Anthrax Sterne strain (34F2) of Bacillus anthracis

What is the Sterne strain (34F2) of Bacillus anthracis?

Bacillus anthracis is a spore-forming bacterium that causes anthrax in humans and animals. B. anthracis has 3 main virulence factors coded on 2 plasmids, pXO1 and pXO2. If one of these two plasmids is missing, the organism cannot produce all of its virulence factors. The resulting organism is attenuated, meaning its virulence and the ability to cause illness in people or animals have been reduced. The pXO1 plasmid controls the production of the anthrax edema and lethal toxins, which are made of three proteins, the edema factor, the protective antigen, and the lethal factor. The pXO2 plasmid codes for the capsule, a layer of polysaccharides outside of the cell wall that protects the bacteria against phagocytosis, or consumption by defensive cells from the immune system. Without its capsule, the bacteria can be phagocytised and destroyed.

The Sterne strain, discovered in the 1930s, has naturally lost its pXO2 plasmid, and consequently its ability to produce a capsule. Compared with normal wild type strains which produce both the toxin and the capsule, the Sterne strain is relatively avirulent, however immunization using the Sterne strain is able to stimulate a protective immune response.

The Sterne strain is currently the predominant strain used for immunization of domesticated animals against anthrax worldwide, and has been used for this purpose for many decades. It is administered to livestock in a dose containing up to 10 million viable spores. The Sterne strain has an excellent safety record, and has been used safely worldwide by laboratory staff involved in its preparation and by hundreds of thousands of veterinarians.

Can the Sterne strain cause infections in people?

No human disease due to anthrax caused by the Sterne strain has been reported.

Theoretically, there are two mechanisms by which the Sterne strain may cause anthrax. First, the toxin alone might be sufficient to cause illness if mechanical protective mechanisms such as skin and mucous membranes are bypassed. In mice, the attenuated strains possess a low degree of virulence, due to toxin production.1-3 The toxin causes occasional losses observed among animals receiving a full dose of vaccine. For this reason, live vaccines have never been considered suitable for human use in the United States.4

Second, the Sterne strain might regain pXO2 and revert to capsule production, resulting in wild-type virulence. No reversion to virulence has been seen in the Sterne strain since its discovery in 1937.

What activities might lead to exposure to the Sterne strain?

Routine manipulation of the strain in a microbiology laboratory is not likely to result in exposure. Risks to personnel handling this organism are no greater than those posed by other category 2 organisms and activities. Using proper laboratory precautions reduces the risk of percutaneous exposure. Good laboratory practices include the use of appropriate PPE (i.e., gloves, gowns, or laboratory coats) and the use of a biosafety cabinet for procedures with the potential to produce aerosols.5 Eye and face protection (i.e., goggles, masks, or face shields) should be used when splashes or sprays are possible outside of the biosafety cabinet.6

It is probably not possible for humans to acquire a Sterne strain infection by the respiratory or oral route.7 However, in the event of an accident during routine manipulation, there is a slight risk of infection. In a worst case accident scenario, the risk exists when spores make contact with a preexisting wound, or if an injury results in a contaminated wound. Worst case accident scenarios are estimated to result in 3×10-3 chance of percutaneous infection. With the addition of precautions such as good laboratory practices, appropriate glove use, and training for immediate first aid action, the estimated probability of percutaneous infection drops to 3×10-10. Additionally, the chance of an infection becoming severe or fatal can be reduced to less than 10-14 with the use of antimicrobial agents when necessary.7

What can those exposed do to decrease their risk of anthrax?

While it is highly unlikely that the Sterne strain will result in infection, cutaneous anthrax can be successfully treated with antimicrobial agents, making it improbable that a localized infection can become severe or fatal. Please consult your personal healthcare professional regarding treatment if you think you have been exposed through an accident which may have contaminated a preexisting wound or caused injury resulting in a contaminated wound.

What is the Sterne strain (34F2) of Bacillus anthracis and am I at risk for being infected?

Bacillus anthracis is a spore-forming bacterium that causes anthrax in humans and animals. The Sterne strain (34F2) of Bacillus anthracis was discovered in the 1930s and has been used successfully as the predominant method worldwide to immunize livestock against anthrax since its discovery.

The Sterne strain is avirulent, meaning its ability to cause illness in people or animals have been reduced. This is because the Sterne strain has lost its ability to produce a capsule, or a layer of polysaccharides, which protects it from being consumed and destroyed by our defensive immune system cells. The Sterne strain is sometimes used in microbiology laboratories when they are testing the ability to accurately identify and diagnose anthrax, and it is sometimes used for anthrax research. The Sterne strain poses almost no risk for infection of laboratory workers who are following good laboratory practices. To date, no human disease due to anthrax caused by the Sterne strain has been reported.


  1. Welkos SL, Keener TJ, and PH Gibbs. Differences in susceptibility of inbred mice to Bacillus anthracis. Infect Immun 1986; 51:795-800.
  2. Welkos SL, and AM Friedlander. Pathogenesis and genetic control of resistance to the Steme strain of Bacillus anthracis. Microbial Pathogenesis 1988; 4:53-69.
  3. Welkos, SL. Plasmid-associated virulence factors of non-toxigenic (pXO1-) Bacillus anthracis. Microbial Pathogenesis 1991; 10:183-198.
  4. Shlyakhov EN and E Rubenstein. Human live anthrax vaccine in the former USSR. Vaccine 1994; 12: 727-730.
  5. CDC. Inadvertent Laboratory Exposure to Bacillus anthracis — California, 2004. MMWR April 54(12):301-304.
  6. U.S. Department of Health and Human Services, Centers for Disease Control and Prevention and National Institutes of Health. Biosafety in Microbiological and Biomedical Laboratories (BMBL). 6th ed. Washington, DC: US Government Printing Office; 2020. (available at
  7. Turnbull, P. Information for Risk Assessments in the Handling of Vaccine Strains of Bacillus anthracis. Internal Report. 2000.