Bacillus are aerobic, spore-forming, gram-positive bacilli. Two pathogenic species are Bacillus anthracis (B. anthracis) and B. cereus. Bacillus anthracis has a unique polypeptide capsule composed of D-glutamate and is associated with cutaneous, gastrointestinal, and pulmonary anthrax. Importantly, B. anthracis is classified as a bioterrorism agent. Bacillus cereus is associated with 2 forms of food poisoning, diarrheal and emetic, both of which are self-limited and require only supportive care.

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Microbiology flowchart gram-positive bacteria classification

Gram-positive bacteria:
Most bacteria can be classified according to a lab procedure called Gram staining.
Bacteria with cell walls that have a thick layer of peptidoglycan retain the crystal violet stain utilized in Gram staining but are not affected by the safranin counterstain. These bacteria appear as purple-blue on the stain, indicating that they are gram positive. The bacteria can be further classified according to morphology (branching filaments, bacilli, and cocci in clusters or chains) and their ability to grow in the presence of oxygen (aerobic versus anaerobic). The cocci can also be further identified. Staphylococci can be narrowed down on the basis of the presence of the enzyme coagulase and on their sensitivity to the antibiotic novobiocin. Streptococci are grown on blood agar and classified on the basis of which form of hemolysis they employ (α, β, or γ). Streptococci are further narrowed on the basis of their response to the pyrrolidonyl-β-naphthylamide (PYR) test, their sensitivity to specific antimicrobials (optochin and bacitracin), and their ability to grow on sodium chloride (NaCl) media.

Image by Lecturio. License: CC BY-NC-SA 4.0

General Characteristics

Basic features of Bacillus spp.

  • Gram-positive rods
  • Spore forming:
    • Endospores (dehydrated cells)
    • Highly resistant to heat and other environmental exposures
    • Can persist in soil in a dormant state for many years
  • Catalase positive
  • Aerobic

Pathogenic species

  • Bacillus anthracis (B. anthracis):
    • Colonies show halo projections known as “medusa heads.”
    • Form long chains when grown in culture
    • Grow on blood agar at 37°C (98.6°F), non-hemolytic
    • Have non-functional flagellae (defective flagellin proteins); non-motile
    • Virulence factors:
      • Polypeptide D-glutamate capsule (stains with India ink)
      • Edema factor (EF)
      • Lethal factor (LF)
      • Protective antigen (PA)
  • B. cereus
    • Grow well on blood or chocolate agar at 25°C–37°C (77°F–98.6°F)
    • Beta-hemolytic
    • Motile, have flagellae
    • Produce 2 types of enterotoxins:
      • Diarrheal toxin (heat labile)
      • Emetic toxin (heat table)
    • Virulence factors:
      • Phospholipase C regulator (PlcR)
      • Controls expression of hemolysins, enterotoxins, phospholipases, and proteases

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B. anthracis


  • Primarily affects animals (herbivores)
  • Sheep, cattle, goats, and horses act as a natural reservoir.
  • Humans usually become infected accidentally through contact with animals or their products.
  • Spores are used as a bioterrorism agent.
  • Some cases were reported secondary to laboratory accidents.


  • Transcutaneous:
    • Infected animals
    • Animal products
  • Ingestion: undercooked meat from animals infected with anthrax
  • Spore inhalation
  • Direct, parenteral injection: rare
  • Does NOT occur person to person
Anthrax Cycle

The anthrax cycle
Bacillus anthracis (anthrax spores) infect humans or mammals via different processes: either via ingestion, inhalation, or through cutaneous pathways by bites from an infected insect. Anthrax spores originate from vegetation in excreted waste from cattle, which is exposed to oxygen.

Image by Lecturio.


  • PA:
    • Named so because PA serves as the target for the anthrax vaccine
    • Attaches to host surface proteins
    • Allows toxin entrance into the cytosol
  • EF:
    • Mimics cyclic adenosine monophosphate (cAMP)
    • Causes characteristic edematous, black eschar in cutaneous anthrax
    • Induces multiorgan hemorrhage
  • LF:
    • Exotoxin that acts as a protease
    • Cleaves mitogen-activated protein (MAP) kinase
    • Causes tissue necrosis
  • Polypeptide capsule: aids aversion of phagocytosis

Clinical presentation

In the United States, 3 clinical manifestations occur.

