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Differences between Anthrax bacilli and Anthracoid bacilli

MN Editors

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

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

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