Bacteria are single-celled organisms which may be present singly or in multiples. Their body cell has no distinct nucleus and for this reason they are prokaryotes - literally "before the nucleus". They are found in almost every Earth habitat including water, in soil, and on or in other organisms. They can also be found in extreme environments.
Bacteria come in many shapes - round, rod, comma, spiral and even square! When in multiple linked colonies they can also take up a variety of shapes. Typically these can be clusters, chains, cubic packets, squares and grape-like "bunched" clusters.
Bacteria are everywhere and you provide an ideal home for around 400 different species in your gut, 600 in your mouth and similar numbers on your skin and in other parts of your body. Don't worry, they are harmless and many are beneficial. In fact, you would die without them. They help process your food, some give you protection against certain unfriendly types of bacteria and others are even thought to be able to provide a calming effect to your body, by producing tranquilizing chemicals. The number of bacteria in your body is greater than the number of cells in your body, by a factor of ten, or maybe a hundred, according to Dr Jeffrey Gordon of Washinton University in St Louis.
Bacteria constitute one of the three domains of life shown above. Strictly speaking, they are termed eubacteria, meaning "true bacteria" to distinguish them from the domain archaea, which is also known as the archaebacteria domain, literally meaning "ancient bacteria".
Many different types of bacteria appear similar in many respects and until the advent of DNA/RNA sequencing were thought to be closely related, if not the same organism. Since the use of the technique in identifying bacteria, however, it has been estimated that there could be as many as a billion different bacterial species on Earth. To date only some thousands have been identified as specific species.
All bacteria are considered dangerous by most of the public, but this is far from true. Many bacteria are wholly beneficial to us, while others are benign - they do nothing that either helps or harms us. Only relatively few bacteria have been identified as dangerous or detrimental to our health.
In the right conditions - a favorable temperature and sufficient nutrients, for instance - bacteria can reproduce as often as every twenty minutes. This is not a simple serial process of one replacing another, but an exponential one, where there is a population explosion of bacteria. This multiplication process can produce something the size of a sugar cube within a day or so. If allowed to continue, this exponential growth would result in a ball of microbes the size of the Earth within something like a week! In practice the attrition (death and destruction) rate of the microbes and the limit imposed by the availability of nutrients, stops the microbial population getting out of hand. This is why the world is not completely overrun with microbes, though some estimates suggest there may be as many as
bacteria on Earth.
Another consequence of the rapid reproduction rate is an impressive rate of evolution. While the generation cycle for people is around 20 years, that for bacteria at every 20 minutes is over 1/2 million times as fast. This is why new bacterial diseases apparently appear on a regular basis and why strains of common bacteria, which we could easily destroy only a few years ago, can become resistant to antibiotic treatments.But that is not the whole reason for their ability to adapt. Bacteria can reproduce rapidly both by simple cell-division and by the exchange of genes between two bacteria in cell to cell contact. Although not strictly sexual reproduction, it is similar in that two individual are involved in providing genetic material from two different ancestories. This latter type of reproduction is another factor that enables them to achieve remarkable rates of evolution. Some bacteria reproduce as spores, which are highly resilient to unfavorable environments. Spores have a tough outer coat which is both mechanically strong and chemically resistant. About half of all bacteria can move by themselves, the remainder are at the mercy of the medium - eg air or water - they are in. Those that move do so in one of four ways.Most bacteria that move can only wander randomly though the media they occupy (eg water), by twitching.Others can control their bouyancy. Some of these have tiny gas-bags (vacuoles) which can inflate to let the bacterium sink or float.Other species have flagella, which beat like whips to drive the bacterium along. Flagella may be arrayed around the cell of the bacteria and these can move in unison, so send the bacteria in a particular direction.Other, called spirochetes, can move in a corkscrew-like motion associated with their shape. They have spiral flagella wrapped around themselves and this modifies their shape so they can adopt the twisting motion.Many members of the latter three groups can move in a particular direction, according to stimuli they receive. This movement is called taxis.
have an uncanny resemblance to the fossilized structures that have been
found in Martian meteorites. Note
also the remarkable similarity between the nanobacteria shown at the University
of Queensland's web site and the images of supposed fossil bacteria that
have been found in the Murchison
meteorite, which came not from Mars but from deep
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