Gram-negative

Definition

Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane containing lipopolysaccharides. They do not retain the crystal violet stain used in the Gram staining procedure, appearing pink or red under a microscope.

5 Must Know Facts For Your Next Test

1. Gram-negative bacteria are characterized by their double-membrane structure.
2. They are often more resistant to antibiotics due to their outer membrane.
3. Examples include Escherichia coli and Salmonella.
4. The lipopolysaccharides in their outer membrane can trigger strong immune responses in humans.
5. They appear pink or red after Gram staining because they do not retain the crystal violet dye.

Review Questions

• Why do Gram-negative bacteria appear pink or red under a microscope?
• What structural feature makes Gram-negative bacteria more resistant to antibiotics?
• Name two examples of Gram-negative bacteria.

Related terms

Gram-positive:

Bacteria with a thick peptidoglycan layer that retains the crystal violet stain, appearing purple under a microscope.

Peptidoglycan: A polymer that forms a mesh-like layer outside the plasma membrane of most bacteria, providing structural support.

Lipopolysaccharide: Molecules found in the outer membrane of Gram-negative bacteria that can elicit strong immune responses.