Saturday, 6 August 2016

Pathogenic Bacterium - Bacillus anthracis

The bacterium Bacillus anthracis is an obligate pathogen that infects both human beings and livestock. It causes anthrax - a disease characterized by skin lesions, breathing difficulty and gastrointestinal infection. It is also the only obligate species in the Bacillus genus (Spencer 183). Likewise, it is regarded as a zoonotic agent because it causes anthrax in animals, and this disease can be transferred to human beings (Brooks et al. 175).
Advanced stage of Cutaneous Anthrax. Photo Credit: Onehealthnetwork.asia
History
In 1855, Aloys Pollender – a German Physician - published his findings on anthrax in which he described a group of stick-shaped bacteria that were present in the blood of infected animals. He is credited with recognizing the pathogen Bacillus anthracis. In 1864, Casimir Davaine – a French physician – studied the bacteria found in the blood of people infected with anthrax, and found that they physically resembled the bacteria described by Dr. Pellender, and thus concluded that the symptoms of anthrax occurred when these bacteria were present in the blood. Later in 1876, Robert Koch provided conclusive evidence that Bacillus anthracis was the cause of anthrax (Théodoridès 159).
Nomenclature
The pathogen belongs to the Bacillaceae family of the kingdom Bacteria. This family belongs to the order Bacillales of the class Bacilli. The class Bacilli is one of the classes of the division Firmicutes (Madigan and Martinko 149).
            The Bacillaceae family encompasses a group of Gram-positive, rod-shaped, and heterotrophic bacteria which produce endospores. Most of these bacteria are non-pathogenic saprophytes with the exception of a Bacillus species known as Bacillus anthracis which causes disease in animals and human beings (Madigan and Martinko 150). The species Bacillus anthracis measures of 3–5 µm in length and 1.0–1.2 µm in width (Spencer 183). In the laboratory, it can be grown in a standard nutrient medium (Brooks et al. 177).
Virulence Factors
The main virulence factors of the bacteria are the capsule, edema factor, lethal factor and the protective antigen. Two plasmids encode these virulence factors. These plasmids are the pX01 which encodes the edema factor, lethal factor and the protective antigen; and the pX02 which encodes the anti-phagocytic capsule. These virulence factors enable the bacteria to infect the host organism and cause the disease anthrax (Spencer 183).
Transmission Vectors
The bacilli form spores which enable them to survive harsh environmental conditions. Animals get infected when they consume these spores which then germinate and multiply within their bodies (Brooks et al. 177). Transmission to humans occurs through various modes which are described below.
First of all, consumption of the bacterial spores or bacilli in contaminated food, such as poorly prepared meat of infected animals, causes intestinal infection which leads to gastrointestinal anthrax. Secondly, inhalation of spores when dealing with animal skin or wool causes respiratory infection which leads to inhalational anthrax. Finally, contamination of an open skin wound or uncovered cut skin with contaminated soil allows the bacteria to enter the body. This form of infection causes cutaneous anthrax (Spencer 183).
Pathogenesis
Its survival in the body immediately following infection is determined by the capsule. The capsule protects the bacteria from the phagocytic processes of the macrophages and other white blood cells, hence its description as a phagocytic capsule. This enables the bacteria to survive within white blood cells. It is these white blood cells that transport the bacteria in the body. While in the macrophages, the spores germinate and the bacilli multiply to such a huge mass that causes individual macrophages to burst and release the bacilli into the blood stream. This causes bacteremia which could ultimately develop into septicemia (Brooks et al. 176).
In the bloodstream, the bacilli release the protective antigen which enables them to bind to a host cell and thereafter gain entry into the cell. Inside the cell, the bacterium uses the edema factor to suppress the immune response. Likewise, the lethal factor serves to inactivate neutrophils. This allows the bacilli to multiply in the host cell, and thereafter cause lysis of the infected cell which permits further dissemination of the bacilli (Brooks et al. 176). This results in the effects described below.
Lysis of the infected cells results in localized necrosis of affected tissues and this leads to regional haemorrhagic lymphadenitis if it occurs within the mediastinal lymph nodes; or alveolar damage in the lungs which impairs respiration and consequently leads to breathing difficulties. In the skin, localized necrosis causes skin rapture and development of skin lesions (Brooks et al. 177).
Symptoms
Anthrax is characterized by the symptoms described hereafter. Cutaneous anthrax is characterized by skin lesions which contain an eschar. The lesion can further develop into a necrotic ulcer (Spencer 183).
Respiratory infection initially manifests as fever which is accompanied by chills, malaise, weight loss, nausea and body aches. Later it develops into pneumonia. Without medical intervention, the condition progresses to respiratory failure which is usually fatal. On the other hand, gastrointestinal infection is characterized by abdominal pain, severe diarrhea, loss of appetite, and vomiting of blood (Spencer 184).
Conclusion
The obligate pathogen Bacillus anthracis is the causative agent of anthrax. It causes infection in animals and human beings. Transmission occurs through the cutaneous, respiratory and gastrointestinal route. Cutaneous anthrax manifests as skin lesions which can sometimes develop into necrotic ulcers. Inhalational anthrax makes breathing difficult and it can lead to respiratory collapse. Finally, gastrointestinal infection causes vomiting of blood.
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Works Cited
Brooks, G. F., J. S. Butel., K.C.Carroll., T.A. Mietzner, and S. A. Morse. Jawetz, Melnick, &
            Adelberg's Medical Microbiology. 26th ed. 2012. New York: McGraw Hill. Print.
Madigan, M., and J. Martinko. "Brock Biology of Microorganisms, 11th edn." (2005): 149-152.
Spencer, Robert C. “Bacillus anthracis”. Journal of Clinical Pathology 56.3 (2003): 182-7. Print.
Théodoridès, Jean. "Casimir Davaine (1812–1882): a precursor of Pasteur."Medical
            History 10.02 (1966): 155-165. Print.

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