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The anthrax agent
Last reviewed: 04.07.2025
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Anthrax is an acute infectious disease of humans and animals (domestic and wild).
The Russian name for the disease was given by S. S. Andrievsky in connection with a large epidemic in the Urals at the end of the 18th century. In 1788, with a heroic experiment of self-infection, he proved the identity of anthrax in humans and animals and finally confirmed its nosological independence. The causative agent - Bacillus anthracis - was repeatedly described by different authors (Pollender A., 1849; Dalen K., 1850; Braun F., 1854), but its etiological role was finally established by R. Koch (1876) and L. Pasteur (1881).
B. anthracis (genus Bacillus) belongs to the family Bacillaceae (class Bacilli). It is a large rod 5-8, sometimes up to 10 µm long, with a diameter of 1.0-1.5 µm. The ends of live rods are slightly rounded, while those of dead rods are cut off and slightly concave. Rods in smears are located in pairs and very often in chains, especially long ones on nutrient media, resembling a bamboo cane. The anthrax rod stains well with all aniline dyes, is gram-positive. It does not have flagella, it forms spores, but only outside the human or animal body in the presence of oxygen and a certain humidity. The optimum temperature for sporulation is 30-35 °C (spore formation does not occur below 12 °C and above 43 °C). Spores are located centrally, their diameter does not exceed the diameter of the bacterial cell. Spore formation occurs when bacteria experience a deficiency of either energy sources, or amino acids or bases. Since these sources of nutrition for bacteria are present in the blood and tissues, sporulation does not occur in the body. The causative agent of anthrax forms a capsule, but only in the body of an animal or a person; it is rarely observed on nutrient media (on media containing blood or serum). Encapsulation of pathogenic bacteria is a protective mechanism. It is induced by factors contained in the blood and tissues, therefore capsules are formed when bacteria are in the body or when grown on media with blood, plasma or serum. The G + C content in DNA varies within 32-62 mol % (for the genus as a whole).
The causative agent of anthrax is an aerobe or facultative anaerobe. The optimum temperature for growth is 37-38 °C, pH of the medium is 7.2-7.6. It is undemanding to nutrient media. On dense media it forms characteristic large matte rough colonies of the R-form. The structure of the colonies, due to the chain arrangement of the rods, which form threads extending from the center, is similar to curls or a lion's mane (Fig. 98). On agar containing penicillin (0.05-0.5 U/ml), after 3 hours of growth, the bacilli disintegrate into individual balls located in the form of a chain, forming the phenomenon of a "pearl necklace". In broth, the rod, which is in the R-form, grows on the bottom, forming a sediment in the form of a lump of cotton wool, while the broth remains transparent. B. anthracis is virulent in the R-form, and loses its virulence when it passes into the S-form. Such rods form round, smooth colonies with smooth edges on a dense medium, and uniform turbidity in broth. In this case, the rods lose the ability to be arranged in chains in smears and acquire the appearance of coccobacteria, arranged in clusters.
B. anthracis is quite active biochemically: it ferments glucose, sucrose, maltose, trehalose with the formation of acid without gas, forms H2S, curdles milk and peptonizes it, is catalase-positive, has nitrate reductase. When sowing by injection into a column of 10-12% meat-peptone gelatin, it causes its layer-by-layer liquefaction.
To distinguish B. anthracis from other species of Bacillus, a set of characteristics is used.
[ Antigenic structure of the anthrax pathogen
The causative agent of anthrax has somatic antigens and a capsular antigen of protein nature (consists of D-glutamic acid), formed mainly in the body of animals and humans. The somatic antigen of polysaccharide nature is heat-stable, and is preserved for a long time in the external environment and in animal corpses. The diagnostic Ascoli thermoprecipitation reaction is based on its detection. The anthrax bacillus also has antigens common to the genus Bacillus.
Pathogenicity factors of the anthrax pathogen
The most important virulence factor of the anthrax bacillus is the capsule. Loss of the capsule leads to loss of virulence. The capsule protects B. anthracis from phagocytosis. Another important virulence factor, which is responsible for the death of animals, is a complex toxin containing three different components: factor I, consisting of protein and carbohydrate; and two factors of a purely protein nature (factors II and III). Synthesis of the complex toxin is controlled by the pXOl plasmid with m. m. 110-114 MD. The pXOl plasmid contains three genes that determine the synthesis of the main components of the exotoxin:
- gene cua - edema factor (EF);
- pag gene - protective antigen (PA);
- lef gene - lethal factor (LF).
The product of the cua (OF) gene is adenylate cyclase, which catalyzes the accumulation of cAMP in eukaryotic cells. The edema factor causes increased vascular permeability.
The protective antigen induces the synthesis of protective antibodies (however, the most immunogenic is the complex of all three components of the neutralized toxin), the lethal factor causes the death of animals. All three components of the toxin act synergistically. Synthesis of the anthrax capsule is also controlled by the plasmid рХ02 with m. m. 60 MD.
