gramnegative

Gramnegative refers to bacteria that do not retain the crystal violet-iodine complex during the Gram staining procedure, and therefore appear pink or red after counterstaining with safranin. They are contrasted with Gram-positive bacteria, which retain the violet stain. The term describes a staining phenotype rather than a single lineage.

In Gram-negative bacteria, the cell envelope comprises an inner cytoplasmic membrane, a thin peptidoglycan layer, and an external outer membrane rich in lipopolysaccharide. The outer membrane and the periplasmic space act as a barrier and host several enzymes and transport proteins, including porins, which influence permeability to nutrients and antibiotics.

During Gram staining, crystal violet-iodine is applied and fixed. Alcohol decolorization dissolves the outer membrane and dehydrates the thin peptidoglycan, causing the cells to lose the violet stain. A counterstain (usually safranin) then colors them pink or red. This differential staining helps distinguish Gram-negative from Gram-positive bacteria in laboratory workflows.

Clinically, Gram-negative bacteria are significant due to their endotoxin, lipopolysaccharide, and their relatively permeable outer membrane, which influence susceptibility to antibiotics. They include diverse groups such as Enterobacterales (Escherichia, Salmonella), Pseudomonadota like Pseudomonas, and Bacteroidota. They are common causes of urinary tract infections, pneumonia, and sepsis, among other infections.

The Gram stain was developed by Hans Christian Gram in 1884. “Gram-negative” describes a staining phenotype rather than a single taxonomic group, encompassing multiple phyla that share the characteristic outer membrane and thin peptidoglycan layer.