Morphology and cultural characteristics
B. anthracis is a large Gram-positive bacillus. In affected tissues, the microorganism appears singly, in pairs, or in short chains. On artificial culture media, the bacteria form long filaments visually resembling a "bamboo cane".
To protect itself from the host's immune system, the pathogen forms a capsule. Upon release into the external environment in the presence of oxygen, the bacillus forms a centrally located spore, the size of which does not exceed the cell diameter. Spores exhibit extreme resilience: they withstand boiling and liquid nitrogen and persist in water bodies for years.
The bacterium is an aerobic or facultatively anaerobic organism that does not require complex media (it grows at pH 7.2–7.6 and t of 35–37 °C). On solid meat-peptone agar, colonies grow large, matte, and irregular, displaying the microscopic phenomenon of a "lion's mane" or "Medusa head". In liquid broth, the pathogen forms a characteristic bottom sediment of cotton-like flakes while leaving the medium clear. A specific identification test is inoculation on a medium with penicillin: cells transform into spheroplasts and align in chains, producing the "string of pearls" phenomenon.
Pathogenicity factors
The virulence of Bacillus anthracis is based on two main components: a capsule and a potent exotoxin.
The capsule consists of a D-glutamic acid polypeptide. It is non-immunogenic but plays a crucial anti-phagocytic role, allowing the bacterium to evade engulfment by immune cells and facilitating tissue adhesion.
The exotoxin is synthesized only inside the host (or on media containing animal proteins) and is encoded by plasmids. Its toxic effect manifests exclusively through the combined action of three components:
- Protective antigen — serves a transport function by binding and delivering the other two factors inside the cell (it is responsible for inducing protective antibodies).
- Lethal factor — a zinc-dependent metalloprotease that inactivates protein kinases and selectively destroys macrophages.
- Edema factor — a calmodulin-dependent adenylate cyclase that sharply increases intracellular cAMP levels, disrupting membrane permeability and causing massive edema.
Epidemiology and pathogenesis
Anthrax is a globally distributed infection tied to livestock-raising regions. The source of infection is sick animals (cattle, small ruminants, horses, pigs), and the reservoir is infected soil. For humans, the disease is a biological dead end, as they do not transmit the infection further.
Main routes of transmission: contact (handling hides, wool, bones), alimentary (consuming contaminated meat), airborne dust (inhaling spores), and rarely vector-borne (insect bites).
Upon entering the body through damaged skin or mucous membranes, spores are engulfed by macrophages and germinate into vegetative forms wrapped in a capsule. Toxin release leads to serous-hemorrhagic inflammation and necrosis. Subsequently, bacilli multiply in the lymph nodes, break into the bloodstream (bacteremia), and cause generalized sepsis. Toxin accumulation in the blood leads to toxic shock and death.
Clinical forms and diagnostics
Depending on the portal of entry, three main forms of the disease are distinguished:
- Cutaneous — the most common. Characterized by the formation of a firm black carbuncle with serous-hemorrhagic discharge. Pathognomonic sign: complete absence of pain in the necrosis zone against a background of marked edema.
- Pulmonary — develops via spore inhalation (can be used as a biological weapon). Follows an extremely severe course with rust-colored sputum and rapid death from shock.
- Gastrointestinal — accompanied by lesions in the intestinal lymphoid tissue, bloody diarrhea, and paresis.
Diagnostics involve microscopy (Gram staining and capsule visualization), culture, PCR, and direct immunofluorescence (DIF). An intradermal allergic skin test with anthraxin is effective for diagnosis during the first days. For post-mortem examination of corpses or old raw hides where isolating a live culture is impossible, the Ascoli thermoprecipitation test is used to detect the thermostable somatic antigen.
Briefly: answers to topical questions
What antigens are identified in the antigenic structure of Bacillus anthracis?
The antigenic structure of Bacillus anthracis includes three main antigens.
- Capsular antigen — a species-specific protein antigen consisting of a D-glutamic acid polypeptide without protective activity.
- Somatic antigen — a genus-specific thermostable cell wall polysaccharide (hapten) used in the Ascoli thermoprecipitation test.
- Protective antigen — a component of the exotoxin responsible for generating protective antibodies and establishing immunity.
What culture media are used to isolate and identify the anthrax pathogen?
Simple nutrient media are used to isolate and identify Bacillus anthracis.
- Meat-peptone agar (MPA) — a solid medium on which large, rough, irregular colonies resembling a "Medusa head" form.
- Meat-peptone broth (MPB) — a liquid medium showing bottom growth in the form of "cotton flakes" while the medium remains clear.
- Penicillin-supplemented medium — used for the diagnostic "string of pearls" test, where bacilli transform into spheroplasts.
What conditions are necessary for spore formation in Bacillus anthracis?
Spore formation in Bacillus anthracis requires exposure to a complex of environmental factors. Sporulation occurs outside a susceptible host when unfavorable conditions arise. Key requirements for vegetative cells to transition into spores are obligatory access to free atmospheric oxygen and temperatures between 15 and 42 °C. Spores do not form inside the body of warm-blooded animals.
What kind of immunity develops after recovering from anthrax?
Following recovery from anthrax, individuals develop a robust post-infection immunity of a cell-humoral nature. Protective antibodies are produced in response to the protective antigen, which is a component of the Bacillus anthracis exotoxin. Due to the high potency of the immune response, reinfection with anthrax is extremely rare.
What products are used for the specific prevention of anthrax in humans?
Specific prevention of anthrax in humans involves vaccines and immunoglobulins, depending on indications.
- Live anthrax vaccine (STI strain) — used for routine active immunization of individuals in occupational risk groups.
- Anthrax immunoglobulin — used for emergency (post-exposure) prophylaxis; it must be administered within the first 5 days after suspected exposure to the pathogen.
What medications are used for etiotropic treatment of anthrax?
Antibiotics are used for the etiotropic treatment of anthrax. Bacillus anthracis is highly susceptible to penicillin and a wide range of other antibiotics. The choice of drug, dosage, and duration of the course depends on the clinical form and variant of the infection.
To distinguish the anthrax pathogen from harmless saprophytic bacilli, remember the "three NOs" rule: Non-motile, Non-lecithinase-producing, Non-hemolytic on blood agar.
Class and exam questions
How does the arrangement of B. anthracis bacteria differ between a pure culture and a patient smear?
In smears from pathological material, bacilli appear singly, in pairs, or in short chains (with a pronounced capsule). In pure culture on nutrient media, they arrange themselves into long chains resembling a bamboo cane.
What is the essence of the Ascoli test and when is it used?
The Ascoli thermoprecipitation test detects a thermostable genus-specific somatic antigen. It is essential for diagnostics when examining hides, raw leather, or decomposed animal carcasses where isolating a live bacterial culture is no longer possible.
What are the main differential characteristics of the anthrax bacillus?
Unlike related opportunistic species (e.g., B. cereus), the anthrax pathogen is non-motile, forms a capsule within the host organism, displays the "string of pearls" phenomenon on penicillin media, and lacks lecithinase activity.
What else to study on this topic
- Detailed biochemical mechanism of action of the lethal and edema factors of the exotoxin.
- Differential diagnosis of Bacillus anthracis and Bacillus cereus in laboratory practice.
- Principles of specific immunotherapy and emergency post-exposure prophylaxis (use of immunoglobulin).
- Sanitary and veterinary regulations for the disposal of infected objects and regulations for cattle burial sites.
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