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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 to10 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 can those exposed do to decrease their risk of anthrax?

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.

Anthrax is an acute infectious disease caused by the spore-forming bacterium Bacillus anthracis. Anthrax most commonly occurs in wild and domestic lower vertebrates (cattle, sheep, goats, camels, antelopes, and other herbivores), but it can also occur in humans when they are exposed to infected animals or tissue from infected animals.

Genomic study of 412 anthrax strains provides new virulence clues

Georgia Institute of Technology

By analyzing genomic sequences from more than 400 strains of the bacterium that causes anthrax, researchers have provided the first evidence that the severity – technically known as virulence – of specific strains may be related to the number of copies of certain plasmids they carry. Plasmids are genetic structures of the cell that can reproduce independently, and are responsible for producing the anthrax toxin and other virulence factors.

The research found that bacteria strains collected from humans and animals tended to have more copies of the virulence plasmids than those collected from environmental sources. The research, a collaboration between scientists at the Georgia Institute of Technology and the Centers for Disease Control and Prevention (CDC), used CDC’s collection of Bacillus anthracis strains gathered from around the world beginning in the 1950s.

“There is a hypothesis that the copy number – number of copies of the plasmids – plays a role in how virulent each strain is,” said Kostas Konstantinidis, a professor in the School of Civil and Environmental Engineering at the Georgia Institute of Technology. “We want to understand which are the more virulent strains, which are less virulent and what explains the difference. This study provides the first evidence that there is a significant difference in plasmid copy number that may be related to the virulence. But more research is needed to test this emerging hypothesis.”

The research, which was sponsored by the CDC and the National Science Foundation, was reported August 14 in the journal mSystems, an open access journal from the American Society for Microbiology. The study involved more than 600 gigabytes of data, which will be shared with other researchers working to understand anthrax.

In B. anthracis, two plasmids – known as pXO1 and pXO2 – are autonomous and independent pieces of DNA that encode the toxin and other virulence factors. In bacteria, plasmids like these tend to move around independently of the organisms’ main chromosomes, and can jump from one strain to another via genetic mechanisms of DNA transfer. Bacterial resistance to antibiotics can also be transferred through plasmid movement, for instance. Interestingly, this study found that the anthrax plasmids show limited genetic exchange between strains; rather, they are inherited from the ancestor, similar to the chromosomes.

“This work and additional analyses performed at Georgia Tech and CDC on these genomes from strains of B. anthracis from around the world help further define the global diversity of this health threat,” said Alex Hoffmaster, chief of CDC’s Zoonotic Select Agent Lab and a co-author on the article. “Understanding more about the strains and their distribution can help us more easily determine whether anthrax cases were caused by natural or man-made sources so we can respond as needed to protect the public’s health.”

The research could also provide information about other organisms, Konstantinidis said. “Beyond B. anthracis, this work could help provide a better understanding of the virulence potential of other organisms carrying similar plasmids. Being able to distinguish between more virulent and less virulent strains is a broader challenge for microbiology.”

The study began with researchers from the CDC’s Division of High-Consequence Pathogens and Pathology, who used next-generation techniques to sequence diverse strains of B. anthracis. The agency provided its data to the Konstantinidis laboratory at Georgia Tech, where Graduate Research Assistant Angela Pena-Gonzalez led the bioinformatics analysis of the data.

“We were able to use the sequencing data to calculate the correlation of the copy number with the source of each strain,” she explained. “We used the whole length of the plasmid to calculate the copy number, and our results were based on the analysis of hundreds of strains obtained from several sources – humans, animals and the environment – and not just a couple of them. This was an advantage of our study.”

Analytical techniques developed at Georgia Tech allowed the whole genome comparisons to be done on the more than 400 genomes – a substantial data science challenge. The research revealed that B. anthracis genomes carried, on average, 3.86 and 2.29 copies of the pXO1 and pXO2 plasmids respectively, and that there was a positive linear correlation between the copy numbers of the two plasmids.

“The technology to do this whole genome sequencing is available, but the processing of the data and the interpretation is not yet very straightforward,” Konstantinidis said. “The very magnitude of the data requires a specific process and considerable experience. The way in which we analyzed this data was not even available two years ago when we started the study.”

Beyond the possible correlation of copy number of virulence, the study also showed the genome of the strains was surprisingly consistent. “The work shows that these plasmids are relatively stable, though we found a few strains that had different varieties of the plasmids that seem to have attenuated the virulence,” he said.

A next step would be to investigate further the possible correlation between copy number and virulence in animal studies.

Konstantinidis hopes to continue collaborating with the CDC to gain a better understanding of virulence and other factors in anthrax and other organisms that have implications for public health.

“The CDC has unique resources like this collection of anthrax strains, and we hope to continue this collaboration to further understand what is going on with this and other pathogens,” he said. “There are a lot of applications to public health and to improving our understanding of the basic biology behind these organisms.”

In addition to those already mentioned, the work included Luis M. Rodriguez-R from the Georgia Tech School of Civil and Environmental Engineering; and Chung K. Marston, Jay E. Gee, Christopher A. Gulvik, Cari B. Kolton, Elke Saile and Michael Frace from the CDC.

This work was supported by United States National Science Foundation (NSF) award number 1356288 and DHHS/PHS/CDC award number RF023, and by Colciencias–Colombian Administrative Department for Science, Technology. The findings and conclusions in this work are those of the authors and do not necessarily represent the official positions of the NSF or CDC.

CITATION: Angela Pena-Gonzalez, et al., “Genomic Characterization and Copy Number Variation of Bacillus anthracis Plasmids pXO1 and pXO2 in a Historical Collection of 412 Strains,” (mSystems 2018) https:/ / msystems. asm. org/ content/ 3/ 4/ e00065-18

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By analyzing genomic sequences from more than 400 strains of the bacterium that causes anthrax, researchers have provided the first evidence that the severity — technically known as virulence — of specific strains may be related to the number of copies of certain plasmids they carry.