Gram-negative bacteria
Say it: GRAM NEG-uh-tiv bak-TEER-ee-uh
Gram-negative bacteria have a thin peptidoglycan wall wrapped in an outer membrane, so they lose the purple dye in a Gram stain and look pink.
Also: Gram-negative bacterium, gram-negative, Gram- bacteria
Definition
What are Gram-negative bacteria?
Gram-negative bacteria are bacteria that turn pink or red in a Gram stain. Their cell envelope has three layers: the inner plasma membrane, a thin sheet of peptidoglycan in a gel-filled space called the periplasm, and an outer membrane. During staining, alcohol washes the crystal violet out of the thin wall, so the cells take up the red counterstain safranin instead.
The outer face of the outer membrane is rich in lipopolysaccharide, or LPS. Its lipid A portion acts as an endotoxin: released from bacteria in the body, it can trigger fever and inflammation and, in severe infections, shock. The outer membrane also works as a barrier that keeps many antibiotics out, one reason these bacteria are often harder to treat. The sugar side chains of LPS vary between strains, and labs use them to tell strains apart.
Well-known Gram-negative bacteria include Escherichia coli, Salmonella, Pseudomonas aeruginosa, Klebsiella, Acinetobacter and the bacterium that causes gonorrhea. In hospitals they cause pneumonia and bloodstream, wound and urinary tract infections, and some strains now resist most or all antibiotics, including carbapenems.
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Terms that go with it
- Gram stainThe Gram stain is a lab staining method that sorts bacteria into Gram-positive (purple) and Gram-negative (pink) groups based on their cell walls.
- Gram-positive bacteriaGram-positive bacteria have a thick peptidoglycan cell wall and no outer membrane, so they keep the purple crystal violet dye in a Gram stain.
- PeptidoglycanPeptidoglycan is the mesh-like polymer of sugars and short peptides that forms the bacterial cell wall and gives bacteria their shape and strength.
- Antibiotic resistanceAntibiotic resistance is the ability of bacteria to survive and multiply despite an antibiotic that once killed them or stopped their growth.
- SepsisSepsis is a life-threatening emergency in which the body's reaction to infection runs out of control and damages its own tissues and organs.
- BacteriumA bacterium is a single-celled prokaryote with no nucleus, found in almost every habitat on Earth, including in and on the human body.
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Written for biofideX in plain words, after checking these references:
- OpenStax Microbiology, 3.3 Unique Characteristics of Prokaryotic Cells
- CDC: About Gram-negative Bacteria
- MedlinePlus Lab Tests: Gram Stain
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