Bacillus anthracis

BACILLUS

Characteristics

  • Gram +
  • Rod
  • Spore forming
  • Obligate aerobic
  • Facultative intracellular

Bacillus anthracis

Characteristics

  • Encapsulated (Capsule could be demonstrated during growth in infected animals)
  • Non-motile
  • Spores are formed in culture, dead animal's tissue but not in the blood of infected animals.
  • Spores are oval and centrally located

· Spores remain viable in soil for decades.

· In World War II in Scotland spores were exploded.

· Survived for >40 years and were eradicated in 1987

· Changing environmental conditions (temp. rain etc.) help in survival and multiplication.

  • Obligate pathogen

Reservoirs

  • Humans (not normal flora)
  • Animals (primarily horses, cattle, sheep and swine)
  • Soil (primarily spores)

Transmission

  • Zoonotic
  • Aerosolized
  • Vectorial (flies)
  • Contaminated soil
  • Contaminated food
  • Contaminated fomites (primarily wool and hides)

Virulence

  1. The Capsular Polypeptide
  • Composed of poly peptide of a high molecular weight consisting of D-glutamic acid.
  1. Polysaccharide Somatic Antigen
  • Composed of N-acetulglucoseamine and D-galactose
  1. Complex Protein Toxin
  • This toxin appear to be responsible for signs and symptoms characteristic of anthrax. Accumulation of the toxin in tissue and its effect on the central nervous system results in death by respiratory failure and anoxia.

Diseases

1. Cutaneous Anthrax

  • Painless necrotic black-based ulcerating papule
  • may complicate by septicemia " septic shock
  • Most common
  • Caused by Bacillus Anthracis infection of the skin

2. Gastrointestinal Anthrax

  • Gastrointestinal necrosis and hemorrhage, abdominal pain, vomiting and bloody diarrhea
  • may complicate by septicemia "septic shock
  • 50% mortality if untreated.
  • caused by ingestion of Bacillus anthracis "Bacillus anthracis infection of the GI-tract

3. Pulmonary Anthrax

  • Severe hilar lymph node necrosis and mediastinal hemorrhage
  • may complicate by septicemia "septic shock
  • 100% mortality if untreated (!)
  • caused by inhalation of Bacillus anthracis "Bacillus anthracis infection of the lungs

Treatment

  • Ciprofloxacin
  • Narrow spectrum penicillins
  • For penicillin-sensitive patients, tetracycline, erythromycin, chloramphenicol and streptomycin may be given as alternative drugs

Diagnosis

Cultural Characteristics:

  • Blood Agar and Nutrient Agar are commonly used to cultivate the bacilli. Plates are incubated aerobically at 37 oC.

On blood agar plates

  • Colonies have irregular borders and are non-hemolytic.

On nutrient agar

  • They are described as "Medusa head" or "Comet tail".

Specimen Collection and Laboratory Diagnosis:

CAUTION: Laboratory safety is very important when working with any materials suspected of containing Bacillus anthracis.

Samples are collected depending on the site affected:

1. Swab samples from cutaneous lesions and blood cultures.

2. Sputum and blood for pulmonary anthrax

3. Gastric aspirate, feces and blood for enteric anthrax.

Gram stained smears:

Made from clinical samples, show large gram positive bacilli in long chains "Bamboo-like appearance".

Giemsa stained smears:

Purple bacilli with red capsule.

Culture

See Cultural characteristics above.

Animal inoculation test:

  • Experimental animals are injected intraperitoneally by a suspension of the test organism "Suspected B. anthracis culture".
  • The animal dies in 48-96 hours due to respiratory failure.
  • Large number of typical bacilli can be found in the blood and tissue of spleen of the infected animal.

Biochemical Identification:

1. Carbohydrate fermentation test:

2. Gelatine liqefaction test: Negative after 7 days. Growth has a characteristic appearance of an inverted pine tree.

3. Nitrate reduction test: Positive

4. Starch hydrolysis test: Positive

5. Voges-Proskauer test: Positive

6. Sensitivity to penicillin. Sensitive

7. Lysis by gamma phages: Positive. This test accurately differentiates B. anthracis from other bacillus species.

Table 1. Shows some distinguishing characteristics of B. anthracis

PEA: PhenylEthyl Alcohol + Brain Heart Infusion Agar.