The rod-shaped bacterium Bacillus anthracis is the causative agent of the severe zoonotic disease anthrax. This organism is definitively classified as a Gram-positive bacterium. It is notable for its capacity to form resistant dormant structures, which is fundamental to understanding its interaction with a host.
Understanding the Gram Stain Classification
The Gram stain procedure is a foundational method in microbiology used to differentiate bacteria into two major groups based on their cell wall characteristics. This technique involves applying crystal violet dye, followed by an iodine solution, a decolorizer, and finally a counterstain, typically safranin. The outcome of this test provides an important clue for bacterial identification and subsequent treatment choices.
Bacillus anthracis is categorized as Gram-positive because its cell structure allows it to retain the initial crystal violet stain, appearing purple when viewed under a microscope. This retention is due to the composition of the Gram-positive cell wall, which features a very thick, multilayered layer of peptidoglycan. This dense meshwork traps the crystal violet-iodine complex, preventing its removal during the alcohol-based decolorizing step.
In contrast, Gram-negative bacteria possess a much thinner peptidoglycan layer sandwiched between two membranes. The decolorizer easily washes the dye out of this less substantial structure, leaving the cells colorless until they pick up the pink or red counterstain.
Defining Features of Bacillus anthracis
The most defining feature of Bacillus anthracis is its ability to create endospores. These spores are not a form of reproduction but a dormant survival mechanism the bacterium employs when environmental conditions become unfavorable. The spore is an extremely dehydrated structure encased in multiple layers of protein coats, making it highly resistant to heat, desiccation, ultraviolet light, and chemical disinfectants.
This spore form is the infectious agent, allowing the organism to persist in the soil for decades until an animal or human is exposed. Once the spores enter a favorable host environment, they undergo germination and revert to the actively growing, or vegetative, bacterial cell. This vegetative form is responsible for causing disease and producing toxins.
The vegetative cell also possesses a unique antiphagocytic capsule, which contributes to its virulence. This capsule is composed of poly-D-glutamic acid (PDGA), an unusual structure made of a single amino acid type. The PDGA capsule prevents immune cells, specifically phagocytes, from recognizing and engulfing the bacterium. The genes responsible for producing this protective capsule are located on one of the two plasmids, pXO2, carried by the bacterium.
The Pathogenesis and Forms of Anthrax
Anthrax is primarily a zoonotic disease, transmitted most commonly through exposure to the environmentally stable spores. The final manifestation of anthrax disease depends entirely on the route by which the spores enter the body. The three main clinical forms are cutaneous, inhalation, and gastrointestinal anthrax.
Cutaneous anthrax is the most frequent form, accounting for over 90% of human cases globally. It occurs when spores enter the body through a cut or abrasion in the skin. The initial lesion develops into a painless ulcer with a characteristic black center, known as an eschar, which is the reason for the disease’s name. This form has a low mortality rate, typically less than 1% with early antibiotic treatment.
Inhalation anthrax is the most dangerous form, contracted by breathing in spores, which are then carried into the lymph nodes in the chest. This form was historically called woolsorter’s disease because of its association with handling contaminated animal hides and wool. The fatality rate is very high, approaching 95% without treatment, and remains around 45% even with modern medical intervention.
Gastrointestinal anthrax, which is rare, results from consuming meat contaminated with spores from an infected animal. The infection can occur in the throat or the lower digestive tract, leading to symptoms like severe abdominal pain and bloody diarrhea. Without treatment, the mortality rate for gastrointestinal anthrax ranges from 25% to 60%.
Regardless of the entry route, the severity of the disease is mediated by two protein exotoxins produced by the vegetative bacteria: lethal factor (LF) and edema factor (EF). These two components combine with a third protein, protective antigen (PA), to form the lethal toxin and the edema toxin, respectively. These toxins disrupt host cell function, leading to tissue damage, massive swelling, and systemic effects that cause death.