Bacillus anthracis

by Kari Rhoades

Bacillus anthracis is the scientific name of the microbe that causes anthrax. B. anthracis is a rod-shaped bacterium that forms strep colonies, or chains.

Gram Stain Results

B. anthracis is a Gram positive bacteria strain. It appears purple under a light microscope due to the absorption of crystal violet stain by its single-layered cell wall, which is made of peptidoglycan.
B_anthracis1.jpg
Gram stain of B. anthracis. Streptobacilli with spores.


Environment and Metabolism

B. anthracis grows well on nutrient agar dishes at temperature ranges of 25 to 30°C.

B. anthracis produces acidic waste products when incubated with glucose. In methyl red testing, the indicator turns red-pink in a 48 hour culture. The acid waste product has been identifyied in literature as lactic acid.

This species of Bacillus is nonmotile, which makes it unique within its genus. Motility was tested with a tube of MIO agar that was stabbed once, then examined for diffuseness of growth.

B. anthracis does not demonstrate hemolysis when grown on sheep's blood agar plates. This is classified as "gamma" in the hemolytic categories.
sba.jpg
B. anthracis shows gamma hemolytic ability when grown on SBA.

B. anthracis is a facultative species, meaning it can growth both with or without oxygen. This was confirmed with a candle-and-jar method. After 24 hours in an oxygen free jar, there was bacteria growth on an inoculated nutrient agar plate.

Antiobiotic Sensitivity

Bauer-Kirby tests were conducted on B. anthracis streaked on nutrient agar. The bacteria proves to be especially sensitive to penicillin and ciprofloxacin. Since penicillin is a cell wall synthesis inhibitor, it appears that it interferes with this Gram positive strain's ability to repair its single-layered cell wall, and the cell's ability to make new cell wall components during mitosis. Cipro prevents the winding of DNA after replication, which in turn prevents further replication and protein synthesis.

Pathogenicity

B. anthracis is the causitive pathogen in anthrax infections. Anthrax can manifest itself as an infection of the skin, digestive system or lungs.

Anthrax is most commonly contracted by humans who live in an agricultural setting by contact with infected cattle. The bacteria is found in both game and domesticated animals. Handling the meat and hides of infected animals can allow the bacteria to enter the skin through wound infection. Contact can also occur by handling soil that contains B. anthracis spores that have been shed in animal feces.

In skin infections, or cutaneous anthrax, the initial sign will resemble an insect bite and itch. Over several days (without treatment), a painless wound forms that necrotizes and turns black. Cutaneous anthrax is not usually fatal.
anthrax.jpg
Cutaneous anthrax lesion.


Eating contaminated meat that has not been properly cooked is the mode of transmission for gastrointestinal anthrax. Symptoms include nausea, fever and bloody vomit.

Inhaling B. anthracis or its spores is the most deadly form of anthrax, and the hardest to treat. Intiially, flu-like symptoms such as fever, sore throat, and generalized "achiness" will be reported by the patient. As the disease progresses, symptoms will worsen, potentially leading to meningitis and shock.

References