Differences between Anthrax bacilli and Anthracoid bacilli


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Anthrax bacilli 

Bacillus anthracis, a Gram-positive, rod-shaped bacterium, causes anthrax. This is a deadly disease that can infect livestock and humans. It is the only permanent (obligated) pathogen in the genus Bacillus. Because it can be transmitted from animals to people, its infection is called zoonosis. Robert Koch, a German physician, discovered it in 1876. It became the first pathogen to be experimentally demonstrated. It was also the first scientific proof for germ theory.

B. anthracis is approximately 3 to 5mm in length and 1 to 1.5mm wide. It has a genome that contains 5,227,293 Bp in one circular DNA. The pathogenicity of B. anthracis is due to its extrachromosal DNA-plasmids pXO1 & pXO2. It forms an endospore, a protective layer that allows it to remain inactive for years. However, when the environment is right, it suddenly becomes infective. The bacterium is a popular biological weapon due to its resilience. The protein capsule (poly-D-gamma-glutamic acid) is key to evasion of the immune response. It uses two secretory siderophore protein IsdX1 or IsdX2 to feed on the heme from blood protein haemoglobin.


Infections caused by B. anthracis can be fatal if left untreated. An inflammatory, black, necrotic (eschar) lesion is a sign of infection. These sores are usually found on the hands, face, neck, arms and legs. Flu-like symptoms can include chest discomfort, diaphoresis and body aches. Louis Pasteur, a French chemist, developed the first anthrax vaccine for animals in 1881. There are many different vaccines for animals and humans. Common antibiotics can be used to treat the infection, including penicillins, Quinzolones and Tetracyclines.

Anthracoid bacilli

Bacillus species that resemble B. anthracis can be collectively called pseudoanthrax bacilli or asanthracoid. These are low-virulence pathogens that can be exploited. These are common organ-isms that can be found in almost all environments. They are also common laboratory contaminants.


B. cereus is the most serious pathogen causing GIinfections, ocular infections and catheter-related infection. Bacillus subtilis can be an opportunistic agent, causing eye infections and septicemia. Bacillus licheniformis was also implicated in food poisoning cases.

Spores from Bacillus Stearothermophilus can be used to determine the effectiveness of sterilization using autoclaves. Some Bacillus species are used to produce antibiotics such as bacitracin and tyrothricin.


Differences between Anthrax bacilli and Anthracoid bacilli – Anthrax bacilli vs Anthracoid bacilli

CharacterAnthrax bacilliAnthracoid bacilli
Known asBacillus anthracisB. anthracis similisB. pseudo-anthracis or “anthrax-like” bacilli and Pseudoanthrax.
Shape (Chain Length)In long chainsIn short chains
Position of sporesCentral, do not bulge the bacilli.Central, subterminal or terminal, may bulge the bacilli.
Under lower power microscopeMedusa head colony seenNot seen
Oxygen requirementStrict aerobeAerobic or facultative anaerobic
Growth at 45°CNo GrowthGrowth usually seen
Blood AgarNo hemolysis (or weak)Hemolytic colonies(usually well marked)
TurbidityNo Turbidity in brothTurbidity seen usually
Solid medium with penicillinString of pearls appearanceNo growth
Gelatin stab agarInverted fir tree appearanceAbsent
In nutrient brothFluffy Cotton wool without pellicleTurbidity and pellicle formation but no fluffy Cotton wool
Salicin FermentationNegativeUsually Positive
Rate of gelatin liquefactionSlowRapid
Lecithinase activity on egg yolk agar— weak+ marked
Reduction of methylene blue in milkReduce methylene blue slowlyRapidly reduce methylene blue
Chloral HydrateGrowth inhibited by  Chloral HydrateNot Inhibited
Susceptibility to Gamma PhageSusceptibleNot susceptible
Penicillin sensitivity 10 unit discSusceptibleResistant
PathogensPathogenicExcept B. cereus, most of them are non-pathogenic or opportunistic pathogens with low virulence.
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.
Pathogenicity for mice or guinea pigsPathogenic (death in 24 – 48 hours)No death
ContaminantsNot a common contaminant.Common contaminants in laboratory cultures.
Organism(s)Bacillus anthracisB. cereus, B. subtilis, B. licheniformis, B. stearothermophilus etc.

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