Notes on Bacterial Cell Walls, Gram-Positive vs Gram-Negative, and Antibiotics

Gram-positive bacteria

Bacteria with a cell membrane and a very thick peptidoglycan layer, lacking an outer membrane. They retain crystal violet stain and appear purple.

Gram-negative bacteria

Bacteria with an inner (plasma) membrane, a thin peptidoglycan layer, and an outer membrane. They lose crystal violet but take up counterstain, appearing pink/red.

Peptidoglycan

A unique bacterial polymer composed of repeating disaccharide units (MurNAc and GlcNAc) cross-linked by short amino acid chains (tetrapeptides). It forms the bacterial cell wall and is a primary drug target.

Lipopolysaccharide (LPS)

A molecule found on the outer membrane of Gram-negative bacteria, consisting of O polysaccharide, core polysaccharide, and Lipid A. Lipid A acts as an endotoxin.

Lipid A

The lipid portion of LPS embedded in the outer membrane of Gram-negative bacteria, responsible for endotoxin activity and can trigger fever and sepsis.

Teichoic acids

Polymers found in Gram-positive bacteria, including wall teichoic acids and lipoteichoic acids, which reinforce structure, provide charge, and act as antigenic surface features.

Periplasmic space

The region between the inner and outer membranes in Gram-negative bacteria, containing various enzymes and transport proteins.

Porins

Large, pore-forming proteins in the outer membrane of Gram-negative bacteria that allow the diffusion of many substances into the periplasm, contributing to selective permeability.

Beta-lactam antibiotics

A class of antibiotics (e.g., penicillins, cephalosporins) that inhibit the cross-linking of peptidoglycan, weakening the cell wall and causing osmotic instability and lysis.

Mycoplasmas

Bacteria that lack a cell wall entirely, making them pleomorphic and requiring membrane reinforcement with sterols. Beta-lactam antibiotics are ineffective against them.

Mycobacteria

Bacteria with a thick, waxy cell wall rich in mycolic acids, which makes them resist Gram staining. They are acid-fast positive and require specialized staining and treatment.

Gram-positive bacteria

Bacteria with a cell membrane and a very thick, multilayered peptidoglycan wall, often containing teichoic acids and lipoteichoic acids, but lacking an outer membrane.

Gram-negative bacteria

Bacteria with an inner (plasma) membrane, a thin peptidoglycan layer, and a second outer membrane that contains porins and lipopolysaccharide (LPS). They have a true periplasmic space.

Peptidoglycan

A unique bacterial polymer forming the cell wall, composed of repeating disaccharide units (MurNAc and GlcNAc) cross-linked by short amino acid chains (tetrapeptides). It provides structural strength and is a primary drug target.

Lipopolysaccharide (LPS)

A molecule found on the outer membrane of Gram-negative bacteria, consisting of an O polysaccharide, a core polysaccharide, and Lipid A. It functions as an endotoxin.

Lipid A

The lipid portion of LPS embedded in the outer membrane of Gram-negative bacteria, responsible for its endotoxin activity and ability to trigger fever and cytokine storm.

Porins

Large, pore-forming proteins in the outer membrane of Gram-negative bacteria that allow the diffusion of many substances into the periplasm.

Teichoic Acids

Polymers found in the cell wall of Gram-positive bacteria, including wall teichoic acids (extend outside peptidoglycan) and lipoteichoic acids (anchor peptidoglycan to the cell membrane), contributing to structure, charge, and antigenicity.

Beta-Lactam Antibiotics

A class of antibiotics (e.g., penicillins, cephalosporins) that inhibit the cross-linking of peptidoglycan, weakening the bacterial cell wall and leading to osmotic instability and lysis.

Mycoplasmas

Bacteria that lack a cell wall entirely, making them pleomorphic and resistant to cell-wall-targeting antibiotics like beta-lactams. They reinforce their membranes with sterols.

Mycobacteria

Bacteria characterized by a thick, waxy cell wall rich in mycolic acids, which makes them resistant to Gram staining and requires acid-fast staining for visualization.

Gram Stain

A differential staining technique that separates bacteria into Gram-positive (purple) and Gram-negative (pink/red) groups based on their cell wall architecture.

Acid-Fast Stain

A staining method used for bacteria like Mycobacteria that have waxy walls (mycolic acids), causing them to appear bright pink/fuchsia.