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Cellular structure::Bacteria

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Cellular structure {{#invoke:Hatnote|hatnote}}

Structure and contents of a typical Gram positive bacterial cell (seen by the fact that only one cell membrane is present).

Intracellular structures

The bacterial cell is surrounded by a cell membrane (also known as a lipid, cytoplasmic or plasma membrane). This membrane encloses the contents of the cell and acts as a barrier to hold nutrients, proteins and other essential components of the cytoplasm within the cell. As they are prokaryotes, bacteria do not usually have membrane-bound organelles in their cytoplasm, and thus contain few large intracellular structures. They lack a true nucleus, mitochondria, chloroplasts and the other organelles present in eukaryotic cells.<ref name=Stryer>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> Bacteria were once seen as simple bags of cytoplasm, but structures such as the prokaryotic cytoskeleton<ref name="Gitai Z 2005 577–86">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref><ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> and the localization of proteins to specific locations within the cytoplasm<ref name="Gitai Z 2005 577–86"/> that give bacteria some complexity have been discovered. These subcellular levels of organization have been called "bacterial hyperstructures".<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

Bacterial microcompartments, such as carboxysomes,<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> provide a further level of organization; they are compartments within bacteria that are surrounded by polyhedral protein shells, rather than by lipid membranes.<ref name=Bobik2007>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> These "polyhedral organelles" localize and compartmentalize bacterial metabolism, a function performed by the membrane-bound organelles in eukaryotes.<ref name=Bobik2006>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref><ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

Many important biochemical reactions, such as energy generation, use concentration gradients across membranes. The general lack of internal membranes in bacteria means reactions such as electron transport occur across the cell membrane between the cytoplasm and the periplasmic space.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> However, in many photosynthetic bacteria the plasma membrane is highly folded and fills most of the cell with layers of light-gathering membrane.<ref name=bryantfrigaard>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> These light-gathering complexes may even form lipid-enclosed structures called chlorosomes in green sulfur bacteria.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> Other proteins import nutrients across the cell membrane, or expel undesired molecules from the cytoplasm.

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Bacteria do not have a membrane-bound nucleus, and their genetic material is typically a single circular DNA chromosome located in the cytoplasm in an irregularly shaped body called the nucleoid.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> The nucleoid contains the chromosome with its associated proteins and RNA. The phylum Planctomycetes<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> and candidate phylum Poribacteria<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> may be exceptions to the general absence of internal membranes in bacteria, because they appear to have a double membrane around their nucleoids and contain other membrane-bound cellular structures. Like all living organisms, bacteria contain ribosomes, often grouped in chains called polyribosomes, for the production of proteins, but the structure of the bacterial ribosome is different from that of eukaryotes and Archaea.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> Bacterial ribosomes have a sedimentation rate of 70S (measured in Svedberg units): their subunits have rates of 30S and 50S. Some antibiotics bind specifically to 70S ribosomes and inhibit bacterial protein synthesis. Those antibiotics kill bacteria without affecting the larger 80S ribosomes of eukaryotic cells and without harming the host.

Some bacteria produce intracellular nutrient storage granules for later use, such as glycogen,<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> polyphosphate,<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> sulfur<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> or polyhydroxyalkanoates.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> Certain bacterial species, such as the photosynthetic Cyanobacteria, produce internal gas vesicles, which they use to regulate their buoyancy – allowing them to move up or down into water layers with different light intensities and nutrient levels.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> Intracellular membranes called chromatophores are also found in membranes of phototrophic bacteria. Used primarily for photosynthesis, they contain bacteriochlorophyll pigments and carotenoids. An early idea was that bacteria might contain membrane folds termed mesosomes, but these were later shown to be artifacts produced by the chemicals used to prepare the cells for electron microscopy. Inclusions are considered to be nonliving components of the cell that do not possess metabolic activity and are not bounded by membranes. The most common inclusions are glycogen, lipid droplets, crystals, and pigments. Volutin granules are cytoplasmic inclusions of complexed inorganic polyphosphate. These granules are called metachromatic granules due to their displaying the metachromatic effect; they appear red or blue when stained with the blue dyes methylene blue or toluidine blue. Gas vacuoles, which are freely permeable to gas, are membrane-bound vesicles present in some species of Cyanobacteria. They allow the bacteria to control their buoyancy. Microcompartments are widespread, membrane-bound organelles that are made of a protein shell that surrounds and encloses various enzymes. Carboxysomes are bacterial microcompartments that contain enzymes involved in carbon fixation. Magnetosomes are bacterial microcompartments, present in magnetotactic bacteria, that contain magnetic crystals.

