Evolution of Bacteria: These primitive organisms were among the first to appear on Earth; bacteria evolved roughly 3.5 billion years ago. The oldest-known fossils are those of bacteria-like organisms.

Discovery of Bacteria: Bacteria were unknown to people until the 1600s, when Antony van Leeuwenhoek first observed them in his newly-made microscope.

Cells of Bacteria: The cells of bacteria are different from those of plants and animals in many ways, the most obvious of which is that bacteria lack a nucleus and other membrane-bound organelles (except ribosomes). Unlike animals and plants, bacteria have pili, flagella, and most have a cell capsule.

basal body - A structure that anchors the base of the flagellum and allows it to rotate.
capsule - A layer on the outside of the cell wall. Most but not all bacteria have a capsule.
cell wall - A thin membrane located outside the plasma membrane and within the capsule.
DNA - The genetic material of the bacterium; it is located within the cytoplasm.
cytoplasm - The jellylike material inside the plasma membrane in which the genetic material and ribosomes are located.
flagellum - A long whip-like structure used for locomotion (movement). Some bacteria have more than one flagellum.
pili - (singular is pilus) Hair-like projections that allow bacterial cells to stick to surfaces and transfer DNA to one another.
plasma membrane - A permeable membrane located within the cell wall. It serves many functions for the cell, including energy generation and transport of chemicals .
ribosomes - Small organelles composed of RNA-rich granules that are sites of protein synthesis. The ribosomes are located within the cytoplasm.

Diet: Bacteria have a wide range of diets. Some are heterotrophs (they eat other organisms) and others are autotrophs (they make their own food).

Most heterotrophic bacteria are saprobes (they absorb dead organic material like rotting flesh). Some of these parasitic bacteria kill their host while others help their host.

Autotrophic bacteria make their own food, either by photosynthesis (which uses sunlight, carbon dioxide and water to make food) or by chemosynthesis (which uses carbon dioxide, water and chemicals like ammonia to make food - these bacteria are called nitrogen fixers and include the bacteria found living in legume roots and in ocean vents).

Types of Bacteria: There are many different types of bacteria. Some bacteria are rod-shaped (these are called bacilli), some are round (called cocci, like streptococcus bacteria), and some are spiral-shaped (spirilli) or are incomplete spirals.

Some bacteria need atmospheric oxygen to live (these are called aerobic bacteria), but others do not (these are called anaerobic bacteria; they get their oxygen from other molecular compounds).

Another way to classify bacteria is by whether or not the bacteria absorbs a dye called "Gram stain" (a violet dye named for its developer, the bacteriologist Christian Gram). Gram positive and Gram negative bacteria have a different type of cell wall, and therefore, a different reaction to the dye and to some other chemicals, including antibiotics (chemicals that can sometimes kill bacteria).

Where are Bacteria Found: Bacteria are found almost everywhere on Earth, including in the seas and lakes, on all continents (including Antarctica), in the soil, and in tissues of plants and animals.

Reproduction: Bacteria grow in colonies and reproduce rapidly by asexual budding or fission, in which the cell increases in size and then splits in two. Bacteria can also undergo conjugation in which two separate bacteria exchange pieces of DNA.

Resting Stages: Under unfavorable environmental conditions, bacteria develop a thick outer wall and enter a dormant phase - in this resting state, the bacterium is called a spore. The bacteria can remain in this dormant state for long periods of time, surviving conditions that kill many other organisms.

Effects of Bacteria: Although bacteria case many illnesses (including dental caries, strep throat, cholera, and tuberculosis) and cause the harmful "red tide" seen in many lakes, bacteria also have many positive effects, including releasing nitrogen to plants and decomposing organic material. Bacteria also have crucial roles in the fermentation process and the manufacture of cheese and yogurt.

Bacteria Deep in the Oceans: Bacteria have recently been found living deep in ocean canyons and trenches that are more than 32,800 feet (10,000 meters) deep. These bacteria live in total darkness by thermal vents at tremendous pressure. These bacteria make their own food (via chemosynthesis) by oxidizing sulfur that oozes from deep inside the Earth.

Classification:
Kingdom Monera (meaning "alone" or "solitary") - all prokaryotic organisms (having no nucleus and virtually no membraned organelles), like bacteria and blue-green algae.
Phylum Schizomycetes - true bacteria.