The genetic information bacteria need to make their proteins is stored as DNA in their chromosome. Like a toolbelt, this chromosome keeps hold of the chunks of DNA that make survival tools for the bacterium. These tools could be anything useful, from proteins that allow bacteria to stick to cells, find or use particular nutrients, or pump out harmful substances like antibiotics. It’s a case of use it or lose it – if it isn’t useful, future generations of those bacteria won’t carry that genetic tool in their chromosome toolbelt.

Also like a toolbelt, these chunks of DNA that code for the survival tools can be swapped in and out according to what other genetic tools are around, the situations the bacterium finds itself in and which tools the bacterium already has. Some tools can be found and moved around by viruses that infect bacteria (bacteriophages). These viruses package their DNA into their head capsules, but often take some of their bacterial host’s DNA too. When the bacteriophage injects its DNA into the next bacterium, it gives the next bacterium some DNA from the virus’ last host. If there are any useful tools there, the next bacterium might keep them.

This is what experts think happened before the 2011 sprouts E. coli outbreak in Germany, which affected 3842 people, and killed 53. The outbreak was caused by a new Shiga toxin producing E. coli (STEC) – a very sticky (enteroaggregative) E. coli strain that had taken a Shiga toxin from a well known STEC. This very sticky E. coli did what it normally does with it’s tools – stick really well to the walls of the gut in high numbers. Coupled with it’s new toxin tool, it was a deadly combination – lots of bacteria meant lots of toxin…