BACILLUS

Bacillus

Bacillus is a genus of Gram-positive, rod-shaped, spore-forming bacteria that are commonly found in soil and water. The genus includes many species, but the most notable in human medicine are Bacillus anthracis, the causative agent of anthrax, and Bacillus cereus, known for causing food poisoning. Bacillus species are known for their ability to produce durable spores that can survive in harsh environments.

Microscopic Appearance

• Shape:

  • Rod-shaped (bacilli).

• Gram Stain:

  • Gram-positive; however, older cultures may appear Gram-variable due to the breakdown of the cell wall.

• Spores:

  • Bacillus species are capable of forming endospores, which are highly resistant to environmental stress such as heat, desiccation, and disinfectants.

  • Spores appear as unstained areas within the bacterial cell when observed under a microscope with Gram staining.

• Arrangement:

  • Typically appear in chains, pairs, or singly.

• Aerobic or facultative anaerobes.

• Non motile.

2 species are important Medically

• Bacillus anthracis

• Bacillus cereus

Virulence Factors

• Capsule (Bacillus anthracis):

  • B. anthracis has a poly-D-glutamic acid capsule that inhibits phagocytosis, aiding in immune evasion.

• Anthrax Toxin (Bacillus anthracis):

  • Composed of three proteins: protective antigen (PA), edema factor (EF), and lethal factor (LF). PA binds to host cells and facilitates the entry of EF and LF, leading to cell death and tissue damage.

• Enterotoxins (Bacillus cereus):

  • B. cereus produces heat-stable and heat-labile enterotoxins, which are responsible for emetic and diarrheal forms of food poisoning, respectively.

• Spore Formation:

  • Endospores allow Bacillus species to survive in extreme conditions and resist common sterilization methods.

Pathogenesis

• Bacillus anthracis (Anthrax):

  • Cutaneous Anthrax:

    • Spores enter through breaks in the skin, leading to a localized infection. The bacteria multiply and produce toxins, causing tissue necrosis and the formation of a characteristic black eschar.

  • Inhalational Anthrax:

    • Inhaled spores are taken up by macrophages and transported to lymph nodes, where they germinate and produce toxins, leading to hemorrhagic mediastinitis, severe respiratory distress, and septicemia.

  • Gastrointestinal Anthrax:

    • Ingested spores lead to ulceration of the intestinal mucosa, with subsequent systemic spread and toxemia.

• Bacillus cereus (Food Poisoning):

  • Emetic Type:

    • Caused by ingestion of a pre-formed heat-stable toxin, often associated with rice dishes. Symptoms include nausea and vomiting within hours of ingestion.

  • Diarrheal Type:

    • Caused by ingestion of heat-labile enterotoxins produced in the intestines. Symptoms include diarrhea and abdominal cramps occurring 6-15 hours after ingestion.

Clinical Manifestations

• Cutaneous Anthrax (Bacillus anthracis):

  • Begins as a painless papule that progresses to a vesicle and then an ulcer with a black necrotic center (eschar). Surrounding edema is common.

• Inhalational Anthrax:

  • Initially presents with non-specific flu-like symptoms (fever, cough, myalgia). Rapid progression to severe respiratory distress, shock, and death if untreated.

• Gastrointestinal Anthrax:

  • Symptoms include nausea, vomiting, abdominal pain, and severe bloody diarrhea. Can progress to septicemia and death.

• Bacillus cereus Food Poisoning:

  • Emetic Type: Nausea and vomiting, typically within 1-6 hours of consuming contaminated food.

  • Diarrheal Type: Watery diarrhea and abdominal cramps, usually 6-15 hours after ingestion.

Lab Diagnosis

• Culture:

  • Bacillus species can be cultured on standard media, such as blood agar, where they typically form large, flat, irregular colonies with a frosted-glass appearance. B. anthracis colonies are non-hemolytic, while B. cereus colonies are beta-hemolytic.

  • Temperature range for growth is 12˚C 45˚C (optimum is

    35˚C 37˚C)

  • On Agar plate:

    Irregular colonies formed with raised dull, opaque, greyish white with a frosted glass appearance, 2 3mm in diameter

  • On Blood agar:

    The colonies are nonhemolytic.

  • In Broth growth occurs as floccular deposits.

• Microscopy:

  • Gram staining reveals large, Gram-positive rods. Spores can be visualized as unstained areas within the cell.

• PCR and Serology:

  • Polymerase chain reaction (PCR) can be used for the rapid identification of B. anthracis.

  • Serological tests, such as ELISA, can detect antibodies against anthrax toxins.

• Toxin Detection:

  • Detection of B. cereus enterotoxins in food or stool samples can confirm the diagnosis of food poisoning.

Treatment

• Anthrax (Bacillus anthracis):

  • Cutaneous Anthrax:

    • Treated with oral antibiotics such as ciprofloxacin or doxycycline. Early treatment is critical to prevent systemic spread.

  • Inhalational Anthrax:

    • Requires aggressive treatment with intravenous antibiotics (ciprofloxacin or doxycycline, often combined with other antibiotics) and supportive care. Anthrax antitoxin may also be used.

  • Post-Exposure Prophylaxis:

    • In cases of suspected exposure to B. anthracis spores, prophylactic antibiotics (ciprofloxacin or doxycycline) are recommended.

• Bacillus cereus Food Poisoning:

  • Treatment is generally supportive, focusing on hydration and symptom management.

  • Antibiotics are not typically required as the disease is self-limiting.

• Prevention:

  • Vaccination against anthrax is available for individuals at high risk of exposure, such as military personnel and laboratory workers.

  • Proper food handling and storage can prevent Bacillus cereus food poisoning, particularly avoiding the reheating of cooked rice and other starchy foods.