Bacillus Anthracis: Morphology, Pathogenesis, and Diagnosis

General Features of Bacillus

• Genus Bacillus consists of aerobic, heat-resistant, spore-forming bacteria
• Gram-positive, but tend to be decolorized easily
• Generally motile with peritrichous flagella, except for Bacillus anthracis
• Most Bacillus species form catalase and produce acid, but not gas from glucose
• The genus includes psychrophilic, mesophilic, and thermophilic species, with a temperature range of 25-75°C and a minimum of 5°C
• Spores are ubiquitous, found in soil, dust, water, and air, and are common contaminants in bacteriological culture media

Species of Bacillus

• There are over 50 species in the genus Bacillus
• Medically important species include Bacillus anthracis and Bacillus cereus
• Bacillus anthracis causes anthrax, while Bacillus cereus is a food poisoning and opportunistic pathogen

History of Bacillus anthracis

• 1849: Pollender was the first to observe Bacillus anthracis under a microscope
• 1850: Davaine showed that anthrax was the first communicable disease transmitted by inoculation of infected blood
• 1876: Robert Koch isolated Bacillus anthracis in pure culture and showed that it possesses spores
• Koch's postulates were based on Bacillus anthracis
• 1881: Louis Pasteur developed the first live attenuated bacterial vaccine, the anthrax vaccine

Morphology of Bacillus anthracis

• One of the largest of pathogenic bacteria, with a size of 3-8 by 1-1.3 μm
• Gram-positive, non-acid fast, straight, sporing bacilli
• Capsule composed of d-glutamic acid
• McFadyean reaction: a characteristic reaction of Bacillus anthracis, where capsular material is visible around the bacilli

Culture of Bacillus anthracis

• NA: irregular, round, 2-3 mm, raised, dull, opaque, grayish-white colonies with a Medusa head appearance
• BA: non-hemolytic, with a narrow zone of hemolysis
• Gelatin: inverted fir tree appearance
• Selective media: PLET, polymyxin, lysozyme, and EDTA

Virulence Factors and Pathogenesis of Bacillus anthracis

• Capsule: composed of poly d-glutamic acid, interferes with phagocytosis, and loss of plasmid controlling capsule production leads to loss of virulence
• Anthrax Toxin: a three-component protein exotoxin consisting of edema factor, protective antigen, and lethal factor
• Toxin fragments are not toxic individually, but in combination, they produce local edema and generalized shock

Clinical Manifestations of Bacillus anthracis

• Transmission: based on mode of infection, human anthrax presents in three ways: cutaneous, inhalation, and ingestion
• Clinical types: cutaneous, pulmonary, and intestinal anthrax
• Cutaneous anthrax: characterized by a malignant pustule, begins as a papule, then becomes a painless vesicle, followed by a coal-black, necrotic eschar surrounded by non-pitting edema
• Pulmonary anthrax: characterized by hemorrhagic pneumonia, bacilli spread by lymphatics or blood, and may lead to bacteremia, hemorrhagic mediastinitis, and hemorrhagic meningitis

Animal Anthrax

• Anthrax is primarily a zoonotic disease
• Animals affected: herbivores, such as cattle, sheep, and horses
• Acquired by ingestion of spores present in soil
• Presentation: fatal septicaemia
• Infective materials: discharges from mouth, nose, and rectum

Vaccines

• Adsorbed (Alum Precipitated) Toxoid Vaccine: prepared from the protective antigen, safe and effective for human use, indicated for pre-exposure and post-exposure prophylaxis
• Anthrax Vaccines: indication, route, and dosing schedule vary depending on the type of vaccine and the population being vaccinated

Bacillus cereus

• Normal habitant of soil
• Widely isolated from vegetables, milk, cereals, spices, meat, and poultry
• Causes food poisoning, with two types: diarrheal and emetic
• Diarrheal type: characterized by diarrhea, fever, and rarely nausea, caused by a heat-labile toxin
• Emetic type: characterized by vomiting, abdominal cramps, and diarrhea, caused by a heat-stable toxin

Laboratory Diagnosis and Treatment

• Sample: feces
• Culture isolation: MYPA (mannitol, egg yolk, polymyxin, phenol red, and agar) and PEMBA (polymyxin B, egg yolk, mannitol, bromothymol blue, and agar)
• Treatment: susceptible to clindamycin, erythromycin, vancomycin, aminoglycosides, and tetracycline, but resistant to penicillin and trimethoprim

Learn about the morphology, pathogenesis, clinical features, and diagnosis of Bacillus anthracis, a Gram-positive, aerobic bacillus that forms heat-resistant spores. Understand its lab diagnosis, prevention, and treatment.