Gram-negative bacteria

Gram-negative bacteria

Microscopic image of gram-negative Pseudomonas aeruginosa bacteria (pink-red rods )

Gram-negative bacteria are a class of bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation,[1] making positive identification possible. The thin peptidoglycan layer of their cell wall is sandwiched between an inner cell membrane and a bacterial outer membrane. In Gram staining, the outer lipid-based membrane of gram-negative bacteria is removed by an alcohol solution. The alcohol also decolorizes the then exposed peptidoglycan layer by dissolving away the previously applied crystal violet. A counterstain (safranin or fuchsine) is then added which recolorizes the bacteria red or pink.

Gram-positive bacteria on the other hand have a thicker peptidoglycan layer in their cell wall outside the cell membrane, which retains the crystal violet stain during the alcohol wash, so long as it is timed correctly. The counter stain may also be absorbed by gram-positive bacteria but the darker crystal violet stain predominates visually.

Contents

  • Characteristics 1
  • Classification 2
  • Outer cell membrane bacterial classification 3
    • Example species 3.1
  • Medical treatment 4
  • Orthographic note 5
  • See also 6
  • References 7
  • External links 8

Characteristics

Gram-negative cell wall structure
Gram-positive and -negative bacteria are differentiated chiefly by their cell wall structure

Gram-negative bacteria display the following characteristics:

  1. Cell membrane (cytoplasmic).
  2. Thin peptidoglycan layer (which is much thicker in gram-positive bacteria)
  3. Outer membrane containing lipopolysaccharide (LPS, which consists of lipid A, core polysaccharide, and O antigen) in its outer leaflet and phospholipids in the inner leaflet
  4. Porins exist in the outer membrane, which act like pores for particular molecules
  5. There is a space between the peptidoglycan layer and the secondary cell membrane called the periplasmic space
  6. The S-layer is directly attached to the outer membrane rather than the peptidoglycan
  7. If present, flagella have four supporting rings instead of two
  8. No teichoic acids or lipoteichoic acids are present
  9. Lipoproteins are attached to the polysaccharide backbone.
  10. Some of them contain Braun's lipoprotein, which serves as a link between the outer membrane and the peptidoglycan chain by a covalent bond
  11. Most, with very few exceptions, do not form spores.
  12. Release some endotoxin

Classification

Along with cell shape, Gram staining is a rapid diagnostic tool and once was used to group species at the subdivision of Bacteria.

Species identification hierarchy in clinical settings
Historically, the kingdom Monera was divided into four divisions based on Gram staining: Firmacutes (+), Gracillicutes (−), Mollicutes (0) and Mendocutes (var.).[2]

Since 1987, the monophyly of the gram-negative bacteria has been disproven with molecular studies.[3] However some authors, such as Cavalier-Smith still treat them as a monophyletic clade and refer to the group as subkingdom "Negibacteria".[4]

Outer cell membrane bacterial classification

Though bacteria are traditionally divided into the two main groups of antibiotic selection pressure.[5][6][9][10] Some bacteria such as Deinococcus, which stain gram-positive due to the presence of a thick peptidoglycan layer, but also possess an outer cell membrane are suggested as intermediates in the transition between monoderm (gram-positive) and diderm (gram-negative) bacteria.[5][10] The diderm bacteria can also be further differentiated between simple diderms lacking lipopolysaccharide, the archetypical diderm bacteria, in which the outer cell membrane contains lipopolysaccharide, and the diderm bacteria, in which outer cell membrane is made up of mycolic acid.[7][8][10][11]

In addition, a number of bacterial taxa (viz. Negativicutes, Fusobacteria, Synergistetes, and Elusimicrobia) that are either part of the phylum Firmicutes or branches in its proximity are also found to possess a diderm cell structure.[8][10][11] However, a conserved signature indel (CSI) in the HSP60 (GroEL) protein distinguishes all traditional phyla of gram-negative bacteria (e.g., Proteobacteria, Aquificae, Chlamydiae, Bacteroidetes, Chlorobi, Cyanobacteria, Fibrobacteres, Verrucomicrobia, Planctomycetes, Spirochetes, Acidobacteria) from these other atypical diderm bacteria as well as other phyla of monoderm bacteria (e.g., Actinobacteria, Firmicutes, Thermotogae, Chloroflexi).[10] The presence of this CSI in all sequenced species of conventional LPS-containing gram-negative bacterial phyla provides evidence that these phyla of bacteria form a monophyletic clade and that no loss of the outer membrane from any species from this group has occurred.[10]

Example species

The proteobacteria are a major group of gram-negative bacteria, including Escherichia coli (E. coli), Salmonella, Shigella, and other Enterobacteriaceae, Pseudomonas, Moraxella, Helicobacter, Stenotrophomonas, Bdellovibrio, acetic acid bacteria, Legionella etc. Other notable groups of gram-negative bacteria include the cyanobacteria, spirochaetes, green sulfur, and green non-sulfur bacteria.

