The pathogenesis of Bacillus anthracis

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Figure1. Vegetative form of Bacillus Anthracis with stain.].

By [Tony Amolo]

From the family Bacillaceae, Anthrax is a highly contagious disease caused by the bacteria named Bacillus anthracis. Bacillus anthracis is an obligate bacillus bacteria pathogen as other Bacillus species are nontoxic saprophytes. Based on phenotype, it can be classed with Bacillus mycoides, Bacillus cereus and Bacillus thuringiensis because it is difficult to differentiate this group based on their 16S rRNA sequences (10). The genus includes psychrophilic, acidophilic, halotolerant, thermophilic, halophilic and alkaliphilic. They are capable of growing at, pH values, salt concentrations and temperatures which very few organisms could survive.(4) Bacillus anthracis was named by Hippocrates for the black skin lesions it causes in its cutaneous form which means coal in Greek (Figure 1). It is an ancient disease and was in two of the ten plagues of the Old Testament.(1) It generally affects warm blooded animals including humans. Aloys Pollender, was credited for discovering the disease and he described the organism as “chyllus corpuscles” in the blood spleen, and carbuncle fluid of cows, which had died of the disease (2). Anthrax was extensively studied in the 1870s by Robert Koch and Louis Pasteur. Koch applied the suspended drop culture method to understand the life cycle of the bacteria and found that the spores formed could survive for long period in harsh environments (3). Anthrax became a matter of public interest after the bioterrorist attacks in the USA during the autumn of 2001 received five letters containing small quantities of anthrax spores contaminated over 30,000 people(13).

Cell Structure and Metabolism

Figure 2. The spore form of Bacillus Anthracis with Shoeffer Fulton Stain .

Bacillus anthracis is an endospore forming aerobic or facultative anaerobic, rod shaped, immobile and capsulated bacteria. It is a gram positive bacteria but in some cultures may turn gram negative with age (4). Bacillus anthracis is 1-1.5micrometer wide by 5-6 micrometer in length. They look like bamboo canes in tissue (13). The diverse abilities of many species of its genus allows them to survive in different environment(4). They have the ability to form only one endospore per cell. Spores are usually formed when vegetative cells are in need of specific nutrients (Figure 2). They are oval and sporulation occurs within 48hours. Bacillus anthracis requires oxygen to sporulate and its spores are resistant to cold, heat, radiation, desiccation and disinfectants(8) Spore formation begins with asymmetric septation then the mother cell engulfs the forespore and protects it with three layers called the coat, cortex and exosporium. The exosporium is a balloon-like, loose-fitting structure enclosing a spore. The exosporium has been studied to understand the use of anthrax as a weapon. Issues about the use of Bacillus anthracis spores as a weapon have resulted in efforts to understand the interactions between Bacillus anthracis and the cells of the immune system, and to develop better detectors for these spores (8). However, Bacillus anthracis has been used as a biological weapon because of its tough resistant endospore and its ability to cause lethal inhalational anthrax.

Habitat and Ecology

Bacillus anthracis is mostly found in the agricultural regions of Central and South America, sub-saharan Africa, central and southwestern Asia, southern and eastern Europe, and the Caribbean. It is a soil borne bacteria. It lives best in black steppe soils, for example chernozem and kastanozem, rich in organic matter and calcium at pH levels between (7-9). Endemic anthrax areas include warmer temperatures, higher soil moisture content and topography (15). Germination and multiplication occurs in the host while spore survival occurs in the soil.

Pathogenesis and Virulence factors

Figure 3. Mechanism of action of the Anthrax Toxin.

