Bacteria: Definition, Types & Examples

••• Dana Chen | Sciencing

Bacteria are the most abundant living organisms on the planet as well as some of the most ancient life forms known. The simplicity and tiny dimensions of bacteria in some ways mask the resilience, antiquity and ubiquity of these life forms.

TL;DR (Too Long; Didn't Read)

Bacteria are single-celled organisms, and they represent one of two domains within the taxonomic category known as prokaryotes. The other is Archaea, which can survive some of Earth's more extreme environmental conditions.

The word "prokaryote" comes from the Greek for "before nucleus," which highlights the main difference between prokaryotes and their more recently emergent counterparts in the biosphere, eukaryotes ("good nucleus").

In short, prokaryotes are single-celled organisms with an anucleate cell, while eukaryotes are multicellular organisms with nucleated cells; rare exceptions exist in both categories.

Why Are Bacteria Important?

Bacteria are active in virtually every known ecosystem on the planet (an ecosystem is a collection of organisms interacting in a common physical environment).

While their primary notoriety lies in their capacity to cause a swath of infectious diseases, many of them potentially fatal, many bacteria actually play beneficial roles in the lives of humans and other eukaryotes.

When two different kinds of organisms live together in ways that are beneficial to both, this is called symbiosis. (This can be contrasted with parasitism, where one of the two organism benefits to the detriment of the other, e.g., tapeworms living in the intestines of mammals and causing human health problems in the process.)

Symbiosis: Examples

One example of bacteria-human symbiosis is the manufacture by a particular species of bacteria of vitamin K, an essential molecule in the clotting of blood.

Other bacteria live symbiotically on human skin and elsewhere in the body, and they can help destroy disease-causing cells as well as aid in the digestive system.

In addition, the culinary landscape would be markedly different without bacteria in the mix. Without them, the world would not have cheese, yogurt and other foods that rely on the controlled and monitored activities of these micro-organisms for their manufacture.

Pathogenic Bacteria

Less than one percent of known bacteria are capable of causing illness in humans.

Bacterial infections, however, remain one of the greatest causes of death and disease worldwide, particularly in areas with poor sanitation, high population density and limited access to the right antibiotics to fight bacteria – public-health issues that, unfortunately, are often found in combination.

Some of the more common types of bacteria that are pathogenic, or disease-causing, in humans are some of the Streptococci and Staphylococci as well as E. coli.

Streptococcus and Staphylococcus are genus names, and each category includes a variety of pathogenic species. E. coli, short for Escherichia coli, is a specific kind of bacteria, so the genus and species name are both included, just as Homo sapiens to refer to modern humans.

Across the taxonomic world, the genus name is always capitalized, while the species name never is.

Nutrient Recycling

Bacteria also contribute positively to the global ecosystem by participating in nutrient recycling (e.g., the carbon cycle, the nitrogen cycle).

These processes return important carbon- and nitrogen-containing molecules that have passed from the top of the so-called food chain to the bacteria at the bottom to the system, making them available for new plant and animal growth; when these organisms die, their carbon and nitrogen atoms find their way back into soil and water, often after bacteria have acted to decompose their remains and extract energy for their own growth.

The History of Bacteria

Bacteria have existed on Earth for about 3.5 billion years, meaning that they have been around for around three-fourths as long as the Earth itself.

(Consider that dinosaurs are believed to have gone extinct about 65 million years ago; this is less than one-fiftieth as deep into geological history as the appearance of bacteria is.)

Their prokaryotic relatives, the archaea, have been present for even longer. You may see the terms capitalized; Archaea and Bacteria are also the names of the taxonomic domains encompassing these organisms.

The "archaeans," if nothing else, do not have to compete with resources with other organisms, for they inhabit only the most adverse environments imaginable: boiling hot or extremely acidic water, extremely saline (salty) pools, sulfur-heavy volcanic openings and deep inside Antarctic ice.

The split of bacteria and archaea is believed to have occurred about 4 billion years ago.

Although it is easy to see bacteria and archaea as close cousins, at the biochemical and genetic level, these two groups of organisms are as different from each other as either is from human beings.

Prokaryotes Before Eukaryotes

Eukaryotes first emerged millions of years after the first bacteria, and their emergence is presumed to be the result of one type of prokaryote engulfing another in a way that "worked out" over time; imagine an AirBnB stay turning into a permanent roommate situation.

Specifically, the organelles inside eukaryotic cells called mitochondria, which are responsible for aerobic metabolism and thus the comparatively massive sizes eukaryotes can reach owing to their reliance on oxygen (aerobic means "with oxygen"), are thought to have once been free-standing bacteria in their own right.

No one person is uniquely credited with the discovery of bacteria, but the 17th-century Dutch scientist Antony von Leeuwenhoek is credited with being the first to use a microscope to conduct extensive studies of these organisms.

