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Bacteria shape names3/20/2024 Having just one membrane, the gram-positive bacteria are also known as monoderm bacteria, while gram-negative bacteria, having two membranes, are also known as diderm bacteria. ( Discuss) (November 2023)īacteria are traditionally classified based on their Gram-staining response into the gram-positive and gram-negative bacteria. It has been suggested that this section be split out into another article titled Gram stain. However some authors, such as Cavalier-Smith still treat them as a monophyletic taxon (though not a clade his definition of monophyly requires a single common ancestor but does not require holophyly, the property that all descendants be encompassed by the taxon) and refer to the group as a subkingdom "Negibacteria". Since 1987, the monophyly of the gram-negative bacteria has been disproven with molecular studies. Historically, the kingdom Monera was divided into four divisions based on Gram staining: Firmacutes (+), Gracillicutes (−), Mollicutes (0) and Mendocutes (var.). Most, with few exceptions, do not form sporesĪlong with cell shape, Gram staining is a rapid diagnostic tool and once was used to group species at the subdivision of Bacteria.Some contain Braun's lipoprotein, which serves as a link between the outer membrane and the peptidoglycan chain by a covalent bond.Lipoproteins are attached to the polysaccharide backbone.Teichoic acids or lipoteichoic acids are absent.If present, flagella have four supporting rings instead of two.The S-layer is directly attached to the outer membrane rather than to the peptidoglycan.Between the outer membrane and the cytoplasmic membrane there is a space filled with a concentrated gel-like substance called periplasm.Porins exist in the outer membrane, which act like pores for particular molecules.Has outer membrane containing lipopolysaccharides (LPS, which consists of lipid A, core polysaccharide, and O antigen) in its outer leaflet and phospholipids in the inner leaflet.A thin peptidoglycan layer is present (this is much thicker in gram-positive bacteria).An inner cell membrane is present ( cytoplasmic).The antibiotics that specifically target gram-negative organisms include aminoglycosides, monobactams (such as aztreonam), and ciprofloxacin.Ĭharacteristics Gram-negative cell wall structure Gram-positive and -negative bacteria are differentiated chiefly by their cell wall structureĬonventional gram-negative (LPS-diderm) bacteria display these characteristics: Many of these antibiotics also cover gram-positive bacteria. Several classes of antibiotics have been developed to target gram-negative bacteria, including aminopenicillins, ureidopenicillins, cephalosporins, beta-lactam- betalactamase inhibitor combinations (such as piperacillin-tazobactam), folate antagonists, quinolones, and carbapenems. This reaction may lead to septic shock, resulting in low blood pressure, respiratory failure, reduced oxygen delivery, and lactic acidosis. Furthermore, the outer leaflet of this membrane contains a complex lipopolysaccharide (LPS) whose lipid A component can trigger a toxic reaction when the bacteria are lysed by immune cells. They pose significant challenges in the medical field due to their outer membrane, which acts as a protective barrier against numerous antibiotics (including penicillin), detergents that would normally damage the inner cell membrane, and the antimicrobial enzyme lysozyme produced by animals as part of their innate immune system. Within this category, notable species include the model organism Escherichia coli, along with various pathogenic bacteria, such as Pseudomonas aeruginosa, Chlamydia trachomatis, and Yersinia pestis. These bacteria are found in all environments that support life on Earth. Their defining characteristic is their cell envelopes, which consists of a thin peptidoglycan cell wall sandwiched between an inner ( cytoplasmic) membrane and an outer membrane. Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Group of bacteria that do not retain the Gram stain used in bacterial differentiation Microscopic image of gram-negative Pseudomonas aeruginosa bacteria (pink-red rods)
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