Bacillus: Classification et Caractéristiques

Classification of Bacillus

• The Bacillus genus is divided into 3 groups
• Group I: Bacilli whose spores do not deform the bacterial body
• Group II: Bacilli whose oval spores deform the bacterial body
• Group III: Bacilli whose round spores deform the bacterial body

Bacteriological Characteristics

• Morphological characteristics: The Bacillus genus includes rod-shaped bacteria, generally motile, and spore-forming, Gram-positive.
• Antibiotic Production: Various Bacillus species produce antibiotics.
  • B. licheniformis: Produces bacitracin
  • B. brevis: Produces tyrothricine
  • B. polymyxa: Produces polymyxin

Bacillus anthracis

• Notably distinct from other Bacillus species due to its high pathogenicity.
• It is the causative agent of anthrax, a zoonotic disease.
• Anthrax is a disease affecting both animals and humans.

Bacillus anthracis - Introduction

• Anthrax is a bacterial infection caused by Bacillus anthracis.

Bacillus anthracis - Bacteriological Characteristics

• Morphological characteristics: Large Gram-positive bacilli. Non-motile (unlike other Bacillus).
  • In pathological samples, it appears as short chains.
  • In cultures, it appears as long chains of bacilli with square ends, resembling bamboo stalks.

Bacillus anthracis - Culture

• Bacillus anthracis grows well on standard culture media.
• Optimal growth temperature is between 30°C and 35°C.
• Optimal pH is between 7 and 7.4
• It prefers aerobic conditions.
• In broth culture, the bacteria form a flocculent precipitate at the bottom of the tube, leaving the supernatant clear.

Bacillus anthracis - Antigenic Structure

• Capsular Antigen: A crucial virulence factor, a polypeptide. It inhibits phagocytosis by macrophages.
• Neutralizes the bactericidal power of serum.
• Stimulates the production of anti-capsular antibodies (protecting only the mouse).

Bacillus anthracis - Somatic Antigen

• Polysaccharides which instigate precipitation reactions (Ascoli reaction)
• Toxin: Induces the creation of neutralizing antibodies for protective immunity.

Bacillus anthracis - Toxin

• The lethal toxin is proteinaceous.
• Composed of three non-toxic factors, which act together.
  • Factor I (EF): edema factor
  • Factor II (PA): protective antigen
  • Factor III (LF): lethal factor
• The combined effect of factors I and II causes edema in rabbits due to elevated cyclic AMP (cAMP).
• Factor III alone is lethal to mice.
• The overall toxicity of the strains varies according to the production of each factor; some produce primarily edema, others primarily the lethal factor.

Bacillus anthracis - Spore

• Under specific conditions, Bacillus anthracis produces an oval, non-deformable spore (characteristic of Group I). This forms after 48 hours of incubation or, in some cases, 72 hours.

Bacillus anthracis - Spore Formation Conditions

• Oxygen presence is needed for spore formation
• Temperature range between 18-42°C
• Appropriate humidity

Bacillus anthracis - Lysotype

• Epidemiologically significant.
• Useful for differential diagnoses.
• Group A phages attack all B. anthracis strains without affecting most B. cereus strains.
• Group AC phages attack B. anthracis and B. cereus strains.
• Group C phages have limited activity against B. anthracis but are effective against some B. cereus strains.

Bacillus anthracis - Epidemiology

• Anthrax occurs across different geographical regions.
• It is widespread in some animal populations (epidemic).
• Some regions have ongoing regular outbreaks (endemic).

Bacillus anthracis - Transmission

• Infected animals release bacilli in their secretions and excretions, contaminating the environment.
• Non-autopsied carcasses lose virulence quickly because spore formation cannot proceed.
• The pathogen can contaminate animal products, e.g., hair, skin, wool, blood, muscle tissue; some products are used in fertiliser production.
• Animals often contract the disease through the digestive system from ingesting contaminated vegetation (like grazing animals).

Bacillus anthracis - Considered a Bioweapon

• This organism is considered a potential biological weapon.

Pathophysiology of Anthrax

• The spores penetrate the host through skin or mucosal lesions.
• The resultant vegetative forms cause inflammatory tissue necrosis and thrombosis.
• Localized infection may resolve in the early stages.
• In serious cases, lymph node involvement followed by systemic spread and tissue invasion will occur.
• The high levels of anthrax toxin produced can lead to a life-threatening toxic shock.

