Bacteria – Concept, types, structure and examples

We explain what bacteria are, the types that exist and what their structure is like. Also, some examples and their differences with the virus.

Bacteria are the most primitive and abundant living beings on planet Earth.

What are Bacteria

Bacteria are a large group of prokaryotic microorganisms (devoid of cell nuclei) of various possible shapes and sizes. Although formerly the term “bacteria” grouped all prokaryotic organisms, today the taxonomy divides them into two categories: the Bacteria domain and the Archea domain. Both are grouped in the Prokaryota superkingdom, made up of all prokaryotic organisms, which constitute the most primitive and abundant living beings on planet Earth, adapted to practically all conditions and habitats. Some bacteria can even survive in harsh conditions, such as outer space.

Modern prokaryotes, which include all bacteria, are immediate descendants of the first unicellular life forms on the planet, which arose under conditions very different from those of today about 4 billion years ago.

Bacteria have been involved, perhaps due to their abundance, in most cellular evolutionary jumps, for example, it is believed that, through endosymbiosis processes, they influenced the origin of mitochondria (organelles present in all eukaryotic cells) or chloroplasts (unique organelles of plant cells).

These living beings have relationships with virtually all life forms on the planet, either through commensalism relationships (such as bacteria that proliferate on the skin), mutualism (such as those that collaborate with the decomposition of food in the intestine) or parasitism (such as those that cause infections and diseases).

Bacterial life is essential in the decomposition processes of organic matter, necessary for the recycling of elements such as carbon or nitrogen, and they constitute the floor of microscopic trophic chains in various environments.

The bacteria reproduce rapidly and by asexual procedures, which consist of the replication of the progenitor cell in two exactly equal to it (binary fission). It is estimated that, in a conducive environment, a bacterium is capable of dividing in just 15-20 or 20-30 minutes, depending on the species.

Types of bacteria

Coconut bacteria
Cocci bacteria are spherical or round in shape.

Bacteria are studied by bacteriology, a branch of microbiology. This discipline has classified them according to various criteria, such as their morphology, their metabolism or the characteristics of their cell wall.

  • According to its morphology:
    • Bacilli. Elongated in shape, like microscopic rods.
    • Coconuts. Spherical or round shapes. Coconut-like bacteria can also occur in pairs (diplococci), in groups of four (tetracocci), in chains (streptococci), and in irregular clusters or clusters (staphylococci).
    • Helical shapes. They may be: vibrios, comma-shaped and slightly curved; spirits, rigid helical or corkscrew; or spirochetes, in the form of a flexible corkscrew.

It is common among bacteria of the same species that they adopt different morphological types, which is called pleomorphism.

  • According to the composition of its cell wall:
    • Gram positive. They acquire a purplish or bluish color when the dye is used, due to the presence of a thickened cell wall.
    • Gram negative. They turn pink or red when the dye is used, due to the presence of a thin cell wall.
  • According to its nutrition:
    • Photoautotrophs. They use sunlight as a source of energy and inorganic substances (mainly CO2) as a source of carbon.
    • Chemoautotrophs. They use reduced inorganic compounds as an energy source and carbon dioxide as a carbon source.
    • Photoheterotrophs. They use light as a source of energy and organic molecules as a source of carbon.
    • Chemoheterotrophs. They use organic molecules as a carbon source, which they also use as a reagent in reactions to obtain energy.

There are other classifications of bacteria, which take into account the habitat or its biochemical components.

Structure of bacteria

Pilis are structures involved in the exchange of genetic material between bacteria.

The single-celled bacterial structure is usually quite simple. Bacteria are made up of a single cell without cell nucleus and almost no defined organelles, but with a nucleoid (irregular region where the circular DNA of prokaryotes is found) and a peptidoglycan cell wall that covers the cell outside the plasma membrane. In addition, they frequently have pili (structures involved in the exchange of genetic material between bacteria) or flagella to move (in the case that they are mobile). Some bacteria also have capsules, a rigid protective structure that is outside the cell wall.

Scattered in the bacterial cytoplasm are ribosomes (in which protein synthesis takes place) and there are also usually plasmids (small non-chromosomal DNA molecules) and small vacuoles (which function as reservoirs for reserve substances). Some bacteria have prokaryotic compartments, primitive organelles surrounded by membranes, destined for specific biochemical tasks within the cell, depending on its metabolism.

Examples of bacteria

Escherichia coli
The escherichia coli it is common in the intestines of warm-blooded living beings.

Bacteria are the most abundant organisms on the planet and are extremely diverse. Throughout evolution they have managed to adapt to all types of environments and that is why they are found in all terrestrial and aquatic habitats, even in the most extreme ones, such as sources of acidic waters and the depths of the ocean.

It is very common to think of bacteria as pathogenic organisms, capable of causing infectious diseases. While some of them are harmful, there are many others that are harmless or even beneficial. For instance:

  • Escherichia coli. It is a gram negative bacterium common in the gastrointestinal tracts of humans and other warm-blooded animals. Some strains of this bacterium are capable at certain times of causing an infection.
  • Neisseria gonorrhoeae. It is a gonococcus that causes gonorrhea, a sexually transmitted infection in humans.
  • Bacillus anthracis. It is an immobile, gram-positive bacterium that produces recognizable black lesions on the skin (carbuncles).
  • Sorangium cellulosum. It is a gram negative myxobacteria that is extremely frequent in soils and has a harmless metabolism.
  • Clostridium botulinum. It is a causal agent of botulism, through a neurotoxin secreted by these bacteria, whose growth is known in canned food (inflated cans that release gas when opened are a clear symptom) and other canned foods.
  • Lactobacillus acidophilus. It is a lactic acid bacterium, a mutualistic inhabitant of the human intestine and other mammals. As a result of its own metabolism, this bacterium provides different benefits as it aids in digestion, increases the bioavailability of nutrients and helps keep the digestive tract free of pathogenic microorganisms.
  • Lactobacillus acidophilus. It is a genus of bacteria that are symbiotic residents of the human digestive tract. Contributes to the production of vitamin K, vitamin B12, folate and biotin.

Differences between viruses and bacteria

Viruses and bacteria they are extremely different, even though they are the best known and most frequent infectious forms for humans.

The main difference has to do with their structure and size: while bacteria are single-celled organisms whose size ranges between 0.5 and 5 micrometers in length, Viruses are acellular beings much simpler and elementary, unable to reproduce except by infecting other cells that function as viral replica factories, after being inoculated with the invading viral DNA.

Currently, the scientific community does not quite agree on whether viruses are really alive, because of how primitive its existence is, which is not much more than a simple DNA or RNA molecule wrapped in a protein layer. For this reason antibiotics do not work on viruses but on bacteria; while antivirals or retrovirals are exclusively used to fight virus infections.