Extracellular structures

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In most bacteria, a cell wall is present on the outside of the cell membrane. The cell membrane and cell wall comprise the cell envelope. A common bacterial cell wall material is peptidoglycan (called "murein" in older sources), which is made from polysaccharide chains cross-linked by peptides containing D-amino acids.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> Bacterial cell walls are different from the cell walls of plants and fungi, which are made of cellulose and chitin, respectively.<ref name=Koch>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> The cell wall of bacteria is also distinct from that of Archaea, which do not contain peptidoglycan. The cell wall is essential to the survival of many bacteria, and the antibiotic penicillin is able to kill bacteria by inhibiting a step in the synthesis of peptidoglycan.<ref name=Koch/>

There are broadly speaking two different types of cell wall in bacteria, a thick one in the gram-positives and a thinner one in the gram-negatives. The names originate from the reaction of cells to the Gram stain, a test long-employed for the classification of bacterial species.<ref name=Gram>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

Gram-positive bacteria possess a thick cell wall containing many layers of peptidoglycan and teichoic acids. In contrast, gram-negative bacteria have a relatively thin cell wall consisting of a few layers of peptidoglycan surrounded by a second lipid membrane containing lipopolysaccharides and lipoproteins. Lipopolysaccharides, also called endotoxins, are composed of polysaccharides and lipid A that is responsible for much of the toxicity of gram-negative bacteria. Most bacteria have the gram-negative cell wall, and only the Firmicutes and Actinobacteria have the alternative gram-positive arrangement.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> These two groups were previously known as the low G+C and high G+C Gram-positive bacteria, respectively. These differences in structure can produce differences in antibiotic susceptibility; for instance, vancomycin can kill only gram-positive bacteria and is ineffective against gram-negative pathogens, such as Haemophilus influenzae or Pseudomonas aeruginosa.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> If the bacterial cell wall is entirely removed, it is called a protoplast, whereas if it is partially removed, it is called a spheroplast. ß-Lactam antibiotics, such as penicillin, inhibit the formation of peptidoglycan cross-links in the bacterial cell wall. The enzyme lysozyme, found in human tears, also digests the cell wall of bacteria and is the body's main defense against eye infections.

Acid-fast bacteria, such as Mycobacteria, are resistant to decolorization by acids during staining procedures. The high mycolic acid content of Mycobacteria, is responsible for the staining pattern of poor absorption followed by high retention. The most common staining technique used to identify acid-fast bacteria is the Ziehl-Neelsen stain or acid-fast stain, in which the acid-fast bacilli are stained bright-red and stand out clearly against a blue background. L-form bacteria are strains of bacteria that lack cell walls. The main pathogenic bacteria in this class is Mycoplasma (not to be confused with Mycobacteria).

In many bacteria, an S-layer of rigidly arrayed protein molecules covers the outside of the cell.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> This layer provides chemical and physical protection for the cell surface and can act as a macromolecular diffusion barrier. S-layers have diverse but mostly poorly understood functions, but are known to act as virulence factors in Campylobacter and contain surface enzymes in Bacillus stearothermophilus.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

Helicobacter pylori electron micrograph, showing multiple flagella on the cell surface

Flagella are rigid protein structures, about 20 nanometres in diameter and up to 20 micrometres in length, that are used for motility. Flagella are driven by the energy released by the transfer of ions down an electrochemical gradient across the cell membrane.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

Fimbriae (sometimes called "attachment pili") are fine filaments of protein, usually 2–10 nanometres in diameter and up to several micrometers in length. They are distributed over the surface of the cell, and resemble fine hairs when seen under the electron microscope. Fimbriae are believed to be involved in attachment to solid surfaces or to other cells, and are essential for the virulence of some bacterial pathogens.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> Pili (sing. pilus) are cellular appendages, slightly larger than fimbriae, that can transfer genetic material between bacterial cells in a process called conjugation where they are called conjugation pili or "sex pili" (see bacterial genetics, below).<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> They can also generate movement where they are called type IV pili (see movement, below).

Glycocalyx are produced by many bacteria to surround their cells, and vary in structural complexity: ranging from a disorganised slime layer of extra-cellular polymer to a highly structured capsule. These structures can protect cells from engulfment by eukaryotic cells such as macrophages (part of the human immune system).<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> They can also act as antigens and be involved in cell recognition, as well as aiding attachment to surfaces and the formation of biofilms.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

The assembly of these extracellular structures is dependent on bacterial secretion systems. These transfer proteins from the cytoplasm into the periplasm or into the environment around the cell. Many types of secretion systems are known and these structures are often essential for the virulence of pathogens, so are intensively studied.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

Endospores

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Bacillus anthracis (stained purple) growing in cerebrospinal fluid

Certain genera of Gram-positive bacteria, such as Bacillus, Clostridium, Sporohalobacter, Anaerobacter, and Heliobacterium, can form highly resistant, dormant structures called endospores.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> In almost all cases, one endospore is formed and this is not a reproductive process, although Anaerobacter can make up to seven endospores in a single cell.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> Endospores have a central core of cytoplasm containing DNA and ribosomes surrounded by a cortex layer and protected by an impermeable and rigid coat. Dipicolinic acid is a chemical compound that composes 5% to 15% of the dry weight of bacterial spores. It is implicated as responsible for the heat resistance of the endospore.

Endospores show no detectable metabolism and can survive extreme physical and chemical stresses, such as high levels of UV light, gamma radiation, detergents, disinfectants, heat, freezing, pressure, and desiccation.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> In this dormant state, these organisms may remain viable for millions of years,<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref><ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> and endospores even allow bacteria to survive exposure to the vacuum and radiation in space.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> According to scientist Dr. Steinn Sigurdsson, "There are viable bacterial spores that have been found that are 40 million years old on Earth — and we know they're very hardened to radiation."<ref name="BBC-2011">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Endospore-forming bacteria can also cause disease: for example, anthrax can be contracted by the inhalation of Bacillus anthracis endospores, and contamination of deep puncture wounds with Clostridium tetani endospores causes tetanus.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>


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