Not until the 1800s did scientists, among them Robert Koch and Louis Pasteur, learn that bacteria could cause disease in people, and it was not until shortly before World War II toward the end of the first half of the 20th century that medical scientists identified and began to make use of antibiotics, which are natural or synthetic chemicals that can stop the reproduction of bacteria in its tracks, with or without killing the organisms outright.

Structure of a Bacterial Cell

Just as animals can take on a dizzying array of physical forms from one species to the next, different types of bacteria span a variety of shapes and sizes, as described in the following section.

Just as all eukaryotic cells have certain features in common, however, many attributes of bacteria are universal.

Perhaps the most important independent structure of a bacterium is the cell wall. (Note that "only" about 90 percent of bacteria actually possess this feature.)

Apart from their function and chemical make-up, the cell wall, which is external to the cell membrane that all cells have, is used to divide bacteria on the basis of the wall's response to a laboratory procedure called the Gram stain.

So called gram-positive (G+) bacteria, which retain most of the dye used in the staining process, have walls that display a purplish color when stained, whereas gram-negative (G-) bacteria, which release most of the dye, appear pink. (Traditionally, "gram-positive" and "gram-negative" are not capitalized despite the root word being a proper noun.)

Both G+ and G- bacterial cell walls contain substances called peptidoglycans that are found nowhere else in nature.

Cell Wall Specifics

About 90 percent of G+ cell walls are made of peptidoglycans, with the remainder consisting of teichoic acid.

In contrast, only about 10 percent of the walls of G- bacterial cells consist of peptidoglycans. G- bacteria also include a plasma membrane on the outside of the cell wall to complement the primary cell membrane beneath it.

Together, the cell wall and the one or two cell membranes of a bacterium make up what is collectively termed the cell envelope.

The genetic information of bacteria is contained in deoxyribonucleic acid (DNA), just as in eukaryotes. Bacterial cells, however, lack nuclei, which is where DNA is found in eukaryotes, so bacterial DNA is found in the cytoplasm (the substance of the cell within the cell membrane) in a loose arrangement of strands called the nucleoid.

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Other Bacterial Cell Elements

External to the cell wall and projecting to the outside environment are various structures that participate in moving the bacteria about and exchanging genetic information with other bacteria.

A flagellum is a whip-like projection that operates much like a propeller on a boat, and it consists of a filament, a hook and a motor, all of which are made of different proteins.

A pilum (plural pili) is a smaller, hairlike projection that may play a small role in locomotion, but it is most often used to attach the bacteria to the surfaces other cells. When this other cell is itself a bacterium, the result can be conjugation, or moving DNA from one bacterial cell to the next.

Ribosomes, which are also present in eukaryotes, are the sites of protein synthesis within cells.

Found scattered in the cytoplasm, these structures use information coded via DNA into messenger ribonucleic acid (mRNA) to build specific proteins from amino acid subunits shuttled to the ribosomes by other proteins.

The Different Types of Bacteria

In addition to dividing bacteria into categories on the basis of their aforementioned cell-wall-staining behavior, bacteria can be distinguished on the basis of their shapes.

There are three basic forms:

  1. Cocci (singular: coccus), which are roughly spherical
  2. Bacilli (bacillus), which are rod-shaped
  3. S_pirilla_ (spirillum), which are twisted into a spiral shape.  

Cocci are often found in colonies.

Diplococci are cocci arranged in pairs; streptococci are found in chains. Staphylococci exist in irregular, grapelike clusters. Bacilli are larger than cocci, and when they divide, the result can be a chain (streptobacilli) or a globular cluster (coccobacilli).

Finally, the spirilla come in three flavors of their own: the vibrio, which is a curved rod, shaped like a comma; the spirochete, a thin and flexible spiral; and the "typical" spirillum, which forms a rigid spiral.

How Bacteria Reproduce

Bacteria reproduce by a process called binary fission, which results in the formation of two daughter bacteria, each virtually identical to the "parent" bacterium in composition and equal to one another in size.

This is an asexual form of reproduction, and it is similar to the mitosis seen in eukaryotic cells.

Mitosis, however, refers strictly to the replication of a cell's genetic material, or DNA. While this occurs almost in concert with the division of whole eukaryotic cells, the cleavage of one eukaryotic cell into two is called cytokinesis.

Recall that the DNA of a bacterium is not packaged into a nucleus, but rather sits in the cytoplasm in a set of loosely organized strands.

In preparation for binary fission, the entire bacterial cell elongates in a coordinated way, with both the cell wall and the cytoplasm becoming more extensive. As this is happening, the cell begins to make a complete new copy of its DNA (replication).

Division Occurs

The "line" along which the bacterium will divide, called a septum, forms in the center of the cell; the synthesis of the septum relies on a protein called FtsZ.

At first, the septum looks like a ring, but then it pushes its way toward opposite sides of the cell, ultimately leading to cleavage and the formation of two daughter bacteria.

Because binary fission results in the formation of two whole, functional organisms, the generation times of bacteria, which are often given in hours, are usually much shorter than those of eukaryotic organisms, which are typically measured in months or years.

Related Topic: Antibiotic Resistance

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