25 Differences between Anthrax bacilli and Anthracoid bacilli

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Last Updated on November 12, 2020 by Sagar Aryal

The saprophytic organisms which simulate the anthrax bacillus closely, both in their morphological and cultural characters within the group of Gram-positive aerobic sporing bacilli are termed as Anthracoid bacilli.

Differences between Anthrax bacilli and Anthracoid bacilli

They have a general resemblance to anthrax bacilli such as producing dry wrinkled colonies and in the smear, appearing as chains of spore-bearing gram-positive bacilli.

However, they differ from anthrax bacilli in many ways as follows:



Anthrax bacilli

Anthracoid bacilli

1.      Known asBacillus anthracisB. anthracis similis, B. pseudo-anthracis or “anthrax-like” bacilli and Pseudoanthrax.
2.      Shape (Chain Length)In long chainsIn short chains
3.      Position of sporesCentral, do not bulge the bacilli.Central, subterminal or terminal, may bulge the bacilli.
4.      CapsulePresentAbsent
5.      MotilityNon-motileMotile
6.      Under lower power microscopeMedusa head colony seenNot seen
7.      Oxygen requirementStrict aerobeAerobic or facultative anaerobic
8.      Growth at 45°CNo GrowthGrowth usually seen
9.      Blood AgarNo hemolysis (or weak)Hemolytic colonies(usually well marked)
10.   TurbidityNo Turbidity in brothTurbidity seen usually
11.   Solid medium with penicillinString of pearls appearanceNo growth
12.   Gelatin stab agarInverted fir tree appearanceAbsent
13.   In nutrient brothFluffy Cotton wool without pellicleTurbidity and pellicle formation but no fluffy Cotton wool
14.   Salicin FermentationNegativeUsually Positive
15.   Rate of gelatin liquefactionSlowRapid
16.   Lecithinase activity on egg yolk agar— weak+ marked
17.   Reduction of methylene blue in milkReduce methylene blue slowlyRapidly reduce methylene blue
18.   Chloral HydrateGrowth inhibited by  Chloral HydrateNot Inhibited
19.   Susceptibility to Gamma PhageSusceptibleNot susceptible
20.   Penicillin sensitivity 10 unit discSusceptibleResistant
21.   PathogensPathogenicExcept B. cereus, most of them are non-pathogenic or opportunistic pathogens with low virulence.
22.   Diseases caused■ Cutaneous anthrax

 ■ Gastrointestinal anthrax

 ■ Inhalational anthrax

 ■ Anthrax meningitis.

■ Bacillus cereus is the most important pathogen causing GI infection, ocular infections, and catheter-related infections.

Bacillus subtilis may act as an opportunistic pathogen, causing eye infections and septicemia.

Bacillus licheniformis has also been incriminated in patients with food poisoning.

23.   Pathogenicity for mice or guinea pigsPathogenic (death in 24 – 48 hours)No death
24.   ContaminantsNot a common contaminant.Common contaminants in laboratory cultures.
25.   Organism(s)Bacillus anthracisB. cereus, B. subtilis, B. licheniformis, B. stearothermophilus etc.


  1. http://ecoursesonline.iasri.res.in/mod/page/view.php?id=66763
  2. https://microbiologyinfo.com/difference-between-anthrax-bacilli-and-anthracoid-bacilli/
  3. http://fac.ksu.edu.sa/sites/default/files/medical_bacteriology_section_4_0.pdf
  4. Parija S.C. (2012). Textbook of Microbiology & Immunology.(2 ed.). India: Elsevier India.
  5. Sastry A.S. & Bhat S.K. (2016). Essentials of Medical Microbiology. New Delhi : Jaypee Brothers Medical Publishers.

Differences between Anthrax bacilli and Anthracoid bacilli

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