The pathogenicity of bacteria causing tuberculosis and leprosy is due to Wax-D. Wax-D, also known as mycolic acid, is a key component of the cell wall of these bacteria, specifically Mycobacterium tuberculosis and Mycobacterium leprae. The cell wall of these bacteria is unique and plays a crucial role in their ability to cause disease.

1. Cell Wall Composition:
The cell wall of Mycobacterium tuberculosis and Mycobacterium leprae is primarily composed of a complex lipid called mycolic acid or Wax-D. This lipid is long and waxy, giving the bacteria their characteristic acid-fast staining property. Mycolic acid makes up a significant proportion of the cell wall and provides structural integrity to the bacteria.

2. Protection from Host Immune System:
The presence of Wax-D in the cell wall of these bacteria provides protection against the host immune system. Wax-D helps in resisting the actions of antimicrobial agents, such as antibiotics, and prevents the entry of toxic molecules into the bacteria. It also contributes to the formation of a thick, impermeable barrier that makes it difficult for the immune system to recognize and eliminate the bacteria.

3. Role in Virulence:
The presence of Wax-D in the cell wall is essential for the virulence of Mycobacterium tuberculosis and Mycobacterium leprae. It allows the bacteria to survive and replicate within the host, leading to the development of tuberculosis and leprosy, respectively. The wax-like nature of mycolic acid helps the bacteria to persist in the host for prolonged periods, leading to chronic infections.

4. Resistance to Antibiotics:
The unique composition of the cell wall, including Wax-D, contributes to the antibiotic resistance observed in these bacteria. The presence of mycolic acid hinders the penetration of antibiotics into the bacterial cell, making them less susceptible to treatment. This resistance is a significant challenge in the treatment of tuberculosis and leprosy.

In conclusion, the pathogenicity of bacteria causing tuberculosis and leprosy is primarily due to the presence of Wax-D or mycolic acid in their cell walls. This lipid provides structural integrity, protection from the host immune system, and contributes to antibiotic resistance, allowing the bacteria to cause disease and evade treatment.