Bacillus anthracis depends on two toxic factors from two plasmids:
the pX01, 182kb,this plasmid encodes the genes for the lethal factor (LF), the edema factor (EF) and the protective antigen (PA),
pX02, 96kb, this plasmid encodes the genes for the biosynthesis of the capsule. The pathogenic effect of bacillus anthracis is through its capsule and through the production of toxin(EF, LF and PA)(13)(Figure 3).The lethal toxin(LT, the combination of PA and LF) and edema toxin (ET, the combination of PA and EF) are enough to produce and cause the anthrax infection.(7) Of the three factors, the PA plays an important role in the toxic action of Bacillus anthracis. Protective antigen (735 amino acids) plays an important role in anthrax intoxication by allowing the attachment and entry of the lethal factor and Edema factor into the cytosol. It contains regions involved in binding to the cell receptor, binding LF and EF, membrane insertion, and translocation of the anthrax toxin. Protective antigen is a major immunogen present in anthrax vaccines(11). The edema factor(89kDa) is a calcium and calmodulin-dependent adenylate cyclase that causes an increase in cytoplasmic cAMP. This leads to an imbalance of water homeostasis in the cell. The edema toxin disrupts the cytokine response of monocytes which may increase the chance of the host being infected by the disease(11). The lethal factor is a Zn2+ dependent metalloprotease that attaches major pathways to surface receptors for the transcription of certain genes within the nucleus(12) while the capsule inhibits the phagocytosis of bacillus anthracis. They study of Anthrax toxins has provided a mechanism of trans-membrane translocation of proteins and the discovery of plasmids pXO1 and pXO2 which allowed the study of toxic factors at a genetic level(5).

Transmission and Pathology

Bacillus anthracis is not an invasive organism. The ways of transmission of the anthrax disease are through wounds, mucous membranes of the mouth, pharynx, gastrointestinal tract, the digestive tract after ingestion of spore contaminated feed or water, skin lesions or abrasions caused by biting flies and the inhalation of dust containing spores. After transmission through one of these route, the spores of Bacillus anthracis are carried from the site of entry to the draining lymph nodes where they begin to multiply rapidly and the produce spores which germinate to produce the toxic factors (13). The pathogenicity of Bacillus anthracis depends on the sensitivity of the host, the infectious dose(amount of toxin produced), the quality of the capsular coat and the route of penetration (13).


Figure 5. Chest x-ray showing early pulmonary edema marked by widening of the thoracic cavity.

Anthrax in Humans
Cutaneous anthrax Cutaneous anthrax is also called malignant pustule. It is usually localized around the face, arms, hands, and neck. It occurs when a contaminated cut creates a source of entry for the organism. A primary lesion is formed with a few days and a ring of vesicles develops around the central papule. This dries up to form a black lesion (Figure 4). The lesion develops for 12-14 days leaving a shallow ulcer that heals in 2-3 weeks. Patients show symptoms of headaches, fever of up to 102 degree Fahrenheit, discomfort and even death.
Pulmonary(inhalation) anthrax Also known as woolsorter’s disease, pulmonary anthrax is caused by inhaling the anthrax spores. Spores are phagocytosed by alveolar macrophages and transported to the lymph nodes where they germinate and multiply (Figure 5). Fatal bacteraemia and toxaemia then occurs. (10) During the initial stage, the patient experiences mild fever, malaise, fatigue and cough. The second stage include symptoms like acute dyspnea, cyanosis and profuse perspiration.
Gastrointestinal anthrax Also known as splenic fever, gastrointestinal anthrax is extremely rare and occurs mainly in Africa, the Middle East and central and Southern Asia. It is caused by eating an insufficiently cooked meat contaminated with spores. Symptoms include diarrhea, ulcer, liver disease and fever. Another rare form of anthrax in man is anthrax meningitis. (6)

