Online Textbook Bacteriology is continuously updated and includes information on Staphylococcus, MRSA, Streptococcus, E. coli, anthrax, cholera, tuberculosis, Lyme disease and other bacterial diseases of humans.
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The Online Textbook of Bacteriology is a general and medical microbiology text and includes discussion of staph, MRSA, strep, Anthrax, E. coli, cholera, tuberculosis, Lyme Disease and other bacterial pathogens.
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Tag words: bacteria, anthrax, Bacillus anthracis, B. anthracis, anthrax bacillus, anthrax toxin, bioterrorism, biowarfare, endospore, spore, inhalational anthrax

Bacillus anthracis

Kingdom: Bacteria
Phylum: Firmicutes
Class: Bacilli
Order: Bacillales
Family: Bacillaceae
Genus: Bacillus
Species: anthracis

Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interest include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

Bacillus cereus bacteria.Print this Page

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Bacillus anthracis and Anthrax (page 1)

(This chapter has 5 pages)

© Kenneth Todar, PhD


The anthrax bacillus, Bacillus anthracis, was the first bacterium shown to be the cause of a disease. In 1877, Robert Koch grew the organism in pure culture, demonstrated its ability to form endospores, and produced experimental anthrax by injecting it into animals.

Figure 1. Robert Koch's original photomicrographs of Bacillus anthracis, the agent of anthrax. Compare the cell morphology and spore position  with the Gram stain below (Figure 2). This is Bacillus anthracis. Beware of phony and mislabeled images of B. anthracis on the internet, including some that are posted by otherwise credible websites. Look for large cells with square ends and centrally-located ellipsoid spores when identifying Bacillus anthracis.

Bacillus anthracis is very large, Gram-positive, sporeforming rod, 1 - 1.2µm in width x 3 - 5µm in length. The bacterium can be cultivated in ordinary nutrient medium under aerobic or anaerobic conditions. Genotypically and phenotypically it is very similar to Bacillus cereus, which is found in soil habitats around the world, and to Bacillus thuringiensis, the pathogen for larvae of Lepidoptera. The three species have the same cellular size and morphology and form oval spores located centrally in a nonswollen sporangium.

Figure 2.  Bacillus anthracis. Gram stain. 1500X. The cells have characteristic squared ends. The endospores are ellipsoidal shaped and located centrally in the sporangium. The spores are highly refractile to light and resistant to staining.

Bacillus thuringiensis is distinguished from B. cereus or B. anthracis by its pathogenicity for Lepidopteran insects (moths and caterpillars) and by production of an intracellular parasporal crystal in association with spore formation. The bacteria and protein crystals are sold as "Bt" insecticide, which is used for the biological control of certain garden and crop pests.

Figure 3. Bacillus thuringiensis. Phase Photomicrograph of vegetative cells, intracellular spores (light) and parasporal crystals (dark). 1000X.

Bacillus cereus is a normal inhabitant of the soil, but it can be regularly isolated from foods such as grains and spices.  B. cereus causes two types of food-borne intoxications (as opposed to infections). One type is characterized by nausea and vomiting and abdominal cramps and has an incubation period of 1 to 6 hours. It resembles Staphylococcus aureus food poisoning in its symptoms and incubation period. This is the "short-incubation" or emetic form of the disease. The second type is manifested primarily by abdominal cramps and diarrhea with an incubation period of 8 to 16 hours. Diarrhea may be a small volume or profuse and watery. This type is referred to as the "long-incubation" or diarrheal form of the disease, and it resembles food poisoning caused by Clostridium perfringens. In either type, the illness usually lasts less than 24 hours after onset.

The short-incubation form is caused by a preformed, heat-stable emetic toxin, ETE. The mechanism and site of action of this toxin are unknown, although the small molecule forms ion channels and holes in membranes. The long-incubation form of illness is mediated by the heat-labile diarrheagenic enterotoxin Nhe and/or hemolytic enterotoxin HBL, which cause intestinal fluid secretion, probably by several mechanisms, including pore formation and activation of adenylate cyclase enzymes.

Figure 4. Bacillus cereus. Gram stain. 450X. Bacilli are large bacteria, so that they are readily observed with the microscope's "high dry objective" ........but you can't detect anything about their spores. This could be a Lactobacillus.

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Kenneth Todar has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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