Bacteria are unicellular (one-celled) organisms and are considered to be primitive living things. Bacteria first appeared 3.5 billion years ago. They are the most numerous and widely spread organisms on earth. Bacteria have a ring of genetic material (one circular chromosome), a cell membrane, and a cell wall containing peptidoglycan (a sugar and polypeptide compound). However, they are prokaryotic cells (cells that lack a nuclear membrane).
To figure out which bacteria are which, researchers grow neighborhoods (or colonies) of these guys on a gooey base material made from certain seaweeds. A bacterial colony is a visible bunch of microorganisms that sprang from a single mother cell, which makes each member genetically identical.
Bacteria like to multiply and "redesign" themselves; it's part of their plan to survive stuff like antibiotics. Identification of a particular strain of bacteria requires a close look at the colony's shape, appearance, color, and smell.
Most bacteria are heterotrophs; they get their energy by consuming organic molecules found in their environment. The majority of bacteria are harmless to humans, but some can hurt us, like anthrax, tuberculosis, and Streptococcus, which causes strep throat. Some bacteria are beneficial, such as nitrogen-fixing bacteria that make nitrogen available to plants, and the bacteria of decay, which help recycle materials in the environment.