Bacillus anthracis as a Bioterrorism Agent
Introduction
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Description and Methods of Infection
Biology of Bacillus anthracis
Bacillus anthracis is the organism which causes the disease of anthrax. It is a large Gram positive aerobic, rod shaped, bacillus bacterium. It ranges from 1-1.5 x 3-10 µm in size and is the only obligate pathogen within the genus bacillus.1
Two virulence plasmids pXO1 and pXO2 encode the major virulence factors for B. anthracis.2 Plasmid pXO1 codes for three toxins which cause hemorrhage, edema, and necrosis. It is 184.5 kilobase pairs (kbp) in size. The exotoxins it codes for are binary, so that the protective antigen acts as a binding domain on the host thus permitting entry of the toxin. pXO2 is a smaller capsule bearing plasmid than pXO1 that is 95.3 kbp in size.It encodes three genes (cap A, cap B, and cap C) which is involved in the synthesis of the polyglutamyl capsule. The capsule prevents the bacteria from phagocytosis and is poorly recognized by the immune system of the host.3 Both pXO1 and pXO2 plasmids are necessary to provide the anthrax toxin and capsule respectively for B. anthracis to have full virulence. An attenuated strain results when either plasmid is lost.4
Anthrax has three routes of infection
Anthrax has three forms in which virulence occurs as cutaneous anthrax, gastrointestinal anthrax, and inhalational anthrax. Cutaneous anthrax comprises over 90% of all human cases. It is obtained via a lesion on the skin in which infection occurs through an abrasion, cut, or insect bite.5 This type of anthrax manifests itself through a black eschar associated with edema. Gastrointestinal anthrax results for the ingestion of undercooked meat from animals who have B. anthracis. This form of the disease has a high mortality rate because it is difficult to make an early diagnosis due to its’ non specific presentation. Inhalation anthrax is the third way in which anthrax can be contracted. It is of utmost concern for its use as an agent of bioterrorism. Inhalation anthrax was traditionally associated with industrial exposure as spores in the textile, meat packing, leather-tanning, bone meal processing, and hair/wool-sorting industries until its use as a bioterrorism agent in 2001. Beginning with “flu-like” symptoms of mild fever, fatigue, malaise, myalgia, and non-productive cough from two to five days after initial exposure, it develops into an acute illness characterized by acute illness characterized by dyspnea, stridor, fever, cyanosis, and pleural effusion. If not treated by multidrug antibiotic regimens and supported care, the disease is more likely to result in coma and death.
Characteristics of B. anthracis as an Agent for Bioterrorism
B. anthracis is a bacterium that can form spores that are resilient to harsh environments, such as ultraviolet and ionizing radiation, heat, and various chemicals. A spore is a dormant and resilient structure that can exist for extended periods of time with a lack of nutrients. A spore can convert to its vegetative state when the environmental conditions are favorable. By converting from a vegetative state to a spore state, the microorganism allows for its survival by withstanding a destructive environmental condition.
B. anthracis is difficult to detect because it has a significant amount of similarity to a variety of other organisms. These organisms include B. cereus, B. thuringiensis and B. mycoides, which along with B. anthracis comprise the B. cereus group of bacteria. Identified through 16S and 23S ribosomal RNA analysis, B. anthracis was shown to be quite similar to other Bacillus. Of the 16S rRNA sequences analyzed between B. cereus and B. anthracis, there was 100% sequence alignment. Additionally there was only a difference of four to nine nucleotides observed between the sequences of B. mycoides and B. thuringiensis. In the 23 rRNA sequence, there was a two nucleotide difference between B. anthracis and B. cereus. Thus due to the great homogeneity between the B. cereus species, B. anthracis is difficult to identify perhaps because the organisms live the majority of their lives as a spore. In this state, spores are not exposed to a DNA-changing events including the presence of phages and constant replication.
With its genetic similarity to other species, B. anthracis is difficult to detect and thus is a good agent for bioterrorism. Most commonly B. anthracis is contracted to infect humans through the form of inhalational anthrax. Inhalational anthrax is the type of infection caused by B. anthracis that is of heightened concern as a biological agent of terrorism. As it displays flu like symptoms initially after the B. anthracis is inhaled, it is difficult to diagnose the disease as anthrax. Once the infection is established, it becomes refractory to treatment; therefore, the success of medical intervention is minimal. If the infection is diagnosed very early on, the disease may be successfully treated with antibiotics. However, even with early medical intervention, the prognosis of patients with inhalational anthrax is dismal as the mortality rate is quite high.
B. anthracis has many genetic similarities among various Bacillus species and can exist in the spore and vegetative state. These features make the detection and specific identification of B. anthracis difficult and have proven to require complex techniques and laborious methods. As an agent for a potential biological threat, B. anthracis is more frequently being studied and methods to detect it are constantly being developed. Due to the heightened interest in the organism, there have been a large number of detection technologies that have recently been developed including methods involving those of immunological and nucleic acid based formats.
Section 3
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Conclusion
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References
Edited by Alison Lerner, a student of Nora Sullivan in BIOL187S (Microbial Life) in The Keck Science Department of the Claremont Colleges Spring 2013.
