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E.coli: Wolf in sheep’s clothing

Updated Tuesday 14th June 2011

Why do helpful bacteria turn harmful? The recent E.coli outbreak in Germany and other parts of Europe highlights the devastating effects this microbe can have. And yet strains also form a vital part of the human gut environment. Lynda Cook, staff tutor in the Faculty of Science at The Open University.

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The recent outbreak of E.coli (Escherichia coli) infections in Germany and other parts of Europe has highlighted the devastating effects this microbe can have on human populations.  There have been 35 deaths and over 3,000 individuals infected with this particular strain of E.coli.  However, E.coli strains also form a vital part of the human gut environment, carrying out essential metabolic processes for our survival. So, how can E.coli seem like a “wolf in sheep’s clothing”?

E.coli bacteria are widespread, rod-shaped microbes (their size is a 1000th of this, -, hyphen).  Most strains of E.coli are harmless, residing peacefully in the gut of animals such as cattle and humans.  Within the human gut, E.coli colonises within two days of birth, contributing to the vast numbers of microbes that form an important community. E.coli performs a valuable function in the human gut – the production of vitamin K. Vitamin K is essential for the normal clotting of blood and its production by gut E.coli supplements the small amount obtained from the diet.

There are, however, many different strains of E.coli, some of which cause harmful effects. One such strain of E.coli causes a bloody diarrhoea, due to the production of a toxin (poison) called Shiga toxin (this strain is known as Shiga-toxin E.coli, STEC). The toxin damages small blood vessels, such as those in the gut and other organs, and in certain cases proceeds to cause kidney failure, by inducing haemolytic-uremic syndrome, which damages red blood cells and can be fatal. STEC is found in the guts of cattle and infection can occur via contact with cow meat or faeces.  So it is not surprising that the source of the German outbreak was thought to be from organic vegetables, where the use of manure as fertiliser is common.

Studying the genes of the E.coli involved in this latest outbreak has revealed that this particular strain carries not only the STEC genes but also genes from another harmful E.coli, known as Entero-aggregative E.coli (EAEC).  EAEC has previously only been found in humans and infection results in persistent diarrhoea. It is the previously unknown combination of genes in a single E.coli strain that appears to have been responsible for the intense virulence and widespread impact of this infection, coupled with the lack of detection of the disease-causing organism.
 
To understand how the evolution of E.coli can produce a killer we need to know the different methods available to bacteria to vary their genetic makeup. Bacteria can ‘share’ their genes, using a process known as conjugation or take up ‘new’ DNA from the environment, by transformation. However, reports suggest that the strain responsible for the German outbreak has evolved due to infection of STEC bacterium with a virus (lambda phage) carrying the EAEC genes. Clearly this potent combination of genes in the E.coli hybrid has resulted in the incredibly high number of ferocious infections in Germany.

The source of the hybrid E.coli outbreak has been traced to beansprouts, from a German organic farm. Further analysis will be needed to fully determine exactly how the hybrid E.coli reached dangerous levels in a human food source.

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