Bacillus anthracis (causes the disease anthrax)

Life > Eubacteria > Phylum: Firmicutes > Class: "Bacilli" > Order: Bacillales > Family: Bacillaceae

Bacillus species form resistant spores that last for ages

Bacillus species are rod-shaped bacteria that are able to turn into resistant spores when conditions get tough (such as when the host dies). Only one spore is formed from each cell. Spores are highly resistant to environmental extremes and can survive in natural environments for decades, being mainly found in the soil. As a result it is impossible to eradicate anthrax from regions where it has had a long history of infecting animals.

Anthrax is a disease mainly of herbivorous mammals

Herbivorous animals become infected with Bacillus anthracis by ingesting spores on forage plants. The spores can end up on the plants by being blown in dust from the soil or can be deposited on leaves by flies that have been feeding on anthrax-infected carcasses. The primary route of infection for herbivorous animals is therefore via the gut. 

There are 3 main ways people can become infected 

• Through the skin (cutaneous). More than 95% of anthrax cases worldwide are caused by cutaneous infection. Infection typically occurs through people such as farmers, veterinarians, knackers and butchers, handling infected animals or animal products. The spores enter through cuts or abrasions in the skin. After 2-3 days a small pimple appears at the site of infection which steadily grows over the next few days into an ulcer with a central region of black, dead tissue (called an eschar). The ulcer is not painful unless there is a secondary infection, and there is a build up of fluid in the tissues around it (i.e. an edema).  In most cases the disease remains limited to the initial site of infection and eventually disappears because of the immune defenses fighting back. However, about 20 percent of untreated cases end up in death, caused by the bacterium and its toxins getting into the blood stream (i.e. septicemia). 
• Through the gut (intestinal). People can become infected with the anthrax bacillus via the gut through eating under-cooked meat from infected animals. Infection occurs by the bacterium entering the lining of the intestine through a pre-existing wound and results in inflammation of the intestine. The bacterium then spreads from the lining of the intestine to the lymphatic system. A person infected through this intestinal route experiences feelings of nausea, loss of appetite, vomiting and fever followed by abdominal pain, vomiting of blood, and severe diarrhea. Death occurs in 25% to 65% of cases of intestinal infection.
• Through the lungs (pulmonary). Infection occurs here through breathing in of spores. Conventionally, people handling animal products such as hides and wool, are susceptible to this form of infection because they are likely to breath in spore-laden dust. It is for this reason that pulmonary anthrax is commonly called wool-sorter's disease (with the current bioterrorism attacks through the mail, mailsorter's disease might be a more appropriate term).

Death is caused by the toxins that Bacillus anthracis produces

Bacillus anthracis cells contain plasmids (independent circles of DNA, not part of the main genome) that produce a toxin consisting of three different proteins.

Treatments available for anthrax

• Vaccinations are available for animals and people, but the ones for people are generally reserved for those at high risk such as those that work with animals and animal products such as skins. The US military has been vaccinating its forces to counteract biological warfare.
• Antibiotics are effective in killing Bacillus anthracis but need to be given early on in the infection before the bacillus has produced lethal amounts of toxin.
• Mourez et al. have recently developed a polyvalent inhibitor (PVI) that stops the two toxic enzymes (EF and LF) produced by B. anthracis from combining with the protective antigen (PA) on the host cell surface thus preventing them from entering the host cell. This substance has been tested successfully on rats but it is not yet available for human use (see Nature Biotechnology 19: 958, October 2001, and a report on this paper in Nature 413: xi, October 2001)

Why anthrax is a favoured organism in biological warfare

• Unless treated early on with antibiotics (before large amounts of toxins are produced), it kills people.
• Large numbers of spores can be produced in the laboratory.
• Spores can survive for decades in storage.
• Missiles, bombs, letters, etc can all be used to distribute the spores.

Names for the disease anthrax

The name 'anthrax' is derived from the Greek word for coal. In medieval times, anthrax was known as black bane. The association of the colour black with anthrax is probably because the ulcer in the skin caused by cutaneous anthrax often has a black central portion of dead tissue. Anthrax is also known in English as wool-sorter's disease because people sorting wool are particularly susceptible to the pulmonary form of infection by anthrax (see above).

In Afrikaans, anthrax is known as miltsiekte, meaning spleen sickness. The spleen the main battle ground in the body where the white blood cells attack invading organisms in the blood. Anthrax toxins become concentrated in the regional lymph nodes and spleen and cause breakdown of the spleen.

History of the disease

• Anthrax has affected humans throughout recorded history. The 5th and 6th plagues of Egypt that are described in the book of Exodus are thought to have been anthrax. 
• Bacillus anthracis is famous in the history of bacteriology because it was the first bacterium that was shown to cause a disease. Through a series of rigorous experiments, the German physician Robert Koch (born 1843, died 1910) discovered the link between this bacterium and the disease anthrax and published his findings in 1876 (As an aside, it is interesting to note that Robert Koch worked in Kimberley, South Africa from 1896-97 developing a vaccine against rinderpest. From 1903-1904 he also investigated an outbreak of East Coast Fever in what was Rhodesia, now Zimbabwe).
• Louis Pasteur (born 1822, died 1895) and his co-workers independently discovered the link between the bacterium and the disease and also found that the spores in dead animals that had died of anthrax survived after the animals had been buried, and were brought to the surface by earthworms. 
• An effective animal vaccine was developed in the 1930's by M. Sterne at the Veterinary Research Institute, Onderstepoort, South Africa. 
• Only in 1954 was it realised that B. anthracis produces toxins and that these are the main reason the host feels sick or dies.
• An effective human vaccine was developed in the mid-1960's.

Links

• Bacillus anthracis (Wikipedia)

• Center for Disease Control (CDC): Anthrax. Contains links to all the latest information and guidelines about anthrax, and information on incidences of anthrax infection in the US.

• Anthrax (USDA Animal and Plant Health Inspection Service). A useful account on anthrax in livestock and its incidence among livestock in the USA.

• World Organisation for Animal Health (OiE): ). Supplies regular updates on outbreaks of various animal diseases, including anthrax, around the world.

• Note: www.anthrax.com will take you to a heavy metal rock group of that name!

References

• Prescott, L.M, Harley, J.P. & Klein, D.A. 1999. Microbiology. 4th edition. McGraw-Hill, Boston.

Text by Hamish Robertson