Anthrax bacilli vs Anthracoid bacilli- 25 Differences

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.

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



Anthrax bacilli

Anthracoid bacilli

1.       Known as Bacillus anthracis B. anthracis similis, B. pseudo-anthracis or “anthrax-like” bacilli and Pseudoanthrax.
2.       Shape (Chain Length) In long chains In short chains
3.       Position of spores Central, do not bulge the bacilli. Central, subterminal or terminal, may bulge the bacilli.
4.       Capsule Present Absent
5.       Motility Non-motile Motile
6.       Under lower power microscope Medusa head colony seen Not seen
7.       Oxygen requirement Strict aerobe Aerobic or facultative anaerobic
8.       Growth at 45°C No Growth Growth usually seen
9.       Blood Agar No hemolysis (or weak) Hemolytic colonies(usually well marked)
10.    Turbidity No Turbidity in broth Turbidity seen usually
11.    Solid medium with penicillin String of pearls appearance No growth
12.    Gelatin stab agar Inverted fir tree appearance Absent
13.    In nutrient broth Fluffy Cotton wool without pellicle Turbidity and pellicle formation but no fluffy Cotton wool
14.    Salicin Fermentation Negative Usually Positive
15.    Rate of gelatin liquefaction Slow Rapid
16.    Lecithinase activity on egg yolk agar — weak + marked
17.    Reduction of methylene blue in milk Reduce methylene blue slowly Rapidly reduce methylene blue
18.    Chloral Hydrate Growth inhibited by  Chloral Hydrate Not Inhibited
19.    Susceptibility to Gamma Phage Susceptible Not susceptible
20.    Penicillin sensitivity 10 unit disc Susceptible Resistant
21.    Pathogens Pathogenic Except 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 pigs Pathogenic (death in 24 – 48 hours) No death
24.    Contaminants Not a common contaminant. Common contaminants in laboratory cultures.
25.    Organism(s) Bacillus anthracis B. cereus, B. subtilis, B. licheniformis, B. stearothermophilus etc.


  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.

About Author

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Sagar Aryal

Sagar Aryal is a microbiologist and a scientific blogger. He is doing his Ph.D. at the Central Department of Microbiology, Tribhuvan University, Kathmandu, Nepal. He was awarded the DAAD Research Grant to conduct part of his Ph.D. research work for two years (2019-2021) at Helmholtz-Institute for Pharmaceutical Research Saarland (HIPS), Saarbrucken, Germany. Sagar is interested in research on actinobacteria, myxobacteria, and natural products. He is the Research Head of the Department of Natural Products, Kathmandu Research Institute for Biological Sciences (KRIBS), Lalitpur, Nepal. Sagar has more than ten years of experience in blogging, content writing, and SEO. Sagar was awarded the SfAM Communications Award 2015: Professional Communicator Category from the Society for Applied Microbiology (Now: Applied Microbiology International), Cambridge, United Kingdom (UK). Sagar is also the ASM Young Ambassador to Nepal for the American Society for Microbiology since 2023 onwards.

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