A bacterium is a unicellular microorganism which represents one of the most basic and primitive forms of life. Bacteria are everywhere, from pools of nuclear waste to deep inside the Earth's crust, and it is believed that bacteria were the first living organisms on Earth. You come into contact with bacteria constantly, although you may not be aware of it. Bacteria exist in such abundance that scientists have barely begun to scratch the surface of the bacterial life on Earth, although some species are well known to humans because they cause infections or disease.
Several things are common to all bacteria, regardless as to how they live their lives. The organisms are classified in the kingdom Prokaryota, which used to be known as Monera. There are only two domains in this kingdom: bacteria and archaea. Bacteria lack a cell nucleus, and they also do not have organelles, like most other celled organisms do. Organelles are small structures inside a cell which have specific functions, like mitochondria. A bacterium has a single DNA molecule, along with RNA strands to help it replicate.
The study of bacteria is known as bacteriology. The more the organisms are studied, the more surprises they yield. A bacterium can come in a range of shapes, although most break down into rod, spiral, or curved shapes. The organisms typically use small hairs attached to their cell walls known as flagella to move around, and a bacterium may have one flagella or a plethora. In most cases, bacteria are surrounded by a hard outer shell which helps to protect them from the elements. This shell allows a bacterium to put itself into stasis, waiting for more congenial conditions to emerge.
A bacterium can live in a number of different ways. Some species are free living, meaning that they exist independently in things like soil, air, and water. Others may form relationships with additional bacteria or other organisms, taking advantage of mutual strengths to survive. In some cases, bacteria may colonize an animal, although they are perfectly capable of living without their host; bacteria use the host as a source of food, not necessarily for shelter. In most cases, bacteria actually assist their hosts, helping them to digest and break down food, and consuming cast off skin and hair. In other instances, as with pathogenic bacteria, the colonization results in an illness such as plague, tuberculosis, or cholera.
Many people are familiar with the rapid multiplication of bacteria, which can be accomplished in a number of ways. Most commonly, a bacterium grows and splits itself, thus creating exponential growth within a colony as each new generation grows and splits, sometimes within minutes. Bacteria can also exchange genetic information, including mutations, with each other. Some bacteria can also reproduce through budding, growing a portion of a parent cell which breaks off and grows into a new bacterium.