Gram-negative
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Bacteria that are Gram-negative are not stained dark blue or violet by Gram staining, in contrast to Gram-positive bacteria. On most Gram-stain preparations, Gram-negative organisms will appear red or pink because they are counterstained.
The difference lies in the cell wall of the two types; in contrast to most Gram-positive bacteria, Gram-negative bacteria have only a few layers of peptidoglycan and a secondary cell membrane made primarily of lipopolysaccharide. The space between the layers of peptidoglican and the secondary cell membrane is called periplasmatic space.
Many species of Gram-negative bacteria are pathogenic. This pathogenic capability is usually associated with certain components of their cell walls, particularly the lipopolysaccharide (endotoxin) layer.
The proteobacteria are a major group of Gram-negative bacteria, including for instance Escherichia coli, Salmonella, and other Enterobacteriaceae, Pseudomonas, Moraxella, Helicobacter, Stenotrophomonas, Bdellovibrio, acetic acid bacteria, Legionella and a great many others. Other notable groups of Gram-negative bacteria include the cyanobacteria, spirochaetes, green sulfur and green non-sulfur bacteria.
Two major subclassifications of Gram-negative bacteria are Gram-negative cocci and Gram-negative rods (bacilli), which they owe to their appearance under a microscope. Their shape has implications for medical antibacterial therapy.
Medically relevant Gram-negative cocci include 3 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 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).
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