Bacteria

Bacteria come in a wide variety of shapes Perhaps the most elemental structural property of bacteriais cell morphology(shape). Typical examples include: *. coccus(spherical) *. bacillus(rod-like) *. spirillum(spiral) *. filamentous(elongated) Cell shape is generally characteristic of a given bacterial species, but can vary depending on growth conditions. Some bacteria have complex life cycles involving the production of stalks and appendages (e.g. Caulobacter) and some produce elaborate structures bearing reproductive spores (e.g. Myxococcus, Streptomyces). Bacteriagenerally form distinctive cell morphologies when examined by light microscopyand distinct colony morphologies when grown on Petri plates. These are often the first characteristics observed by a microbiologistto determine the identity of an unknown bacterial culture. Perhaps the most obvious structural characteristic of bacteriais (with some exceptions) their small size. For example, Escherichia colicells, an "average" sized bacterium, are about 2 micrometres(μm) long and 0.5 μm in diameter, with a cell volume of 0.6 - 0.7 μm3. [ 1 ]This corresponds to a wet mass of about 1 picogram(pg), assuming that the cell consists mostly of water. The dry mass of a single cell can be estimated as 20% of the wet mass, amounting to 0.2 pg. About half of the dry mass of a bacterial cell consists of carbon, and also about half of it can be attributed to proteins. Therefore, a typical fully grown 1-liter culture of Escherichia coli(at an optical density of 1.0, corresponding to ca. 109cells/ml) yields about 1 g wet cell mass. [ 2 ]Small size is extremely important because it allows for a large surface area-to- volume ratiowhich allows for rapid uptake and intracellular distribution of nutrients and excretion of wastes. At low surface area-to-volume ratios the diffusion of nutrients and waste products across the bacterial cell membrane limits the rate at which microbial metabolism can occur, making the cell less evolutionarily fit. The reason for the existence of large cells is unknown, although it is speculated that the increased cell volume is used primarily for storage of excess nutrients. Cell wall The structure of peptidoglycan. The cell envelopeis composed of the plasma membrane and cell wall. As in other organisms, the bacterial cell wallprovides structural integrity to the cell. In prokaryotes, the primary function of the cell wall is to protect the cell from internal turgor pressurecaused by the much higher concentrations of proteins and other molecules inside the cell compared to its external environment. The bacterial cell wall differs from that of all other organisms by the presence of peptidoglycanwhich is located immediately outside of the cytoplasmic membrane. Peptidoglycan is made up of a polysaccharide backbone consisting of alternating N-Acetylmuramic acid(NAM) and N-acetylglucosamine(NAG) residues in equal amounts. Peptidoglycanis responsible for the rigidity of the bacterial cell wall and for the determination of cell shape. It is relatively porous and is not considered to be a permeability barrier for small substrates. While all bacterial cell walls (with a few exceptions e.g. extracellular parasites such as Mycoplasma) contain peptidoglycan, not all cell walls have the same overall structures. Since the cell wall is required for bacterial survival, but is absent in eukaryotes, several antibioticsnotably the ( penicillinsand cephalosporins) stop bacterial infections by interfering with cell wall synthesis, while having no effects on human cellswhich have no cell wall only a cell membrane.