Definition
Gram-negative bacteria are prokaryotes that do not retain the crystal violet–iodine complex during the Gram staining procedure, appearing pink after counterstaining. This staining reaction is due to a cell wall structure featuring a thin peptidoglycan layer and an outer membrane rich in lipopolysaccharide.
Explanation
The cell envelope of Gram-negative bacteria is composed of an inner cytoplasmic membrane, a periplasmic space with a single layer of peptidoglycan and an outer membrane. The outer membrane contains lipopolysaccharide (LPS), phospholipids and porin proteins, which confer permeability barriers and contribute to virulence. During Gram staining, the alcohol or acetone decolorizer dissolves the outer membrane and allows the crystal violet complex to escape through the thin peptidoglycan, so a counterstain such as safranin colours the cells pink. Gram-negative organisms also have periplasmic enzymes, such as β-lactamases, that degrade certain antibiotics, and their outer membrane limits penetration of large molecules, making them intrinsically more resistant to some antimicrobial agents compared with Gram-positive bacteria. The LPS lipid A component acts as endotoxin, triggering inflammatory responses and septic shock when released into the host circulation. Gram-negative bacteria inhabit diverse environments, including soil, water and the intestines of animals. In clinical microbiology, recognizing the Gram-negative cell wall aids in selecting appropriate culture conditions and therapeutic strategies.
Notable examples and facts
Important Gram-negative pathogens include Escherichia coli and Klebsiella pneumoniae (members of the Enterobacterales), Pseudomonas aeruginosa (an opportunistic waterborne organism), Neisseria meningitidis and Neisseria gonorrhoeae (diplococci causing meningitis and gonorrhoea), and Vibrio cholerae (the cause of cholera). The obligate intracellular bacteria Rickettsia and Chlamydia also have Gram-negative envelopes but may stain poorly. Bacteroides species are abundant anaerobes in the human gut and are significant in intra-abdominal infections. Endotoxin released from Gram-negative bacteria is responsible for many features of septic shock, including fever, hypotension and disseminated intravascular coagulation. Identification of Gram-negative organisms in clinical specimens often involves additional tests such as oxidase reactions, carbohydrate fermentation patterns and molecular assays.
Gram-negative bacteria constitute a large and diverse group distinguished by their cell wall architecture. Understanding their structural and physiological traits informs diagnostics, infection control and antimicrobial therapy.
Related Terms: Gram Positive, Gram Stain, Peptidoglycan, Lipopolysaccharide, Outer membrane