Bacteria

Bacteria** (singular: **bacterium**) are unicellular [|microorganisms]. Typically a few [|micrometres] in length, bacteria have a wide range of shapes, ranging from [|spheres] to rods to spirals. Bacteria are ubiquitous in every [|habitat] on [|Earth], growing in soil, [|acidic hot springs], [|radioactive waste], seawater, and deep in the [|Earth's crust]. There are typically 40 million bacterial [|cells] in a gram of soil and a million bacterial cells in a millilitre of fresh water; in all, there are approximately five [|nonillion] (5×1030) bacteria on Earth, forming much of the world's [|biomass].Bacteria are vital in recycling nutrients, and many important steps in [|nutrient cycles] depend on bacteria, such as the [|fixation of nitrogen] from the [|atmosphere]. However, most of these bacteria have not been characterized, and only about half of the [|phyla] of bacteria have species that can be [|cultured] in the [|laboratory].hThe study of bacteria is known as [|bacteriology], a branch of [|microbiology]. There are approximately ten times as many bacterial cells as [|human] cells in the human body, with large numbers of bacteria on the [|skin] and in the [|digestive tract]. Although the vast majority of these bacteria are rendered harmless by the protective effects of the [|immune system], and a few are [|beneficial], some are [|pathogenic b] [|acteria] 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 a year, mostly in [|sub-Saharan Africa]. In [|developed countries], [|antibiotics] are used to treat bacterial infections and in various agricultural processes, so [|antibiotic resistance] is becoming common. In industry, bacteria are important in processes such as [|sewage treatment], the production of [|cheese] and [|yoghurt], and the manufacture of antibiotics and other chemicals. Bacteria are [|prokaryotes]. Unlike cells of animals and other [|eukaryotes], bacterial cells do not contain a [|nucleus] and rarely harbour [|membrane-bound] [|organelles]. Although the term //bacteria// traditionally included all prokaryotes, the [|scientific classification] changed after the discovery in the 1990s that prokaryotic life consists of two very different groups of organisms that [|evolved] independently from an ancient common ancestor. These [|evolutionary domains] are called Bacteria and [|Archaea].
 * __BACTERIA__

Unlike multicellular organisms, increases in the size of bacteria ([|cell growth]) and their reproduction by [|cell division] are tightly linked in unicellular organisms. Bacteria grow to a fixed size and then reproduce through [|binary fission], a form of [|asexual reproduction].[|[84]] Under optimal conditions, bacteria can grow and divide extremely rapidly, and bacterial populations can double as quickly as every 9.8 minutes.[|[85]] In cell division, two identical [|clone] daughter cells are produced. Some bacteria, while still reproducing asexually, form more complex reproductive structures that help disperse the newly-formed daughter cells. Examples include fruiting body formation by //[|Myxobacteria]// and arial [|hyphae] formation by //[|Streptomyces]//, or budding. Budding involves a cell forming a protrusion that breaks away and produces a daughter cell.  A growing colony of //[|Escherichia coli]// cells[|[86]] In the laboratory, bacteria are usually grown using solid or liquid media. Solid growth media such as [|agar plates] are used to isolate pure cultures of a bacterial strain. However, liquid growth media are used when measurement of growth or large volumes of cells are required. Growth in stirred liquid media occurs as an even cell suspension, making the cultures easy to divide and transfer, although isolating single bacteria from liquid media is difficult. The use of selective media (media with specific nutrients added or deficient, or with antibiotics added) can help identify specific organisms.[|[87]] Most laboratory techniques for growing bacteria use high levels of nutrients to produce large amounts of cells cheaply and quickly. However, in natural environments nutrients are limited, meaning that bacteria cannot continue to reproduce indefinitely. This nutrient limitation has led the evolution of different growth strategies (see [|r/K selection theory]). Some organisms can grow extremely rapidly when nutrients become available, such as the formation of [|algal] (and cyanobacterial) blooms that often occur in lakes during the summer.[|[88]] Other organisms have adaptations to harsh environments, such as the production of multiple [|antibiotics] by //[|Streptomyces]// that inhibit the growth of competing microorganisms.[|[89]] In nature, many organisms live in communities (e.g. [|biofilms]) which may allow for increased supply of nutrients and protection from environmental stresses.[|[39]] These relationships can be essential for growth of a particular organism or group of organisms ([|syntrophy]).[|[90]] [|Bacterial growth] follows three phases. When a population of bacteria first enter a high-nutrient environment that allows growth, the cells need to adapt to their new environment. The first phase of growth is the [|lag phase], a period of slow growth when the cells are adapting to the high-nutrient environment and preparing for fast growth. The lag phase has high biosynthesis rates, as proteins necessary for rapid growth are produced.[|[91]] The second phase of growth is the [|logarithmic] phase (log phase), also known as the exponential phase. The log phase is marked by rapid [|exponential growth]. The rate at which cells grow during this phase is known as the //growth rate// (//k//), and the time it takes the cells to double is known as the //generation time// (//g//). During log phase, nutrients are metabolised at maximum speed until one of the nutrients is depleted and starts limiting growth. The final phase of growth is the //stationary phase// and is caused by depleted nutrients. The cells reduce their metabolic activity and consume non-essential cellular proteins. The stationary phase is a transition from rapid growth to a stress response state and there is increased expression of genes involved in [|DNA repair], [|antioxidant metabolism] and [|nutrient transport].[|[92]]