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.
