Bacteria

Germ is a general term used to designate any minute pathogenic agent. The term germ became widely used after Louis Pasteur's development of the germ theory of disease in the 19th century. The term is applied to disease-producing micro organisms, such as bacteria, Protozoa, and fungi, and to pathogenic agents of uncertain classification, such as Rickettsia and viruses.

Bacteria

Bacteria are micro organisms that lack internal cell membranes. The most common and ancient organisms on earth, bacteria are intimately connected to the lives of all organisms.

Most bacteria are less than 1 micron (0.001 mm/0.00004 in.) in length. Hundreds of thousands of bacteria can fit into a space the size of the period at the end of this sentence. However, colonies of bacteria, such as on a laboratory culture plate or on the surface of salt marsh mud, can easily be viewed without a microscope.

A large number of bacteria, such as the green bacteria, purple bacteria, and cyanobacteria, are called Phototrophs. These bacteria are able to convert the sun's energy into food in a process called photo synthesis. Phototrophic bacteria have dominated earth's seas and landscapes for hundreds of millions of years and remain common today.

Micro biologists (scientists who study life forms that can only be observed with the aid of a microscope) also classify bacteria according to whether or not they require oxygen to survive. Bacteria that require oxygen are called aerobic bacteria, or aerobes. Bacteria that live without oxygen are called anaerobic bacteria, or anaerobes. Both aerobes and anaerobes can be either phototrophic or nonphototrophic.

In the late 17th century, Antoni van Leeuwenhoek, a custodian and town caretaker of Delft, Holland, became the first person to make a systematic study of bacteria. Leeuwenhoek spent hundreds