in Greenland, dated 3.8 billion years old.
In contrast, the oldest cyanobacteria-like fossils known are nearly 3.5 billion years old. So according to the fossil record, it seems that archaea comes first. But scientists make new discovery every day, you never know when they will discover the bacteria fossil order than archaea’s.
Reference: Allwood, A. Evidence of life in Earth’s oldest rocks. Nature 537, 500–501 (2016).
Do bacteria and archaea have a nucleus?
No, they don’t have membrane-bound nuclei and belong to prokaryotes.
Do bacteria and archaea have cell organelles?
They don’t have membrane-bound organelles such as mitochondria, chloroplasts, endoplasmic reticulum (ER), and Golgi apparatus. However, they have cell walls, cell membrane, cytoplasm, ribosomes, flagella, and pili.
Do bacteria and archaea have ribosomes?
Yes, they have ribosomes of the same size. But structurally, archaeal ribosomes are closer to Eukarya’s ribosomes.
Do bacteria and archaea have mitochondria?
No, they don’t have mitochondria. They generate energy in the cytoplasm and across the cell membrane.
Are bacteria and archaea harmful?
Some pathogenic bacteria could cause diseases. No archaea are found to be pathogens. Many bacteria and archaea are beneficial to our health.