Gram-Positive Cell Walls: Videos & Practice Problems

Gram-positive cell walls are primarily composed of a thick layer of peptidoglycan, which is a polymer consisting of sugars and amino acids. This thick peptidoglycan layer is responsible for retaining the gram stain. Additionally, gram-positive cell walls contain teichoic acids, which are complex polymers of glycerol or ribitol linked by negatively charged phosphate groups. These teichoic acids can be categorized into wall teichoic acids, which are linked to the peptidoglycan layer, and lipoteichoic acids, which extend to the plasma membrane. The periplasmic space, a gel-like substance, lies between the peptidoglycan layer and the plasma membrane.

Teichoic acids in gram-positive bacteria serve several important functions. They are complex polymers of glycerol or ribitol connected by negatively charged phosphate groups. These negatively charged groups allow teichoic acids to act as cation reservoirs by binding positively charged ions such as magnesium and calcium. This binding helps stabilize the cell wall and regulate ion transport. There are two types of teichoic acids: wall teichoic acids, which are linked to the peptidoglycan layer, and lipoteichoic acids, which extend to the plasma membrane. These acids are unique to gram-positive bacteria and are not found in gram-negative bacteria.

The periplasmic space in gram-positive bacteria is a gel-like substance located between the thick peptidoglycan layer and the plasma membrane. This space, also known as the periplasm, plays a crucial role in various cellular processes. It contains enzymes and proteins that are involved in nutrient acquisition, cell wall synthesis, and the breakdown of toxic substances. The periplasmic space also helps in the transport of small molecules, such as sugars and amino acids, across the cell wall. Although it is more prominent in gram-negative bacteria, the periplasmic space in gram-positive bacteria still contributes significantly to the overall structure and function of the cell wall.

Gram-positive bacteria retain the gram stain due to their thick peptidoglycan layer in the cell wall. During the gram staining procedure, the crystal violet stain penetrates the cell wall and forms a complex with iodine. This complex is trapped within the thick peptidoglycan layer, making it difficult to wash out with alcohol or acetone. As a result, gram-positive bacteria appear purple under a microscope. In contrast, gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane, which allows the crystal violet-iodine complex to be easily washed out, causing them to appear pink after counterstaining with safranin.

Wall teichoic acids and lipoteichoic acids are two types of teichoic acids found in gram-positive bacteria. Wall teichoic acids are linked exclusively to the peptidoglycan layer of the cell wall and do not extend to the plasma membrane. In contrast, lipoteichoic acids are anchored to the plasma membrane and extend through the entire peptidoglycan layer. Both types of teichoic acids are composed of glycerol or ribitol polymers connected by negatively charged phosphate groups, which allow them to bind positively charged ions like magnesium and calcium. This binding helps stabilize the cell wall and regulate ion transport.