MICROORGANISMS

Microorganisms

Microorganisms

Bacteria

Bacteria (bacterium, singular), micro-organisms that lack internal cell membranes. The most common and ancient organisms on earth. Most bacteria are less than 1(m 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.

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 ...