Bacillus anthracis is a highly pathogenic bacterium that causes disease. It is a rod-shaped, Gram-positive bacterium, measuring about 3 to 5 micrometers long and 1 to 1.2 micrometers wide. This bacterium is the sole cause of anthrax, a severe infectious disease that can affect both animals and humans.
The Nature of Bacillus anthracis
Bacillus anthracis appears as large, box-car shaped rods that often arrange in chains. A distinctive feature is its ability to form highly resilient spores, known as endospores. These endospores are metabolically inactive and possess a robust structure that allows them to endure harsh environmental conditions for extended periods, even decades.
The spores are the primary infectious form and are naturally found in soil worldwide. While they are dormant outside a host, these spores can become infective under suitable environmental conditions. Unlike many other Bacillus species, Bacillus anthracis is nonmotile.
How Bacillus anthracis Causes Disease
Its pathogenicity stems from its ability to produce powerful toxins once it enters a host. When spores are introduced into a host, they encounter favorable conditions, such as a temperature of around 37°C, carbon dioxide, and serum proteins, which trigger their germination into active, vegetative bacteria. This transition can occur rapidly. These multiplying bacteria then secrete virulence factors, including a protein capsule that helps them evade the host’s immune system by preventing ingestion by macrophages.
The most significant virulence factors are the anthrax toxins, which are encoded on two extrachromosomal DNA plasmids, pXO1 and pXO2. The pXO1 plasmid specifically encodes the three main components of anthrax toxin: protective antigen (PA), edema factor (EF), and lethal factor (LF). Individually, these proteins are not toxic, but they work together to exert their damaging effects within host cells.
The process begins when the protective antigen (PA) binds to specific receptors on the surface of host cells. Once bound, PA undergoes a change and forms a pre-pore, which then associates with the edema factor (EF) and/or lethal factor (LF). This entire complex is then internalized by the cell through a process called endocytosis. Inside an acidic compartment within the cell, the low pH causes a conformational change in the PA pre-pore, forming a channel that allows EF and LF to enter the cell’s cytoplasm.
Once inside the cytosol, the edema factor disrupts cellular signaling by elevating intracellular cyclic AMP, leading to severe edema or swelling. The lethal factor targets and cleaves signaling molecules within the cell, leading to cellular damage, tissue destruction, and ultimately, cell death, particularly in immune cells like macrophages. The combined actions of these toxins disrupt immune cell functions, allowing the bacteria to proliferate unchecked. This leads to systemic illness, multi-organ failure, and septic shock, which can be fatal if untreated.
Manifestations of Anthrax Disease
Anthrax disease can manifest in several forms, depending on the route by which Bacillus anthracis spores enter the body. The symptoms and severity vary significantly across these forms.
Cutaneous Anthrax
Cutaneous anthrax is the most common form, occurring when spores enter through a break in the skin, often from contact with infected animal products like wool or hides. Initially, a small, itchy sore develops, resembling an insect bite, which progresses into a blister and then a painless ulcer with a characteristic black center, known as an eschar. Swelling often surrounds the lesion, and while typically not painful, untreated cases can spread systemically and become fatal in about 20% of instances.
Inhalation Anthrax
Inhalation anthrax, also known as woolsorter’s disease, develops when spores are breathed into the lungs. This form is often severe and can be highly deadly. Initial symptoms can resemble a common cold or flu, including a sore throat, mild fever, cough, and muscle aches, which may briefly resolve. A more severe second stage follows, characterized by a rising fever, difficulty breathing, excessive sweating, and shock, often leading to significant lung damage and ultimately death if not promptly treated.
Gastrointestinal Anthrax
Gastrointestinal anthrax occurs when individuals consume undercooked meat contaminated with anthrax spores. Spores can infect any part of the digestive tract. Symptoms typically appear within a week and can include nausea, loss of appetite, vomiting (which may contain blood), abdominal pain, fever, and bloody diarrhea. This form can progress to severe stomach pain and internal ulceration, and if untreated, it has a high fatality rate, with over half of infected people dying.
Injection Anthrax
Injection anthrax is a less common form, primarily associated with injecting contaminated drugs. Symptoms are similar to cutaneous anthrax, with redness, swelling, and a cluster of small blisters or bumps at the injection site, which can develop into a black-centered sore. However, this form can lead to deep abscesses under the skin or in the muscle and can spread throughout the body more rapidly than cutaneous anthrax, making it more challenging to diagnose and treat.