Resistance of B. anthracis
In its vegetative form, the anthrax pathogen has the same resistance to environmental factors and chemicals as other non-spore-forming bacteria - at temperatures above 75 °C it dies in 5-10 minutes, in animal carcasses under the influence of waste products of putrefactive bacteria - in a few days. The spores of the anthrax bacillus are extremely stable: they survive in soil for decades, in water - for several years, under the influence of direct sunlight they die in 20 or more days, when boiled they are destroyed within 45-60 minutes, when autoclaved at 110 °C - in 5 minutes, dry heat (140 °C) can be withstood for up to 3 hours. The spores survive for a long time in the wool and skins of animals used for various tanning, and in salted meat.
Epidemiology of anthrax
The main source of anthrax are sick herbivores. Throughout the disease period, they excrete the pathogen with urine, excrement, and saliva into the soil, infecting it. Therefore, soil, especially rich in organic matter, becomes an additional reservoir of the pathogen. Animals are infected mainly through the alimentary route (through food and drinking water contaminated with spores), less often - through transmission - through the bites of flies, ticks, horseflies, which carry the pathogen from sick animals, corpses, and infected objects of the external environment; very rarely - by air. The pathogen is not transmitted through direct contact from a sick animal to a healthy one.
Humans become infected with anthrax through direct contact with animal carcasses, when cutting up carcasses of animals that were killed by force, when caring for sick animals, when eating meat or meat products obtained from sick animals, or when in contact with wool, skins, leather, or bristles infected with the pathogen or its spores. Infection of a healthy person from a sick person occurs extremely rarely.
The portals of entry for the infection are the skin and mucous membranes of the intestinal and respiratory tracts. Depending on the portal of entry, human anthrax occurs in the form of cutaneous (most often, up to 98% of all cases of the disease), intestinal or pulmonary forms. The incubation period varies from several hours to 6-8 days, most often - 2-3 days. The cutaneous form manifests itself in the form of an anthrax carbuncle, which is usually localized on open parts of the body (face, neck, upper limbs), less often - on areas of the body covered by clothing. A carbuncle is a kind of hemorrhagic necrosis focus, at the top of which a vesicle with serous-bloody contents or a dense black-brown scab forms. The skin and subcutaneous tissue of the carbuncle and around it are edematous, saturated with serous-bloody exudate, but suppuration and abscesses are usually not observed. In inflamed tissues and exudate there is a large number of bacilli surrounded by a capsule.
In the intestinal form, general intoxication with catarrhal and hemorrhagic manifestations from the gastrointestinal tract (nausea, vomiting with blood, bloody diarrhea, pain in the abdomen and lower back) is observed. The disease lasts 2-4 days and most often ends in death.
The pulmonary form of anthrax is extremely rare and occurs as bronchopneumonia with deep general intoxication, chest pain, general malaise, high temperature, cough with sputum production, initially mucous, then bloody. Death occurs on the 2nd-3rd day. As a rule, all forms of anthrax are accompanied by high temperature (39-40 °C). The most severe form of anthrax is in the septic form, which can be either primary or a consequence of a complication of another form of the disease. It is characterized by an abundance of hemorrhagic manifestations and the presence of a large amount of the pathogen in the blood, cerebrospinal fluid and in a number of organs of the sick person. Anthrax diseases among people are sporadic.
Post-infectious immunity is associated with the appearance of antitoxins and antimicrobial (protective) antibodies.
Laboratory diagnostics of anthrax
The following serve as the material for the study: in the cutaneous form - the contents of the vesicles, discharge from the carbuncle or ulcer; in the intestinal form - feces and urine; in the pulmonary form - sputum; in the septic form - blood. Various environmental objects (soil, water), food products, raw materials of animal origin and other materials can be subjected to the study. To detect the pathogen, a bacterioscopic method is used: detection of gram-positive rods surrounded by a capsule (in material from animals or humans) or containing spores (environmental objects). The main diagnostic method is bacteriological - isolation of a pure culture and its identification, with mandatory testing for pathogenicity for laboratory animals. In cases where the material being studied is heavily contaminated with accompanying, especially putrefactive, microflora, a biological test is used: white mice or guinea pigs are infected subcutaneously. In the presence of B. anthracis, mice and guinea pigs die in 24-26 hours, rabbits - in 2-3 days, with signs of general sepsis; the spleen is sharply enlarged, at the site of injection of the material - infiltrate. In smear preparations from blood and organs - capsular rods.
Among the serological reactions, the Ascoli thermoprecipitation reaction is used primarily for diagnostic purposes. It is used in cases where it is difficult to count on isolating a pure culture of the pathogen (in particular, when examining wool, skins, bristles and other objects). The Ascoli reaction is based on the detection of thermostable antigens of the pathogen, which are preserved much longer than viable vegetative cells and spores of the anthrax bacillus. For retrospective diagnostics of anthrax, an allergic test with anthraxin is used.
Specific prophylaxis of anthrax
The first vaccine against anthrax was obtained by L. Pasteur in 1881, in our country - by L. S. Tsenkovsky in 1883 from weakened strains of B. anthracis. Currently, in Russia, a live spore-free capsule-free vaccine STI is used to prevent anthrax in humans and animals. It is prepared from an avirulent strain of the anthrax bacillus. The vaccine against anthrax is highly effective. Vaccinations are administered once, either cutaneously or intradermally, to those individuals who, due to their profession, may be infected with anthrax. Revaccination is administered after a year.