Those of you who live in Germany, as I do, are probably concerned about the recent and ongoing E coli outbreak that has killed 10 people (so far) and sickened more than 1000. The news coverage provides updates on the whos, whats, wheres and whys of this outbreak, but discussions of how such events occur are lacking or incomplete, so I thought I'd fill you in.
Contrary to what you might think after seeing most news reports, not all E coli (shortened from their scientific name, Escherichia coli) are toxic: in fact, most strains are beneficial to their hosts. E coli are normal gut flora whose preferred habitat is the large intestine. They are protective because shortly after a baby is born, E coli become established in the gut, crowding out disease-causing pathogenic bacteria by filling up all available space with their sheer numbers. E coli are also beneficial; producing vitamin K2, which has a variety of functions in the body.
Like most organisms, when E coli get into the wrong place in the body, even non-pathogenic strains can cause problems, such as urinary tract infections. But E coli die fairly quickly after they've left the body so they generally don't cause problems in the environment. But besides being disgusting, the presence of E coli in the environment is a clear indication that fecal contamination has occurred. Your local health officials rely on this as a handy method to test the cleanliness of food and water by looking for the presence of E coli.
But E coli are not restricted to human colons: farm animals' guts are home to their own strains of E coli. So identifying whether contaminating feces originated from a malfunctioning municipal sewage treatment plant or from farm runoff requires DNA testing. Nevertheless, there's one complication for narrowing down the source for contaminating E coli and that stems from a trait that all bacteria share: I refer to this trait as "DNA swapping".
DNA swapping is my picturesque terminology to describe the fascinating and strange ability of bacteria to behave as if they are a super-organism comprised of different cell types instead of just different species of single-celled organisms. Basically, bacteria do not respect conventional species concepts. By this, I am referring to their habit of sharing small segments of their DNA with neighboring bacteria, regardless of "species". Because bacterial DNA swapping is different to what we see when parents provide copies of their DNA to their offspring (vertical gene transfer), this process has another name: horizontal gene transfer. More than simply hybridising a horse with a donkey to create a sterile mule, bacterial "species" that are as different from each other as a human and a parrot can share select segments of their DNA and still be capable of reproducing.
Bacteria swap DNA using three distinct processes, but the most likely way that the E coli that are attacking northern Germany picked up their toxic genes is through a process known as bacterial conjugation -- "bacterial sex":
Bacterial conjugation is where one bacterium -- the donor -- transfers DNA to another bacterium -- the recipient -- using a "sex pilus" (the word "pilus" is misspelled in the above diagramme). But conjugation is not identical to sexual reproduction because, instead of sharing half its genome with its offspring, donor bacteria only transfer a small segment of DNA to recipient bacteria -- which are not its offspring. This small segment of bacterial DNA is not contained within the bacterial genome at all. Instead, it is a small ring of extrachromosomal DNA known as a "plasmid". Plasmids do not contain any of the hundreds of basic housekeeping genes that are essential for life. Instead, they contain just a few "extra" genes that may come in handy under certain circumstances; genes such as those encoding resistance to a particular type of antibiotic, for example.
Almost like miniature stamp collectors, bacteria are avid collectors and hoarders of plasmids. Further, they are not picky about the sorts of DNA they acquire nor are they particular about the plasmid DNA's source. Friendly gut-dwelling E coli can pick up genes that, in a Jekyll-and-Hyde scenario, can transform them into little monsters that are harmful or fatal to their hosts. One such plasmid provides the genetic blueprints for generating a toxin that is strikingly similar in form and function to that produced by the pathogenic bacteria, Shigella dysenteriae.
The Shigella toxin specifically attacks and kills mucous membrane cells, particularly those in the intestines, lungs and kidneys. This triggers a massive wave of dead and dying cells followed by widespread haemorrhage -- the cause of bloody diarrhoea (dysentery), haemorrhagic colitis, and haemolytic uremic syndrome -- any of which can be fatal. Scientists have identified the E coli strain that is sickening Germans (and visitors to northern Germany) as STEC -- Shiga toxin-producing E coli.
Since friendly E coli have already filled up all available spaces in the human colon, deadly Shigella dysenteriae cannot easily gain a foothold there -- but its toxin can. Encoded on a plasmid, instructions for producing the shiga toxin can be slipped into friendly gut bacteria through the Trojan horse-like efficiency of bacterial conjugation, transforming our formerly protective E coli into little monsters. Further, also due to bacterial conjugation, the infective dose of this particular strain of E coli is astonishingly small -- as few as 10 to 100 bacteria (Greig et al, 2010).
But STEC can't arise de novo: it has to come from somewhere. The evidence suggests that the source of this pathogenic E coli infection is raw vegetables (probably raw tomatoes, fresh cucumber and lettuces) imported into Germany from Spain. How were the vegetables exposed to this disease? Did a sick person handle them at some point? Considering the hundreds of people involved with producing, transporting and preparing food; from the farmers, pickers, packers, and truckers, to the grocery shelf stockers, the chefs, and even what happens in most people's kitchens whilst a meal is prepared, it's surprising that this sort of outbreak doesn't happen more often.
But how did these fresh and healthy vegetables become infected? Common sense suggests that they weren't infected: vegetables don't have colons for E coli to colonise. E coli infections originate in domestic livestock, whilst the vegetables were the vector -- the mechanical means for getting STEC into people. Furthermore, the sheer size of this outbreak and the variety of vegetables involved suggests a basic, common source: polluted groundwater.
Epidemiologists have documented that some E coli strains that are pathogenic in humans can be found naturally in cattle (where they do not cause disease, and therefore are beneficial to them). Since cattle require a lot of water every day (whenever possible, they enjoy standing around knee-high in creeks ... erm ... defecating) and because cattle are often raised in highly crowded conditions, they can be a point-source for E coli infections. It is possible that sewage runoff from cattle farms located either upstream or uphill from vegetable farms could be the source for this contamination. But that said, it is only fair to point out that at this time, Spanish authorities claim that water testing has not revealed the presence of STEC.
So even though fresh lettuces, tomatoes and cucumbers have been removed from restaurant menus and store shelves, and people have been cautioned to follow careful hygienic practices, there are ten people and their families out there who are not so lucky, and there are hundreds more who are facing a future of dialysis and other challenges. This deadly E coli outbreak serves as a tragic reminder that we are only as strong as our weakest link.
EU Reaction Mixed as E. coli Outbreak Spreads (accessed 0818 BST 30 May 2011)
European Centre for Disease Prevention and Control (27 May 2011). Outbreak of Shiga toxin-producing E. coli (STEC) in Germany [free PDF]
J.D. Greig, E.C.D. Todd, C. Bartleson, and B. Michaels. (March 25, 2010). "Infective Doses and Pathogen Carriage", pp. 19-20, USDA 2010 Food Safety Education Conference [free PDF].