Gram-Positive vs. Gram-Negative Cell Walls: Videos & Practice Problems

Gram-positive and gram-negative bacteria have distinct cell wall structures. Gram-positive bacteria have a thick peptidoglycan layer and teichoic acids, which provide rigidity and structural support. In contrast, gram-negative bacteria have a thin peptidoglycan layer, an outer membrane containing lipopolysaccharides (LPS), and porins. The outer membrane in gram-negative bacteria contributes to their resistance to certain antibiotics and detergents. Additionally, the presence of LPS in gram-negative bacteria is associated with increased pathogenicity. These structural differences are crucial for the bacteria's interaction with their environment and their response to antibiotics.

Gram-negative bacteria have an outer membrane to provide an additional protective barrier. This outer membrane contains lipopolysaccharides (LPS), which contribute to the bacteria's pathogenicity and can trigger strong immune responses in hosts. The outer membrane also contains porins, which are protein channels that allow the passage of small molecules and nutrients while restricting harmful substances. This structure helps gram-negative bacteria resist certain antibiotics and detergents, making them more resilient in hostile environments compared to gram-positive bacteria.

The peptidoglycan layer in gram-positive bacteria is significantly thicker than in gram-negative bacteria. In gram-positive bacteria, the peptidoglycan layer can be up to 20-80 nm thick, providing substantial structural support and rigidity. In contrast, gram-negative bacteria have a much thinner peptidoglycan layer, typically around 2-7 nm thick. This thin layer is located between the inner plasma membrane and the outer membrane, which contains lipopolysaccharides (LPS) and porins. The thick peptidoglycan layer in gram-positive bacteria is a key feature that differentiates them from gram-negative bacteria.

Teichoic acids are important components of the cell wall in gram-positive bacteria. They are polymers of glycerol or ribitol phosphate and are embedded within the thick peptidoglycan layer. Teichoic acids play several roles, including maintaining cell wall structure, regulating cell growth, and providing rigidity. They also contribute to the bacteria's ability to adhere to surfaces and interact with the host's immune system. There are two types of teichoic acids: wall teichoic acids, which are linked to the peptidoglycan, and lipoteichoic acids, which are anchored to the plasma membrane. These acids are absent in gram-negative bacteria.

Lipopolysaccharides (LPS) are crucial components of the outer membrane in gram-negative bacteria. LPS molecules consist of a lipid A, a core polysaccharide, and an O-antigen. Lipid A is responsible for the toxic effects of LPS, making it an endotoxin that can trigger strong immune responses in hosts. The core polysaccharide and O-antigen contribute to the structural integrity and surface properties of the bacteria. LPS plays a significant role in the pathogenicity of gram-negative bacteria, helping them evade the host's immune system and resist certain antibiotics. This makes infections caused by gram-negative bacteria particularly challenging to treat.