Anthrax - Clinical Manifestations (External)

• Cutaneous Anthrax: The primary route involves a skin abrasion or cut; the incubation period is 1-15 days.
• A characteristic skin lesion appears: papule → pustule → ulcer (eschar).
• The lesion is surrounded by a red ring (erythema).
• Systemic symptoms include headache, fever, and nausea.
• Complication: If present near the neck, the swelling can lead to airway obstruction and suffocation.

Anthrax - Clinical Manifestations (Internal)

• Inhalation Anthrax: The disease is acquired through spore inhalation from contaminated materials.
• Incubation period: 10 days - 6 weeks.
• Initial signs: fever, non-productive cough, muscle aches.
• Later, symptoms rapidly evolve to life-threatening difficulty breathing, distressing coughs, and chills.
• Without treatment, death may be rapid, occurring within 3 days.

Anthrax - Gastrointestinal Anthrax Symptoms

• Appears 2-5 days after ingestion of contaminated food (especially meat).
• Results in gastrointestinal ulceration, extensive bacterial proliferation, and diarrhea, progressing to severe symptoms such as abdominal necrosis, edema, ascites.
• There is a high mortality rate (without treatment).

Anthrax - Additional Forms

• This infection can also manifest in the oropharynx, meninges or with systemic involvement (observed in autopsy cases and people with unusual histories).

Anthrax - Diagnostic Details

• The definitive diagnosis for Anthrax is through bacterial laboratory analysis.
• This organism is classified as a Category 3 biological risk agent.

Anthrax - Sample Collection/Preparation

• Animal Samples: Samples from internal organs /bone (after autopsy), or finely ground materials such as bone, food, and tissues.
• Human Samples: Samples taken from infected skin or lesions, blood, and cerebrospinal fluid (CSF).

Anthrax - Microscopy

• Smear Examination: Prepared slides reveal the presence of large, immobile bacilli with square ends shaped like bamboo stalks.
• Spore presence is visualized after 48-72 hours.
• Gram staining renders the bacilli Gram-positive.

Anthrax - Biological Diagnostic

• Identifying the causative organism using culture techniques.
• Culture on standard media is straightforward, and visualization of the bacterium from vesicle fluid or blood samples provides a clear diagnosis.
• The characteristic appearance of colonies, coupled with biochemical testing, facilitates definitive identification.

Anthrax - Differential Diagnosis

• Differentiating anthrax is important since it is necessary to distinguish from similar species.

Anthrax - Identification

• Positive Tests: Catalase-positive, Nitrate-positive, with a weak proteolytic action.
• Slower Tests: Slow gelatin liquefaction; slow coagulation serum digestion

Anthrax - Antimicrobial Susceptibility Tests

• Anthrax is typically sensitive to penicillin, amoxicillin, ampicillin, and related medications; however, acquired resistance attributed to beta-lactamases can occur.
• It is also sensitive to some other drug classes such as chloramphenicol, macrolides, tetracycline and aminoglycosides.

Anthrax - Advanced/Other Diagnostics

• PCR: Evaluation of virulence factors (e.g., plasmids).
• Sequencing: For epidemiological research.
• Guinea Pig Inoculation: For assessing potency;
• The results are observed after 24 to 72 hours, showing edema and death, particularly when using high bacterial concentrations.

Anthrax - Serological Diagnostic

• ELISA and Western blot: Useful for epidemiological and retrospective investigations.
• Ascoli test: Identifies polysaccharide antigens in serum samples.
• A sample of the suspected tissue (e.g., skin pieces, minced), is diluted in sterile saline, and boiled for 5 minutes before filtering it. Some serum containing anthrax antibodies is added to the filtered fluid. If the organism is present, a visible precipitate will form at the interface between the two liquids.

Anthrax - Allergic Reaction Diagnostic (Skin Test)

• A skin test(e.g., PPD skin test), to determine the presence of an allergic anthrax response is quick, cheap and allows early diagnosis.

Anthrax - Treatment

• Antibiotics: Ciprofloxacin, tetracycline
• Severe edema cases may need corticosteroids.

Anthrax - Prophylaxis

• Animal Surveillance
• Proper Animal Carcase Disposal: Burial (with lime coverings) or incineration is crucial.
• Vaccination: For animals (attenuated spores) and people (cell-based vaccine) in high-risk zones
• Post-infection immunity is usually long-lasting.

Bacillus cereus

• A less significant cause of food poisoning.
• Two forms of food poisoning are associated with this bacterium:

  • Emetic syndrome: Results from eating reheated food; symptoms arise within 6 hours

  • Diarrheal syndrome: Onset of symptoms is within 8-15 hours after suspect food ingestion; characterised by watery diarrhea.