Archaea and bacteria

The archaea and bacteria are prokaryotes, unicellular living whose genetic material is not contained in an intracellular compartment.

Archaea were initially considered bacteria, and in fact they were known as archaea. Thanks to the studies of Carl R. Woese and technological advances in genetic sequencing, archaea and bacteria were separated into different phylogenetic groups. Now living organisms are classified into three domains:

  • Bacteria Domain : where the bacteria are.
  • Archaea domain : where archaea are included.
  • Eukarya domain : where all eukaryotes (plants, fungi and animals) are included.
Carbon bond of lipidsEtherEster
Lipid phosphate columnGlycerol-1-phosphateGlycerol-3-phosphate
MetabolismSimilar to bacteriaBacterial
LocationExtensive, they are located in extreme environmentsExtensive
Transcription apparatusSimilar to eukaryotesBacterial
Nucleus and organellesAbsentAbsent
MetanogénesisHere I’mAbsent
Ribosomal RNA subunit16S16S
Cellular wallDoes not contain peptidoglycanContains peptidoglycan
SporesThey do not form sporesSome bacteria form spores
ExamplesHalobacteriumEscherichia coli


Archaea are microscopic organisms that were discovered just 130 years ago, although they were initially thought to be bacteria. Archaea constitute the third branch of the tree of life, between bacteria and eukaryotes.

The first fully sequenced genome of an archaea was that of Methanococcus jannaschii published in 1996.

Characteristics of archaea

Archaea have a similar structure to bacteria: circular DNA, plasma membrane, cell wall, cytoplasm, and ribosomes. However, the cell membrane of archaea is characterized by the incorporation of isoprenoid lipids with ether bonds attached to a glycerol-1-phosphate base.

They have information processing systems like bacteria and eukaryotes, that is, DNA replication, transcription and translation, although they are more like the latter.

They are microscopic in size and can be as small as 400-500 nanometers. They have shapes similar to bacteria: rounded (cocci), cylindrical (bacilli) and irregular shapes. In fact, the first square microorganism ( Haloquadratum walsbyi ) was an archa discovered in 1980 on the Sinai Peninsula.

Archaea do not photosynthesize and do not form spores . They produce methane from biological compounds through the process of methanogenesis.

Archaea are beings that can live in extreme environments : either very high or very low temperatures. That is why they are classified as Extremophiles. However, not all extremophilic organisms are archaea, nor are all archaea extremophilic.

So far no pathogenic archaea, that is, that causes disease in animals or plants, are known.

Dominio Archaea

The classification of archaea as a distinct domain arose from the studies of Carl Woese in the late 1960s using the ribosomal RNA sequence as a marker. Thus, these organisms form a separate domain from bacteria and eukaryotes, the Archaea domain , which in turn presents several main divisions or rows that grow as new specimens are studied.


Most are hyperthermophilic and thermoacidophilic . Thermoacidophils (including hyperthermophiles, which grow faster above 80ºC) colonize volcanic terrestrial environments and deep-sea hydrothermal vents. They can grow in the presence or absence of oxygen and be heterotrophic or autotrophic.

Examples of Crenarchaeota are Metallosphaera sedula (isolated from a volcano in Italy) and Thermoproteus neutrophilus ( found in hot springs).


A large number of families with varied habitats are grouped on this edge. For example, methanogens are found in anaerobic aquatic environments and in the gastrointestinal tract of animals, where they participate in the conversion of organic matter by utilizing the metabolic products of bacteria (for example CO 2 , hydrogen H 2 , acetate, and formate). and convert them to methane (CH 4 ).

On the other hand, haloarchaeas live in hypersaline environments (such as salt flats, lakes, and the Dead Sea) where they grow as heterotrophs, often in association with phototropic algae. The square archa Haloquadratum walsbyi is a representative halophilic.


Nanoarcheum equitans belongs to this group , the smallest archaea (400 nm) found so far. It was identified as small dots that grew next to another arches ( Ignicoccus hospitalis ).


This division was recognized in 2008 and its members are widely spread in medium-temperature marine environments. An example is Nitrosopumilus maritimus , found in a tropical marine tank at the Seattle Aquarium in Washington (USA).


