Gram-negative

(adjective)

Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain (commonly safranin) is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color.

Related Terms

  • symbionts
  • pathogens
  • Deltaproteobacteria
  • Proteobacteria
  • Epsilonproteobacteria
  • aerobic

Examples of Gram-negative in the following topics:

  • Gram-Negative Outer Membrane

    • The Gram-negative cell wall is composed of an outer membrane, a peptidoglygan layer, and a periplasm.
    • In the Gram-negative Bacteria the cell wall is composed of a single layer of peptidoglycan surrounded by a membranous structure called the outer membrane.
    • The gram-negative bacteria do not retain crystal violet but are able to retain a counterstain, commonly safranin, which is added after the crystal violet.
    • The safranin is responsible for the red or pink color seen with a gram-negative bacteria.
    • Together, the plasma membrane and the cell wall (outer membrane, peptidoglycan layer, and periplasm) constitute the gram-negative envelope.
  • The Deltaproteobacteria

    • All species of this group are, like all Proteobacteria, Gram-negative.
    • Myxobacteria are included among the delta group of proteobacteria, a large taxon of Gram-negative forms.
  • The Entner–Doudoroff Pathway

    • Pseudomonas, a genus of Gram-negative bacteria, and Azotobacter, a genus of Gram-negative bacteria use the Entner-Doudoroff pathway
    • This pathway is generally found in Pseudomonas, Rhizobium, Azotobacter, Agrobacterium, and a few other Gram-negative genera.
    • Very few Gram-positive bacteria have this pathway, with Enterococcus faecalis being a rare exception.
    • Most organisms that use the pathway are aerobes due to the low ATP yield per glucose such as Pseudomonas, a genus of Gram-negative bacteria, and Azotobacter, a genus of Gram-negative bacteria.
  • Spectrum of Antimicrobial Activity

    • Narrow spectrum antibiotics act against a limited group of bacteria, either gram positive or gram negative, for example sodium fusidate only acts against staphylococcal bacteria.
    • Broad spectrum—antibiotics act against gram positive and gram negative bacteria, for example amoxicillin.
    • Gram staining (or Gram's method; is a method of differentiating bacterial species into two large groups (Gram-positive and Gram-negative).
    • A Gram positive results in a purple/blue color while a Gram negative results in a pink/red color.
    • This is a microscopic image of a Gram stain of mixed Gram-positive cocci (Staphylococcus aureus, purple) and Gram-negative bacilli (Escherichia coli, red).
  • Basic Structures of Prokaryotic Cells

    • Bacteria are divided into two major groups: gram-positive and gram-negative, based on their reaction to gram staining.
    • Gram-positive organisms typically lack the outer membrane found in gram-negative organisms .
    • Bacteria are divided into two major groups: gram-positive and gram-negative.
    • Both groups have a cell wall composed of peptidoglycan: in gram-positive bacteria, the wall is thick, whereas in gram-negative bacteria, the wall is thin.
    • Porins, proteins in this cell membrane, allow substances to pass through the outer membrane of gram-negative bacteria.
  • Selective and Differential Media

    • Eosin methylene blue (EMB) that contains methylene blue – toxic to Gram-positive bacteria, allowing only the growth of Gram negative bacteria.
    • Mannitol salt agar (MSA) which is selective for Gram-positive bacteria and differential for mannitol.
    • Xylose lysine desoxyscholate (XLD), which is selective for Gram-negative bacteria buffered charcoal yeast extract agar, which is selective for certain gram-negative bacteria, especially Legionella pneumophila.
  • Peptidoglycan Synthesis and Cell Division

    • The peptidoglycan layer is substantially thicker in Gram-positive bacteria (20 to 80 nanometers) than in Gram-negative bacteria (7 to 8 nanometers), with the attachment of the S-layer.
    • Peptidoglycan forms around 90% of the dry weight of Gram-positive bacteria but only 10% of Gram-negative strains.
    • For both Gram-positive and Gram-negative bacteria, particles of approximately 2 nm can pass through the peptidoglycan.
    • Gram-positive and Gram-negative bacteria are sensitive to different types of antiobiotics.
    • Simplified sc hematic of a cell wall in a Gram-positive bacteria
  • Bacterial Pneumonias

    • Other commonly isolated bacteria include: Haemophilus influenzae in 20%, Chlamydophila pneumoniae in 13%, and Mycoplasma pneumoniae in 3% of cases; Staphylococcus aureus; Moraxella catarrhalis; Legionella pneumophila and Gram-negative bacilli.
    • Gram Positive.
    • Gram Negative.
    • Gram-negative bacteria are seen less frequently: Haemophilus influenzae , Klebsiella pneumoniae, Escherichia coli, Pseudomonas aeruginosa, and Moraxella catarrhalis are the most common.
    • In reality, they are "atypical" because they do not gram stain as well as gram-negative and gram-positive organisms.
  • Gram-Positive Cell Envelope

    • Gram-positive bacteria are stained dark blue or violet by Gram staining .
    • A Gram-positive results in a purple/blue color while a Gram-negative results in a pink/red color.
    • They lack the outer membrane envelope found in Gram-negative bacteria.
    • Teichoic acids give the Gram-positive cell wall an overall negative charge due to the presence of phosphodiester bonds between teichoic acid monomers.
    • One idea is that they provide a channel of regularly-oriented, negative charges for threading positively-charged substances through the complicated peptidoglycan network.
  • Actinobacteria (High G + C Gram-Positive Bacteria)

    • Actinobacteria are a group of Gram-positive bacteria with high guanine and cytosine content in their DNA.
    • Actinobacteria are a group of Gram-positive bacteria with high guanine and cytosine content in their DNA.
    • Unlike the Firmicutes, the other main group of Gram-positive bacteria, they have DNA with a high GC-content, and some Actinomycetes species produce external spores.
    • Classification of Gardnerella is controversial, and MeSH catalogues it as both a gram-positive and gram-negative organism.
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