Bacteria (singular: bacterium) constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste and the deep portions of Earth's crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals. They are also known to have flourished in manned spacecraft.

There are typically 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water. There are approximately 5X10 30th bacteria on Earth, forming a biomass which exceeds that of all plants and animals. Bacteria are vital in recycling nutrients, with many of the stages in nutrient cycles dependent on these organisms, such as the fixation of nitrogen from the atmosphere and putrefaction. In the biological communities surrounding hydrothermal vents and cold seeps, bacteria provide the nutrients needed to sustain life by converting dissolved compounds such as hydrogen sulphide and methane to energy.

On March 17, 2013, researchers reported data that suggested bacterial life forms thrive in the Mariana Trench, which with a depth of up to 11 kilometres is the deepest part of the Earth's oceans. Other researchers reported related studies that microbes thrive inside rocks up to 580 metres below the sea floor under 2.6 kilometres of ocean off the coast of the northwestern United States. According to one of the researchers,"You can find microbes everywhere - they're extremely adaptable to conditions, and survive wherever they are."

Most bacteria have not been characterized, and only about half of the phyla of bacteria have species that can be grown in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology.

There are approximately ten times as many bacterial cells in the human flora as there are human cells in the body, with the largest number of the human flora being in the gut flora, and a large number on the skin. The vast majority of the bacteria in the body are rendered harmless by the protective effects of the immune system, and some are beneficial. However, several species of bacteria are pathogenic and cause infectious diseases, including cholera, syphilis, anthrax, leprosy, and bubonic plague. The most common fatal bacterial diseases are respiratory infections, with tuberculosis alone killing about 2 million people per year, mostly in sub-Saharan Africa.

In developed countries, antibiotics are used to treat bacterial infections and are also used in farming, making antibiotic resistance a growing problem. In industry, bacteria are important in sewage treatment and the breakdown of oil spills, the production of cheese and yogurt through fermentation, and the recovery of gold, palladium, copper and other metals in the mining sector, as well as in biotechnology, and the manufacture of antibiotics and other chemicals.

Once regarded as plants constituting the class Schizomycetes, bacteria are now classified as prokaryotes. Unlike cells of animals and other eukaryotes, bacterial cells do not contain a nucleus and rarely harbor membrane-bound organelles. Although the term bacteria traditionally included all prokaryotes, the scientific classification changed after the discovery in the 1990s that prokaryotes consist of two very different groups of organisms that evolved from an ancient common ancestor. These evolutionary domains are called Bacteria and Archaea. Read more ...

In the News ...

Bacteria 'see' like tiny eyeballs   BBC - February 9, 2016
Biologists say they have solved the riddle of how a tiny bacterium senses light and moves towards it: the entire organism acts like an eyeball.

Newfound groups of bacteria are mixing up the tree of life   Science Daily - June 15, 2015
Bacteria, one of the three major branches of the tree of life, are a fuzzy bit of foliage. While scientists know there are many unidentified species, they have not been classified or characterized because no one can culture them. Now biologists have sequenced a community of bacteria, assembled almost 800 nearly complete genomes and found that many of them represent completely new phyla: more than 35 in all.

Even bacteria use social networks   PhysOrg - July 19, 2013

Myxococcus xanthus, a single-cell soil bacterium also uses a social network. The scientists believe M. xanthus uses its network to quietly transfer proteins and other molecules from one to another. This could enable the bacteria to coordinate social activities - such as evading bacterial enemies and snaring prey - without revealing its location.

Human Microbiome Project: a map of every bacterium in the body - August 18, 2010

In front of the doctors' eyes, the young girlŐs life was draining away. The two-year-old patient at Massachusetts General Hospital was suffering from life-threatening diarrhea, and nothing the pediatricians could do seemed to work. A last-resort therapy using an antibiotic called vancomycin had failed. They turned to experimental, unlicensed antibiotics but these, too, had no effect. Running out of time and options, the doctors in Boston decided to take drastic action: rather than trying to get rid of the bacteria that were causing the disease, they introduced more - taken, in what must have been a distinctly unpleasant exercise, from fecal matter found in the gut of a healthy individual. Within weeks, the child had made a full recovery. Scientists suspect that some patients fail to respond to conventional treatment for intestinal infections because they are lacking certain bacteria, which have a protective role, found in the normally functioning gut.

Bacteria Employ Type Of DNA Modification Never Before Seen In Nature Science Daily - December 10, 2007
A team of MIT researchers and others has discovered that bacteria employ a type of DNA modification never before seen in nature. For several decades, researchers have known that it is possible to modify synthetic oligonucleotides (short strands of DNA) by adding sulfur to the sugar-phosphate DNA backbone as a phosphorothioate. Researchers often use such modifications in the laboratory to make DNA resistant to nucleases (enzymes that snip DNA in certain locations) as a step toward gene and antisense therapies of human diseases. Dedon said he and his co-workers were surprised to discover that a group of bacterial genes, known as the dnd gene cluster, gives bacteria the ability to employ the same modification on their own.

Oldest Living Thing Found in Ice Live Science - August 28, 2007

The oldest living thing has been discovered in the form of 600,000-year-old bacteria extracted from ice cores, scientists claim. The finding gives hope that if Mars or other frigid worlds ever supported life similar to these ancient bacteria, they might be alive still. The trick, the discovery suggests, is for organisms to keep some metabolism going over the millennia, so that they can keep their DNA in repair while awaiting a more favorable environment in which to multiply. The other approach, complete dormancy, would cause DNA to be damaged and perhaps destroyed over such long periods of time. Recently in separate work, scientists collected the oldest ever recovered DNA, estimated to be up to 800,000 years old, below more than a mile of Greenland ice.