  1. Cutaneous anthrax:
    • Painless papule surrounded by vesicles
    • Black eschar at the site of contact
    • Most common manifestation, self-limited course
    • Rarely, can progress to bacteremia and death
  2. Gastrointestinal anthrax:
    • Ulcerative lesion that produces abdominal pain, vomiting, and diarrhea
    • Lesion may then perforate.
    • Rare and lethal
  3. Pulmonary anthrax (wool-sorter’s disease):
    • Flu-like symptoms
    • Long incubation initially, up to 60 days
    • Rapid progression to fever, pulmonary hemorrhage, mediastinitis, and shock
    • Chest X-ray: widened mediastinum or pleural effusion
    • Approximately 50% of patients will develop hemorrhagic meningitis.


  • Cultures showing encapsulated bacilli:
    • Blood
    • Pleural fluid
    • Cerebrospinal fluid
  • Serologic testing during the convalescent period


  • Antimicrobials (fluoroquinolones and doxycycline)
  • Antitoxin (raxibacumab or anthrax immunoglobulin)
  • Drainage of pleural effusions
  • Supportive care, glucocorticoids


  • Primarily through animal vaccination
  • Human anthrax vaccine is available for:
    • Certain veterinary, laboratory, or military personnel
    • Post-exposure prophylaxis

B. cereus


  • Abundant in soil, fresh and marine water
  • Frequent food contaminant
  • Normally transiently present in the human gastrointestinal tract (up to 43%)
  • B. cereus bacteremia can occur in injection drug users, neonates, immunosuppressed patients, and patients with prosthetic devices.


  • Ingestion:
    • Meats
    • Vegetables
    • Reheated rice/grains
  • Spores:
    • Can survive cooking temperatures
    • As food cools, spores germinate and produce toxins.
  • Direct parenteral injection: IV drug users 


  • Food poisoning when ingested:
    • Symptoms are related to toxin production.
    • Diarrheal toxin is produced by ingested bacteria in vegetative or spore forms.
    • Emetic toxin is produced in the food and ingested directly.
  • Bacteremia (bloodstream infection) causes various organ seeding.
  • Soft-tissue infections can occur in traumatic wounds on direct contact. 

Clinical presentation

  • Diarrheal food poisoning:
    • Occurs 6–14 hours after ingestion
    • Associated with meats and vegetables
    • Symptoms secondary to heat-labile enterotoxin
    • Watery diarrhea and abdominal cramps for a median of 24 hours
  • Emetic food poisoning:
    • Rapid onset, occurs 2–3 hours after ingestion
    • Associated with reheated rice and grains
    • Symptoms secondary to heat-stable enterotoxin
    • Nausea and vomiting for a median of 9 hours
    • Remember to differentiate from Staphylococcus aureus (S. aureus) food poisoning
  • Rare manifestations:
    • Meningitis
    • Necrotizing soft tissue infections
    • Endocarditis
    • Keratitis: associated with post-cataract surgery or contact lens usage


  • Cultures:
    • Stool (for cases of diarrheal poisoning)
    • Blood (if systemic infection suspected)
  • Assays to detect diarrheal toxin 


  1. Noonan L., & Freeman J. (2020). Bacillus cereus and other non-anthracis Bacillus species. UpToDate. Retrieved December 30, 2020, from
  2. Wilson, K. (2020). Clinical manifestations and diagnosis of anthrax. UpToDate. Retrieved December 30, 2020, from
  3. Wilson, K. (2020). Microbiology, pathogenesis, and epidemiology of anthrax. UpToDate. Retrieved December 30, 2020, from
  4. Bacillus anthracis.
  5. Tankeshwar, A. (2020). Bacillus anthracis: Properties, Pathogenesis and Laboratory Diagnosis. Retrieved 30 December 2020, from 
  6. Grace Ucar. (2011). Bacillus cereus.

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