Gram-Negative Bacteria

Gram-negative bacteria are bacteria that do not keep the violet dye of the Gram stain and instead appear pink or red. This reflects a distinctive cell envelope with a thin wall and an outer membrane, and the group includes many important pathogens.

A Gram-negative cell has an inner membrane, a thin layer of peptidoglycan in a space called the periplasm, and an outer membrane. The outer leaflet of that membrane is made of lipopolysaccharide (LPS), and protein channels called porins let small molecules through. Gram-positive bacteria lack the outer membrane and have a thick peptidoglycan wall that holds the stain, a difference found by Hans Christian Gram in 1884.

The outer membrane blocks many antibiotics and detergents, so Gram-negative infections are often more difficult to treat. LPS is an endotoxin, and when it enters the blood it triggers the intense inflammation of septic shock. Examples include Escherichia coli, Salmonella, Pseudomonas aeruginosa and Klebsiella, and many cause hospital-acquired infections.

Efflux pumps, porin changes and enzymes such as beta-lactamases make many Gram-negative species resistant, and resistance genes can pass between cells on plasmids. Carbapenem-resistant types are listed among the most serious threats by health agencies. See antimicrobial resistance and Escherichia coli.

• Bacteria — the domain they belong to
• Peptidoglycan — the thin wall layer
• Antimicrobial Resistance — a growing clinical problem