Gram-negative bacteria

Microscopic image of gram-negative Pseudomonas aeruginosa bacteria (pink-red rods)
Gram-negative cell wall structure
Gram-positive and -negative bacteria are differentiated chiefly by their cell wall structure

Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation.

- Gram-negative bacteria

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Bacterial outer membrane

Structure of gram-negative cell envelope

The bacterial outer membrane is found in gram-negative bacteria.


Structure of a lipopolysaccharide (LPS)
The saccharolipid Kdo2-Lipid A. Kdo residues in (core), glucosamine residues in, acyl chains in black and phosphate groups in.
LPS final assembly: O-antigen subunits are translocated across the inner membrane (by Wzx) where they are polymerized (by Wzy, chain length determined by Wzz) and ligated (by WaaL) on to complete Core-Lipid A molecules (which were translocated by MsbA).
LPS transport: Completed LPS molecules are transported across the periplasm and outer membrane by the proteins LptA, B, C, D, E, F, and G
Toll-like receptors of the innate immune system recognize LPS and trigger an immune response.

Lipopolysaccharides (LPS) are large molecules consisting of a lipid and a polysaccharide composed of O-antigen, outer core and inner core joined by a covalent bond; they are found in the outer membrane of Gram-negative bacteria.

Chlamydia trachomatis

Bacterium that causes chlamydia, which can manifest in various ways, including: trachoma, lymphogranuloma venereum, nongonococcal urethritis, cervicitis, salpingitis, pelvic inflammatory disease.

Life cycle

Chlamydia trachomatis is a gram-negative bacterium that can replicate only within a host cell.


2-deoxystrept-amine, 2D representation, oxygens, nitrogens (with attached hydrogens) in red, blue.
Streptomycin in complex with a bacterial ribosome. X-ray crystallographic structure of the 30S ribosomal subunit with bound drug (purple, space-filling model, at center) protein secondary structure elements such as alpha-helices in bright green, and the RNA phosphodiester backbone shown in orange (and the ladder of base pairs in dark green and blue)
Kanamycin A
Neomycins B, C
Neomycin E (paromomycin)

Aminoglycoside is a medicinal and bacteriologic category of traditional Gram-negative antibacterial medications that inhibit protein synthesis and contain as a portion of the molecule an amino-modified glycoside (sugar).

Cell envelope

The cell envelope comprises the inner cell membrane and the cell wall of a bacterium.

Schematic of typical gram-positive cell wall showing arrangement of N-Acetylglucosamine and N-Acetylmuramic acid; Teichoic acids not shown.
Schematic of typical gram-negative cell wall showing arrangement of N-Acetylglucosamine and N-Acetylmuramic acid and the outer membrane containing lipopolysaccharide.

In gram-negative bacteria an outer membrane is also included.

Gram-positive bacteria

Traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall.

Rod-shaped gram-positive Bacillus anthracis bacteria in a cerebrospinal fluid sample stand out from round white blood cells, which also accept the crystal violet stain.
Violet-stained gram-positive cocci and pink-stained gram-negative bacilli
Gram-positive and gram-negative cell wall structure
Structure of gram-positive cell wall
The structure of peptidoglycan, composed of N-acetylglucosamine and N-acetylmuramic acid
Colonies of a gram-positive pathogen of the oral cavity, Actinomyces sp.

Conversely, gram-negative bacteria cannot retain the violet stain after the decolorization step; alcohol used in this stage degrades the outer membrane of gram-negative cells, making the cell wall more porous and incapable of retaining the crystal violet stain.

Gram stain

A Gram stain of mixed Staphylococcus aureus (S. aureus ATCC 25923, Gram-positive cocci, in purple) and Escherichia coli (E. coli ATCC 11775, Gram-negative bacilli, in red), the most common Gram stain reference bacteria
Gram stain of Candida albicans from a vaginal swab. The small oval chlamydospores are 2–4 µm in diameter.
Purple-stained gram-positive (left) and pink-stained gram-negative (right)

Gram stain or Gram staining, also called Gram's method, is a method of staining used to classify bacterial species into two large groups: gram-positive bacteria and gram-negative bacteria.


Breaking down of the membrane of a cell, often by viral, enzymic, or osmotic (that is, "lytic" ) mechanisms that compromise its integrity.


If the cell wall is completely lost and the penicillin was used on gram-positive bacteria, then the bacterium is referred to as a protoplast, but if penicillin was used on gram-negative bacteria, then it is called a spheroplast.


The cephalosporins (sg.

Core structure of the cephalosporins
Structure of the classical cephalosporins

Successive generations of cephalosporins have increased activity against Gram-negative bacteria, albeit often with reduced activity against Gram-positive organisms.

Septic shock

Potentially fatal medical condition that occurs when sepsis, which is organ injury or damage in response to infection, leads to dangerously low blood pressure and abnormalities in cellular metabolism.

Sepsis is one of the most common causes of death in critically ill patients in Intensive Care Units. (Oil by Gabriël Metsu).
Thrombocytopenia with purpura on right hand in patient with septic shock

Most cases of septic shock are caused by gram-positive bacteria, followed by endotoxin-producing gram-negative bacteria, although fungal infections are an increasingly prevalent cause of septic shock.