So-called toxin-antitoxin systems, a kind of on-off switch in many bacterial DNA genomes, are increasingly being found to defend bacteria against attack by bacteriophages — viruses that infect bacteria. Activation of toxins allows bacterial populations to go into a kind of lockdown that limits growth and therefore the spread of the virus. As such, understanding the diversity, mechanisms and evolution of these systems is critical for the eventual success of phage therapy to treat antibiotic resistance infections. — Toxin-antitoxin pairs consist of a gene encoding a toxin that dramatically inhibits bacterial growth and an adjacent gene encoding an antitoxin that counteracts the toxic effect. It is like keeping a bottle of poison on a shelf next to a bottle of the antidote. While toxin-antitoxin pairs have been seen to evolve to associate with new toxins or antitoxins before, the scale of the neutralisation ability seen with Panacea — so called hyperpromiscuity — is unprecedented, explains researcher and group leader Gemma Atkinson at Lund University, who has led the study.

PhD student and co-first author Chayan Kumar Saha made a computer program for analysing the kinds of genes that are found next to each other in bacterial genomes. The team then used this tool to predict new antitoxin genes found next to some very potent toxins that they have previously worked on. “We were startled by the discovery that one particular antitoxin protein fold can be found in toxin-antitoxin-like arrangements with dozens of different kinds of toxins. Many of these toxins are new to science.”

The other first author Tatsuaki Kurata, Lund University, has confirmed experimentally that several of these systems are genuine toxins neutralized by the neighbouring antitoxin genes.

The study shows that what we know so far about the diversity of toxin-antitoxin systems probably is just the tip of the iceberg, and that there could be a range of similar systems that have gone under the radar until now. — As well as being important for understanding the weird and wonderful world of bacterial biochemistry, the discovery of new toxin-antitoxin systems is important for so-called phage therapy against antibiotic resistant infections. As bacteria have increasingly become resistant to antibiotics, other approaches are needed for eliminating infections.

The principle of phage therapy is to treat patients with cocktails of bacteriophages — viruses that infect bacteria — in order to kill the bacteria causing infection. However bacteria carry various defence systems to protect themselves from phages, and this includes toxin-antitoxin systems.

“Thus identifying toxin-antitoxin systems of pathogens may help us in the future design phage therapy that can counter this layer of defence,” explains Gemma Atkinson.

So, what is the next research step?

“We are now trying to find novel toxin-antitoxin systems on a universal scale, and understand their involvement in phage defence. We are also interested in possible biotechnological applications of toxin-antitoxin systems, given that these systems can be thought of as on-off switches of core aspects of bacterial biology. The full set of toxin-antitoxin systems could be a molecular toolbox for tweaking bacterial metabolism and controlling bacterial cell resources. This can be important in industrial and pharmaceutical manufacture situations where bacteria are used to produce molecules of interest.”

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