Mycolic acids are waxy lipid molecules that form an external layer in the cell walls of certain gram-positive bacteria, such as those in the family Mycobacteriaceae. Because of this waxy layer, specialized acid-fast stains must be used to penetrate the cell wall for microscopy, leading to these bacteria being referred to as acid-fast.

Mycolic Acids

Teichoic acids (TAs) are carbohydrate chains commonly embedded within the peptidoglycan layers of gram-positive bacteria, extending through and beyond the cell wall. TAs function to stabilize the peptidoglycan layer by increasing its rigidity. Additionally, in pathogenic gram-positive bacteria like *Streptococcus*, TAs enhance the ability to cause infection by binding to specific proteins on the surface of host cells.

Teichoic Acids (TAs)

In gram-positive bacteria, the cell wall consists of a thick peptidoglycan layer, totaling $${}30-100$$ nm in thickness. Within this thick layer, the N-acetylmuramic acid (NAM) molecules are connected by tetrapeptide chains linked via pentaglycine cross-bridges. This differs from gram-negative bacteria, where the tetrapeptide chains extending from each NAM unit are directly cross-linked. The peptidoglycan layers in gram-positive bacteria are commonly embedded with teichoic acids (TAs) on the external side, which increase the rigidity of the cell wall.

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Types of Bacterial Cell Walls

Gram positive bacteria have a specific cell wall with a thick peptidoglycan layer. They have been identified from the Gram stain because of their cell wall retains the Gram stain. They have also been further classified by their genomes: 
- High G+C gram positive bacteria
- Low G+C gram positive bacteria

Gram Positive Bacteria

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Gram Positive Cell Walls

Gram-negative cells possess a cell envelope with a significantly thinner layer of peptidoglycan—no more than about $${}4$$ nm thick—compared to gram-positive cells. Their overall cell envelope structure is highly complex, featuring a periplasmic space and an outer membrane located external to the peptidoglycan layer.

Gram Negative Cell Walls

The class *Actinobacteria* is also called the high G+C gram positive bacteria because their genomes have over 50% guanine and cytosine base pairs. While their class is very diverse in size and biomes they inhabit, one of their important features is that they have distinct and different peptidoglycans in their cell walls.

 High G+C gram positive bacteria: *Actinobacteria*

Low G+C gram positive bacteria have less than 50% cytosines and guanines their genomes. This group of bacteria also contains many pathogens. They consist of many different bacteria: 
- Clostridia
- Lactobacillales
- Bacilli
- Mycoplasmas 

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