Pictures of Bacteria:                                    

Characteristics of Bacteria:                            

Bacteria are one of the three major lineages of life, being the most important group for sustaining life on Earth, although they go completely unnoticed by people. This is due to their small size, as they usually measure no more than one-thousandth of a millimeter, so even though they are omnipresent, they are never seen. Even when using a microscope, they are usually observed as tiny translucent dots. These unicellular organisms have a thick wall surrounding their cells, giving them a defined shape, and usually their morphology is simple, with spheres or rods being common. Sometimes they may have filamentous protein structures in their wall, which serve to anchor them to certain substrates, allow them to attack cells if they are pathogenic, move, or swim if those structures have movement (flagella). Their morphology is very limited, but the diversity of this group is extraordinarily high, much higher than any other domain of life. While the Eukaryota domain has a single type of metabolism highly conserved in all its species, bacteria have a very high metabolic diversity, being able to obtain energy from organic matter, light, methane, sulfides, nitrates, iron, sulfuric acid, arsenic, or invisible infrared light, among many other methods. They can inhabit places as varied as ocean floors at depths of more than 11 km, Antarctic ice, the interior of nuclear power plants, hyperacid volcanic lakes, waters at over 120°C, or the interior of rocks buried kilometers deep.

Bacteria are essential organisms for higher life on earth, as they control the cycles of the chemical elements essential for building the bodies of organisms. For example, the nitrogen present in all organisms, which is essential for forming proteins, was at some point taken from the atmosphere by a bacterium that transformed it into molecules usable for forming proteins (such as nitrate for plants or amino acids for animals). In addition, a group of bacteria (Cyanobacteria) was the origin of the photosynthetic capacity of all photosynthetic eukaryotes, by living in symbiosis inside their cells forming chloroplasts. Moreover, the ability to obtain energy by breathing oxygen possessed by eukaryotes like us, essential for life, is thanks to another bacterium that, at the origin of eukaryotes, began to live inside their cells, forming mitochondria. Thus, we can say that the metabolism of eukaryotes is nothing more than a mixture of bacterial metabolisms.

The species category in bacteria is different from that of organisms such as animals or plants; for example, the same species of bacteria can have individuals genetically as different as the difference between a person and a fly. This is because bacteria are capable of exchanging large fragments of DNA between individuals of the same species or of another species, consequently the genetic variability within a species is very high. Two individuals can be considered to belong to the same species even if they only share 30% of their DNA, i.e., which is for example the difference there is between a person and an alga. Thanks to this ability to exchange DNA, an individual can live breathing nitrate inside our intestines and suddenly have the ability to infect us and cause our death, as may be the case with the species Escherichia coli. This capacity makes the evolutionary history of bacteria appear not as a phylogenetic tree but rather as a network, in which DNA fragments pass from one organism to another or even very different organisms merge to give rise to other organisms with new properties.

There is still much to be known among the numerous and very diverse kingdoms of bacteria, which currently have a changing taxonomy due to the new species discovered and new studies conducted. The following phylogenetic tree shows the evolutionary relationships of some of the major lineages of bacteria.