Anthrax bacteria get help from viruses and worms to survive

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When the bacteria that cause anthrax (Bacillus anthracis) aren’t ravaging livestock or being used in acts of bioterrorism, they spend their lives as dormant spores. In these inert but hardy forms, the bacteria can weather tough environmental conditions while lying in wait for their next host. This is the standard explanation for what B.anthracis does between infections, and it’s too simple by far. It turns out that the bacterium has a far more interesting secret life involving two unusual partners – viruses and earthworms.

A dying animal can release up to a billion bacterial cells in every single millilitre of blood. This torrent of microbes provides a feast of riches for bacteriophages – viruses that infect bacteria. Raymond Schuch and Vincent Fischetti from the Rockefeller University  have found that the anthrax bacterium depends on becoming infected by phages. They began by isolating several strains of phages that specifically infect B.anthracis. The viruses hailed from a range of sources, including the soil, plant roots and worm guts. <

When these phages find bacterial targets, they inject their own DNA, which insinuates itself into the genome of the host. This process is called lysogeny and it is essential for the bacterium’s survival. The added viral DNA encodes proteins called sigma factors that change how bacterial genes are switched on. In doing so, they change the behaviour of the bacteria, giving them new abilities that boost their survival and allow them to colonise an intermediate host – the earthworm.

With their newly incorporated viral DNA, some bacteria formed spores while others were actually prevented from doing so, depending on the phage. Regardless, all the anthrax bacteria grew at almost twice the rate. The phage DNA brought out the social side of the bacteria, inducing them to cluster in groups. It also made them more likely to secreted more complex sugar molecules that form the building blocks of biofilms – the bacterial equivalent of towns and cities. Amid this matrix of sugars, the cells find shelter and protection.

Small wonder then that the infected bacteria are much better are surviving for long durations. Their advantage was so great in comparison to virus-free strains that Schuch and Fischetti suggest that phage infections may actually be necessary if anthrax bacteria are to survive in soil. Indeed, duo identified three bacterial genes that are activated by the phages and that are necessary for eking out a living in soil. When they inactivated these genes, the bacteria survived in these environments for the briefest of times.

The bacteria don’t have to survive in isolation either. Schuch and Fischetti speculate that their biofilms act as a staging ground from which to find a new host. Again, their viral hitchhikers come into play, giving them the ability to set up long-term colonies in the guts of earthworms. That’s hardly an easy environment, for it’s extremely low in oxygen and most bacteria are digested or excreted. Any permanent hangers-on must be able to stick tightly to the walls of the gut. The genetic manipulations of the virus could activate some latent ability of the bacteria to do just that.

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The idea of worms as alternative hosts for anthrax bacteria, in between their decimations of livestock, was first put forward by Louis Pasteur in the 19th century, after he noticed that the soil near anthrax carcasses were rife with earthworms. Ignored for over a century, Pasteur’s idea has finally been confirmed.

The viruses within the bacteria aren’t totally dormant. Within a small proportion of cells, they multiply as viruses typically do, bursting out of their host and shedding thousands of infectious daughter virses into the environment.  This process may kill a few of the anthrax bacteria, but it provides a route for the survivors to trade genetic material between each other.

As phage DNA hops in and out of bacterial genomes, they could take snippets of local DNA with them, transferring them from host to host and increasing the genetic diversity of the population. Don’t underestimate how extreme these changes can be: in a previous study, Jonathan Kiel showed that a phage taken from a related species, Bacillus cereus, managed to change a strain of anthrax bacteria so greatly that it was no longer genetically recognisable as the original strain, or even as the right species!

The picture painted by this new study is a far cry from the somewhat dull idea of anthrax bacteria lying dormant in the soil. Instead, it seems that the bacteria lead a secret life, and a most dynamic one, involving hidden potential unleashed by bacterial invaders-turned-partners.  

Reference: Schuch, R., & Fischetti, V. (2009). The Secret Life of the Anthrax Agent Bacillus anthracis: Bacteriophage-Mediated Ecological Adaptations PLoS ONE, 4 (8) DOI: 10.1371/journal.pone.0006532

Images: Cow by Daniel Schwen

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