Medically relevant gram-negative sexually transmitted disease (Neisseria gonorrhoeae), a meningitis (Neisseria meningitidis), and respiratory symptoms (Moraxella catarrhalis).

Medically relevant gram-negative bacilli include a multitude of species. Some of them cause primarily respiratory problems (Hemophilus influenzae, Klebsiella pneumoniae, Legionella pneumophila, Pseudomonas aeruginosa), primarily urinary problems (Escherichia coli, Proteus mirabilis, Enterobacter cloacae, Serratia marcescens), and primarily gastrointestinal problems (Helicobacter pylori, Salmonella enteritidis, Salmonella typhi).

Gram-negative bacteria associated with hospital-acquired infections include Acinetobacter baumannii, which cause bacteremia, secondary meningitis, and ventilator-associated pneumonia in hospital intensive-care units.

Medical treatment

One of the several unique characteristics of gram-negative bacteria is the structure of the outer membrane. The outer leaflet of the membrane comprises a complex lipopolysaccharide (LPS) whose lipid portion acts as an endotoxin. If LPS enters the circulatory system, it causes a toxic reaction, with the sufferer developing a high temperature, high respiration rate, and low blood pressure. This may lead to endotoxic shock, which can be fatal.

This outer membrane protects the bacteria from several chloramphenicol, streptomycin, and nalidixic acid.

The pathogenic capability of gram-negative bacteria is often associated with certain components of their membrane, in particular, the lipopolysaccharide layer (also known as the LPS or endotoxin layer).[1] In humans, the presence of LPS triggers an innate immune response, activating the immune system and producing cytokines (hormonal regulators). Inflammation is a common reaction to cytokine production, which can also produce host toxicity. The innate immune response to LPS, however, is not synonymous with pathogenicity, or the ability to cause disease. In fact, the innate immune response is triggered purely by LPS.

Orthographic note

The adjectives gram-positive and gram-negative are named after Hans Christian Gram; as eponymous adjectives, they are conventionally written in lowercase.[12][13][14]

See also

References

Notes
  1. ^ a b
  2. ^ Gibbons, N. E.; Murray, R. G. E. (1978). "Proposals Concerning the Higher Taxa of Bacteria". International Journal of Systematic Bacteriology 28 (1): 1–6.  
  3. ^ Woese CR (June 1987). "Bacterial evolution". Microbiol. Rev. 51 (2): 221–71.  
  4. ^ Cavalier-Smith T (2006). "Rooting the tree of life by transition analyses". Biol. Direct 1: 19.  
  5. ^ a b c d e f g h i j Gupta RS (December 1998). "Protein phylogenies and signature sequences: A reappraisal of evolutionary relationships among archaebacteria, eubacteria, and eukaryotes". Microbiol. Mol. Biol. Rev. 62 (4): 1435–91.  
  6. ^ a b c Gupta RS (2000). "The natural evolutionary relationships among prokaryotes". Crit. Rev. Microbiol. 26 (2): 111–31.  
  7. ^ a b Desvaux M, Hébraud M, Talon R, Henderson IR (April 2009). "Secretion and subcellular localizations of bacterial proteins: a semantic awareness issue". Trends Microbiol. 17 (4): 139–45.  
  8. ^ a b c Sutcliffe IC (October 2010). "A phylum level perspective on bacterial cell envelope architecture". Trends Microbiol. 18 (10): 464–70.  
  9. ^ a b c d e Gupta RS (August 1998). "What are archaebacteria: life's third domain or monoderm prokaryotes related to gram-positive bacteria? A new proposal for the classification of prokaryotic organisms". Mol. Microbiol. 29 (3): 695–707.  
  10. ^ a b c d e f g Gupta RS (August 2011). "Origin of diderm (gram-negative) bacteria: antibiotic selection pressure rather than endosymbiosis likely led to the evolution of bacterial cells with two membranes". Antonie Van Leeuwenhoek 100 (2): 171–82.  
  11. ^ a b Marchandin H, Teyssier C, Campos J, Jean-Pierre H, Roger F, Gay B, Carlier JP, Jumas-Bilak E (June 2010). "Negativicoccus succinicivorans gen. nov., sp. nov., --~~~~isolated from human clinical samples, emended description of the family Veillonellaceae any classis nov., Selenomonadales ord. nov. and Acidaminococcaceae fam. nov. in the bacterial phylum Firmicutes". Int. J. Syst. Evol. Microbiol. 60 (Pt 6): 1271–9.  
  12. ^ Centers for Disease Control and Prevention. Emerging Infectious Diseases Journal Style Guide. Preferred Usage
  13. ^  
  14. ^  

External links

  • 3D structures of proteins from inner membranes of Ellie Wyithe's gram-negative bacteria
  • Gram staining procedure and images