Question

Anthrax is caused by a bacterium, Bacillus anthracis, that avoids the body's defenses against disease by synthesizing an outer glycoprotein covering made from D-glutamic acid. This covering is not digestible by white blood cells that normally engulf bacteria. Why is the covering indigestible?

Short answer

The outer glycoprotein covering of Bacillus anthracis is synthesized from D-glutamic acid. Human enzymes, including those in white blood cells, are adapted to breaking down and digesting L- amino acids, not the D- form. This makes the bacterium's covering indigestible to white blood cells.

Step-by-step solution

Understanding the role of white blood cells

White blood cells, also known as leukocytes, serve the body's immune system by fighting off foreign substances like bacteria and viruses. They do this through a process called phagocytosis, where they engulf and digest harmful entities.

Exploring the properties of D-glutamic acid

D-glutamic acid is a type of amino acid. One of its unique properties is its stereochemistry. The D- form of amino acids, including glutamic acid, are not typically recognized or utilized by human enzymes, which are adapted to handle L- version of amino acids.

Connecting the bacterium's strategy to its survival

Bacillus anthracis synthesizes a glycoprotein covering from D-glutamic acid. Since human enzymes (including those in white blood cells) are not adapted to breaking down D- amino acids, this makes the bacterium's outer covering indigestible to white blood cells, allowing it to circumvent the body's immune defenses.

Key concepts

Understanding Bacillus anthracis

The bacterial species Bacillus anthracis is notorious as the causative agent of anthrax, a serious infectious disease that can affect both humans and animals. This bacterium is rod-shaped, and under stressful conditions, can produce protective spores that allow it to survive in a dormant state for years until conditions are favorable for growth.

The virulence of Bacillus anthracis stems not only from its ability to form spores but also from its capacity to secrete lethal toxins and to avoid being destroyed by the host's immune system. One of its primary defense mechanisms is the production of an antiphagocytic capsule. This capsule, made of the amino acid D-glutamic acid, serves as a protective shield, effectively rendering the bacterium invisible to the phagocytes of the immune system.

Evasion of the Immune System

The immune system is adept at identifying and neutralizing pathogens through various mechanisms, including phagocytosis. However, Bacillus anthracis employs a cunning strategy to evade this system. It synthesizes a unique outer glycoprotein covering, or capsule, that is composed of D-glutamic acid.

Unlike the L-isomers of amino acids that make up the proteins in the human body, D-isomers are not commonly recognized by human enzymes. This means that the usual processes the body uses to break down and digest proteins are ineffective against this D-amino acid polymer. As a result, the capsule is resistant to the digestive enzymes of phagocytic white blood cells, letting the bacterium evade destruction and continue to proliferate within its host.

The Role of Phagocytosis

Phagocytosis is a critical process performed by white blood cells, which are essential components of the immune system. During phagocytosis, these cells, primarily macrophages and neutrophils, engulf and digest pathogens and dead or damaged cells.

The process involves several steps: recognition and attachment of the white blood cell to the target, ingestion of the target into a specially formed cellular vesicle called a phagosome, fusion of the phagosome with a lysosome to form a phagolysosome, and finally, digestion of the ingested material by enzymes. Bacillus anthracis hinders this process by presenting an indigestible capsule, thereby disrupting the crucial step where the pathogen would typically be digested and neutralized.