Bacteria are prokaryotic unicellular microorganisms , that is, they do not have a nucleus defined by a nuclear membrane. It is widely distributed in the biosphere and they were the first ancestral life forms.

There are more bacterial cells in the human body than human cells. The bacteria that reside in the intestine are called the gastrointestinal microbiome and play a fundamental role in the individual’s state of health.

Of the great variety of known bacterial species, only a few are pathogenic for humans, the vast majority are harmless. Examples of pathogenic species are Haemophilus influenza (which can cause meningitis and pneumonia in children under five years of age) and Vibrio cholerae (which causes cholera).

Bacteria characteristics

Bacterial cells possess circular chromosomal DNA, plasmids, cell membrane, cytoplasm, ribosomes, and cell wall.

The bacterial cell wall contains peptidoglycans composed of polysaccharide chains that are interconnected with unusual peptides. It works as a protective layer and shapes the bacteria. The forms of bacteria are varied; They can be spherical, cylindrical, spiral, or comma-shaped.

Some bacteria have a capsule outside the cell wall. The capsule allows bacteria to adhere to surfaces, guarding against dehydration and against attack by phagocytic cells.

The plasmids are small DNA fragments are separated from the main DNA (chromosomal DNA) and can be transmitted between bacteria.

Some species have flagella that are used for locomotion and pili that are used to adhere to surfaces.

Dominio Bacteria

Bacteria are divided into two large groups according to their reaction to a staining technique: Gram-positive and Gram-negative. This stain was invented by Hans Christian Gram (1853-1938).

The Gram-positive bacteria have a cell wall composed of up to 90% of peptidoglycan and teichoic acids for the rest. Examples of Gram-positive bacteria are Staphylococcus aureus found on the skin.

The Gram-negative bacteria have a relatively thin cell wall with only 10% of peptidoglycan covered by an envelope made of external lipopolysaccharide and lipoprotein. Examples of Gram-negative bacteria are the meningococci Neisseria meningitidis , the causative agent of meningococcal meningitis.

The Bacteria domain (formerly called Eubacteria) represents the first branch of the tree of life division. This group presents a great variety of row of which we can mention:

  • Proteobacteria: Gram-negative organisms such as Escherichia coli and Salmonella sp .
  • Chlamydias: Gram-negative aerobic pathogens such as Chlamydia trachomatis and Chlamydia pneumoniae .
  • Spirochetes: bacteria with wavy shapes such as Spirochaeta halophila .
  • Cyanobacteria: bacteria that carry out photosynthesis.
  • Gram-positive bacteria: such as lactobacilli, which produce lactic acid and are used in making yogurt.

You may be interested in the kingdoms of nature .

Differences between archaea and bacteria

The main differences between archaea and bacteria are in the composition of the cell membrane and wall, metabolism and genetic machinery.

Composition of the plasma membrane

The cell membrane of archaea differs from bacteria in the type of phospholipids that make it up. Bacterial membrane phospholipids are made up of two linear chains of fatty acids, linked by ester bonds to a glycerol with a phosphate group on the third carbon. Two layers of these phospholipids make up the membrane. That is why it is called a lipid bilayer and is similar to the membrane structure of eukaryotes.

For their part, the phospholipids in the archaea membrane are made up of long (20 to 25 carbons) and branched chains of isoprenoids, which are joined at each end by ether bonds to a glycerol, which in this case has a phosphate group on the first carbon. This type of phospholipid forms a lipid monolayer.

Cellular wall

Unlike bacteria, the cell wall of archaea does not contain peptidoglycans and is made up of proteins, polysaccharides or glycoproteins. Some archaea have a pseudopeptidoglycan with different sugars in the polysaccharide.


One characteristic that distinguishes certain species of archaea from bacteria is their ability to generate methane from carbon dioxide and other organic compounds such as acetate and formate. Although archaea can generate their energy source from light, they do not carry out the photosynthesis process, as cyanobacteria do.

Genetic machinery

The processing of genetic information in archaea is more like eukaryotes than bacteria. While there is a replication origin site in bacterial DNA, archeal DNA has several replication initiation sites. The first amino acid in protein synthesis in bacteria is formyl-methionine, while in archaea it is methionine.

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