Bacteria (bacterium, singular), micro-organisms that lack internal cell membranes. The most common and ancient organisms on earth.
Most bacteria are less than 1mm in length. Hundreds of thousands of bacteria can fit into a space the size of the full stop at the end of this sentence. However, colonies of bacteria, such as on a laboratory culture plate can easily be viewed without a microscope.
Grouping organisms helps scientists study, understand, and discuss them more effectively. Life on earth is frequently described as being either prokaryotic (microscopic and lacking cells with internal membranes) or eukaryotic (macroscopic or microscopic but with clearly defined internal compartments). Bacteria are prokaryotic organisms, or prokaryotes.
Microbiologists further classify bacteria according to their basic shapes. Spherical bacteria are called cocci, corkscrew-shaped are called spirilla or spirochetes, rod-shaped are called bacilli, and threadlike bacteria are called filamentous. Some bacteria, called pleiomorphic, take various forms depending on conditions.
Structure Like all cells, bacteria contain DNA. However, whereas cellular DNA is arranged in strands, bacterial DNA has a circular arrangement. Bacteria also often have additional genes on smaller rings called plasmids. Bacterial DNA is not enclosed in a nucleus, as is the DNA of eukaryotic cells. Like eukaryotic cells, bacteria have ribosomes—round structures active in protein synthesis—but they are smaller and have different density characteristics than eukaryotic ribosomes.
Many bacteria feature small protrusions from their outside cell surface known as pili (singular, pilus). These hairlike outgrowths assist the bacteria in attaching to certain surfaces.
Flagella (singular, flagellum) are hairlike extensions. They are much longer than pili and can be found at either or both ends of a bacterium or all over its body surface. Flagella beat in a circular motion to help the bacterium move. Bacterial flagella are completely different from eukaryotic flagella, which are made up of different proteins and move the cell by sliding and contracting.
Some bacteria have structures known as endospores around their DNA and other cell portions. These thick-walled bodies are extremely resistant to environmental stresses and can live in a kind of dormant condition for decades or even centuries.
Growth and Reproduction A bacterium reproduces by means of a process called binary fission, in which a copy of the DNA is made and then the outer membrane of the bacterium begins to grow inward and divides the bacterium into two identical cells.
Binary fission does not provide bacteria with a way to exchange genetic information between individuals. Bacteria transfer genetic material by means of a process called conjugation, in which one bacterium attaches itself to another bacterium and introduces DNA directly into it by means of a pilus.