What is bacteria? what are bacteria?


What is bacteria? what are bacteria?

The word bacteria is the plural of bacterium. Grammatically the headline should just say "What are bacteria?" The incorrect usage has been included in the headline to remind readers that it is wrong - and hopefully help correct an increasingly common mistake in the English language.

Bacteria are tiny living beings (microorganisms) - they are neither plants nor animals - they belong to a group all by themselves. Bacteria are tiny single-cell microorganisms, usually a few micrometers in length that normally exist together in millions.

A gram of soil typically contains about 40 million bacterial cells. A milliliter of fresh water usually holds about one million bacterial cells.

Planet Earth is estimated to hold at least 5 nonillion bacteria. Scientists say that much of Earth's biomass is made up of bacteria.

5 nonillion = 5,000,000,000,000,000,000,000,000,000,000 (or 5x1030)

(Nonillion = 30 zeros in USA English. In British English it equals 54 zeros. This text uses the American meaning)

Bacteria come in three main shapes:

1) Spherical (like a ball)

These are usually the simplest ones. Bacteria shaped like this are called cocci (singular coccus).

2) Rod shaped

These are known as bacilli (singular bacillus).

Some of the rod-shaped bacteria are curved; these are known as vibrio.

3) Spiral

These known are as spirilla (singular spirillus).

If their coil is very tight they are known as spirochetes.

There are many variations within each shape group.

Bacteria morphology. Image by Vojtěch Dostál

Bacteria are found everywhere

Bacteria can be found in:

  • Soil
  • Radioactive waste
  • Water
  • Plants
  • Animals
  • Deep in the earth's crust
  • Organic material
  • Arctic ice
  • Glaciers
  • Hot springs
  • The stratosphere (between 6 to 30 miles up in the atmosphere)
  • Ocean depths - they have been found deep in ocean canyons and trenches over 32,800 feet (10,000 meters) deep. They live in total darkness by thermal vents at incredible pressure. They make their own food by oxidizing sulfur that oozes from deep inside the earth.

Scientists who specialize in bacteria - bacteriologists - say bacteria are found absolutely everywhere except for places that humans have sterilized. Even the most unlikely places where temperatures may be extreme, or where there may be a high concentration of toxic chemicals, have bacteria. These bacteria are known as extremophiles (an extremophile is any organism adapted to living in conditions of extreme temperature, pressure, or/and chemical concentrations) and can survive where no other organism can.

Bacteria cells

A bacterial cell differs somewhat from the cell of a plant or animal. Bacterial cells have no nucleus and other organelles (sub-units within a cell with a specific function) bound by a membrane, except for ribosomes. Bacteria have pili, flagella and a cell capsule (most of them), unlike animal or plant cells. An organism without a nucleus is called a prokaryote.

A diagram of a bacterial cell.

A bacterial cell includes:

  • Basal body - this anchors the base of the flagellum, allowing it to rotate.
  • Capsule - a layer on the outside of the cell wall. Some bacteria don't have a capsule.
  • Cell wall - a thin layer (membrane) outside the plasma membrane, and within the capsule.
  • DNA (Deoxyribonucleic acid) - contains all the genetic instructions used in the development and functioning of the bacterium. It is inside the cytoplasm.
  • Cytoplasm - a gelatinous substance inside the plasma membrane. Genetic material and ribosomes lie inside.
  • Flagellum - this is used for movement; to propel the cell. Some bacterial cells have more than one.
  • Pili (singular: pilus) - these spikes allow the cell to stick to surfaces and transfer genetic material to other cells. A study revealed that pili are involved in causing traveler's diarrhea.
  • Plasma membrane - it generates energy and transports chemicals. Substances can pass through the membrane (permeable). It is located within the cell wall.
  • Ribosomes - this is where protein is made (synthesized). Ribosomes are small organelles made up of RNA-rich granules.

The origins and evolution of bacteria

Modern bacteria's ancestors - single-celled microorganisms - appeared on earth about 4 billion years ago. Scientists say they were the first life forms on Earth. For the following 3 billion years all life forms on Earth were microscopic in size, and included two dominant ones: 1. Bacteria, and 2. Archaea (classified as bacteria, but genetically and metabolically different from all other known bacteria).

There are fossils of bacteria. However, because their form and structure (morphology) are not distinctive it is virtually impossible to date them, making it extremely hard to study the process of bacterial evolution with any degree of accuracy. However, with the help of gene sequences, it is now possible to know that bacteria diverged from their original archaeal/eukaryotic ancestry (Eukaryotic = pertaining to an eukaryotice; a single-celled or multicellular organism whose cells contain a distinct membrane-bound nucleus).

Archaea is bacteria's most recent common ancestor - it was most likely hyperthermophile, an organism that thrived in extremely hot environments, approximately 2.5 - 3.2 billion years ago. Bacteria were also involved in the divergence of archaea and eukaryotes. Eukaryotes came from a very early bacteria which had an endosymbiotic association (when an organism lives within the body or cells of another organism) with the predecessors of eukaryotes cells, which were probably related to the Archaea. Biologists say that some algae probably originated from later endosymbiotic relationships.

Put simply - bacteria were the first organisms to appear on earth, about 4 billion years ago. Our oldest known fossils are of bacteria-like organisms.


On the next page we look at a short history of bacteriology, how bacteria feed themselves and what kinds of environments bacteria inhabit. On the final page we discuss how bacteria reproduce and the effects of bacteria.

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