Gramnegatif

Gram-negative bacteria are a broad group of microorganisms that do not retain the crystal violet stain during the Gram staining procedure and instead appear pink after counterstaining with safranin. This staining pattern reflects fundamental differences in their cell envelope compared with Gram-positive bacteria.

The Gram-negative cell envelope consists of an inner cytoplasmic membrane, a thin layer of peptidoglycan, and an outer membrane that contains lipopolysaccharide (LPS). The outer membrane and the periplasmic space between the membranes influence the bacterial interaction with the environment and contribute to intrinsic resistance to many antibiotics. Porins in the outer membrane regulate the movement of ions and molecules into and out of the cell.

Taxonomically, Gram-negative bacteria include numerous phyla and many medically important genera. Notable examples are Escherichia, Salmonella, Vibrio, and Pseudomonas within the Proteobacteria, as well as other groups such as Neisseria and Bacteroides. They inhabit diverse habitats, ranging from soil and water to animal and human hosts. In humans, Gram-negative bacteria can be harmless residents of the gut or skin, but several species are notable pathogens, causing meningitis, pneumonia, urinary tract infections, gastroenteritis, sepsis, and sexually transmitted infections.

Lipopolysaccharide, a component of the outer membrane, can act as an endotoxin and contribute to inflammatory responses in hosts. Treatment of Gram-negative infections is complicated by multidrug resistance in many species. Despite clinical challenges, Gram staining remains a rapid, foundational tool for initial bacterial classification and guides subsequent diagnostic testing.