Vaccines and Treatment

The first bacterial disease that a preventive treatment (prophylaxis) was developed for was the Anthrax disease(11). Prophylaxis helped in controlling anthrax in animals and preventing the spread of the infection. Several vaccines have been produced for the anthrax disease since Louis Pasteur first produced the attenuated anthrax vaccine in 1881 (2). The vaccines used to fight anthrax are composed of spores from weakened strains of Bacillus anthracis. They are classified into two categories namely;
Live attenuated vaccines, capsulated and atoxigenic cap
Live attenuated spore vaccines, not capsulated and toxigenic cap. For example, Sterne and sexually transmitted infection (STI) vaccines. (13)
Other vaccines include non-living vaccines, In vitro protective antigen, Boor and Tresselt vaccine, production in non-proteinaceous media, UK vaccine, American vaccines(Aerobic antigen and Anaerobic antigen) and Russian antigen (6). Special therapy can also be provided to treat anthrax if it is applied immediately to the infected animal or human. Aside from vaccines, antitoxin and antibiotics have been used to treat the disease. Penicillin, doxycycline and ciprofloxacin have been used for treatment of susceptible strains of anthrax (14). Several methods have been adopted to create an improved human vaccine. Recombinant DNA methodology is being used to create live vaccine strains of Bacillus anthracis. The goal is to create a new vaccine that is safe, non reactogenic, efficacious against all disease and requires a little number of inoculations to achieve maximum lasting immunity (11). The challenging and difficult aspect of evaluating the efficacy of anthrax vaccine is making an assessment in humans because most of these test are safe to be tested on humans. Animals can be used as a model but the process can be dangerous.

Application to Bioterrorism

Out of many pathogenic bacteria, Bacillus anthracis is one that can be used as for bioterrorism. It makes a good model as a weapon for bioterrorism because it spores can be produced in the lab,last for a long time in the environment and can be found easily in nature. Also because of its small size, it can be placed in food and letters like in 2001 when powdered anthrax was mailed to the U.S postal office. Twenty-two cases of anthrax infections were confirmed. Half of these cases involved cutaneous anthrax and no death resulted. The remaining eleven were identified as inhalational anthrax and produced five deaths. A global awareness should be created to reject the development of such weapons and provide means of preventing developing a vaccine for this deadly disease


1] Annabel Guicharda, Victor Nizetb, c, Ethan Biera "New insights into the biological effects of anthrax toxins: linking cellular to organismal responses." 2012.

2] Joseph A Witkowski,Lawrence Charles Parish "The story of anthrax from antiquity to the present: a biological weapon of nature and humans."2002.

3] George Sternbach "The history of anthrax."2003.

4] Peter C. B. Turnbull "Bacillus."1996.

5] Maxime Schwartz "Dr. Jekyll and Mr. Hyde: A short history of anthrax."2009.

6] Peter Hambleton, J.Anthony Carman, Jack Melling "Anthrax: the disease in relation to vaccines."1984.

7] Mahtab Moayeri, Stephen H Leppla "The roles of anthrax toxin in pathogenesis."2004.

8] Jeremy A. Boydston, Ling Yue, John F. Kearney, and Charles L. Turnbough, Jr. "The ExsY Protein Is Required for Complete Formation of the Exosporium of Bacillus anthracis."2006.

9] W. Beyer, P.C.B. Turnbull. "Anthrax in animals."2009.

10] Les Bailliea, , Timothy D Read. "Bacillus anthracis, a bug with attitude!."2001.

11] Stephen F Little, Bruce E Ivins. "Molecular pathogenesis of Bacillus anthracis infection."1999.

12] Laurence Abrami, Nuria Reig, F. Gisou van der Goot. "Anthrax toxin: the long and winding road that leads to the kill."2005.

13] Antonio Fasanellaa, Domenico Galantea, Giuliano Garofoloa, Martin Hugh Jones. "Anthrax undervalued zoonosis."2010.

14] Martin Hugh-Jonesa, Jason Blackburn. "The ecology of Bacillus anthracis."2009.

15] Jocelyn C. Mullins, Giuliano Garofolo, Matthew Van Ert, Antonio Fasanella, Larisa Lukhnova, Martin E. Hugh-Jones, Jason K. Blackburn. "Ecological Niche Modeling of Bacillus anthracis on Three Continents: Evidence for Genetic-Ecological Divergence?."2013.

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Edited by student of Joan Slonczewski for BIOL 238 Microbiology, 2009, Kenyon College.