Gram-negative

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Gram-negative bacteria are those that do not retain crystal violet dye in the Gram staining protocol. Gram-positive bacteria will retain the dark blue dye after an alcohol wash, whereas Gram-negative do not. In a Gram stain test, a counterstain is added after the crystal violet, which colors all Gram-negative bacteria a red or pink color. The test itself is useful in classifying two distinctly different types of bacteria based on structural differences in their cell walls.

Many species of Gram-negative bacteria are pathogenic, meaning they can cause disease in a host organism. This pathogenic capability is usually associated with certain components of Gram-negative cell walls, in particular the lipopolysaccharide (also known as LPS or endotoxin) layer.

1 Characteristics
2 Example species
3 The outer membrane
4 References

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Characteristics

The following characteristics are displayed by Gram-negative bacteria:

Cell walls only contain a few layers of peptidoglycan (which is present in much higher levels in Gram-positive bacteria)
Cells are surrounded by an outer membrane of lipopolysaccharide outside the peptidoglycan layer
There is a space between the layers of peptidoglycan and the secondary cell membrane called the periplasmatic space
The S-layer is directly attached to the outer membrane, rather than the peptidoglycan
If present, flagella have four supporting rings instead of two
No teichoic acids are present

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Example species

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

Medically relevant Gram-negative cocci include three organisms, which cause a sexually transmitted disease (Neisseria gonorrhea), a meningitis (Neisseria meningitidis), and respiratory symptoms (Moraxella catarrhalis).

Medically relevant Gram-negative bacilli include a multitude of species. Some of them primarily cause 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).

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The outer membrane

One of the several unique characteristics of gram-negative bacteria is the outer membrane. This outer membrane is responsible for protecting the bacteria from several antibiotics, dyes, and detergents which would normally damage the inner membrane or cell wall (peptidoglycan). Because of the outer membrane, these bacteria are resistant to lysozyme and penicillin. Fortunately, alternative medicinal treatments such as lysozyme with EDTA and ampicillin have been developed to combat the protective outer membrane of pathogenic Gram-negatives.

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