Ames strain
The Ames strain is one of 89 known strains of the anthrax bacterium (Bacillus anthracis). It was isolated from a diseased 14-month-old Beefmaster heifer that died in Sarita, Texas in 1981. The strain was isolated at the Texas Veterinary Medical Diagnostic Laboratory and a sample was sent to the United States Army Medical Research Institute of Infectious Diseases (USAMRIID).[1] Researchers at USAMRIID mistakenly believed the strain came from Ames, Iowa because the return address on the package was the USDA's National Veterinary Services Laboratories in Ames and mislabeled the specimen.[2]
The Ames strain came to wide public attention during the 2001 anthrax attacks when seven letters containing it were mailed to media outlets and US Senators on September 18, 2001, and October 9, 2001.
Because of its virulence, the Ames strain is used by the United States for developing vaccines and testing their effectiveness. Use of the Ames strain started in the 1980s, after work on weaponizing the Vollum 1B strain ended and all weaponized stocks were destroyed after the end of the U.S. biological warfare program in 1969.[3]
Virulence[]
Virulence plasmids[]
Researchers have identified two specific virulence plasmids in B. anthracis, with the Ames strain expressing greater virulence compared to other strains. The virulence of B. anthracis results from two plasmids, pXO1 and pXO2. Plasmid pXO2 encodes an antiphagocytic poly-D-glutamic acid capsule, which allows B. anthracis to evade the host immune system. Plasmid pXO1 encodes three toxin proteins: edema factor (EF), lethal factor (LF) and protective antigen (PA). Variation in virulence can be explained by the presence or absence of plasmids; for example, isolates missing either pXO1 or pXO2 are considered attenuated, meaning they will not cause significant infection. One possible mechanism that may be responsible for the regulation of virulence is the copy number of plasmids per cell. The number of plasmids among isolates varies, with as many as 243 copies of pXO1 and 32 copies of pXO2 per cell. Studies have shown that pXO2 contributes significantly to the observed variation in virulence, as mutants producing greater amounts of the capsule show a higher level of virulence.[4] Virulent strains that were cured of the pXO1 plasmid, but had the Ames pXO2 plasmid were still fully virulent for mice; thus, the Ames pXO2 plasmid specifically appears to give a higher level of virulence, as strains that are missing one of the plasmids are usually attenuated. Additionally, isolates that carried the Ames pXO2 were found to be more virulent than those with the Vollum 1B strain pXO2, also a virulent strain.[5] Another well-known strain of anthrax, the Sterne strain, is avirulent, meaning it does not cause significant illness in animals or humans.
Antibiotic resistance[]
The Ames strain is resistant to antibiotics, while other strains do not show the same level of resistance. The Ames strain is difficult to treat because it contains both an inducible