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Food safety: reducing the healthy carriage of enterohaemorrhagic E. coli  by cattle

INRA scientists have demonstrated the fundamental role of certain sugars in the survival of E.coli O157:H7 in cattle intestine. Restricting substrate availability for these pathogens would reduce their presence in the digestive tract of animals and hence the risks of the contamination of meat or dairy products. 

Salers cattle in the Aveyron (south-west France). © INRA, WEBER Jean
Updated on 09/23/2013
Published on 09/12/2013

Enterohaemorrhagic E. coli bacteria (EHEC)

Food poisoning due to enterohaemorrhagic Escherichia coli (EHEC) first appeared on the global stage in the early 1980s.  Since then, numerous outbreaks have occurred in Europe, the USA and Asia, mainly due to the O157:H7 serotype.  This type of food poisoning can cause severe complications, particularly in children, such as haemorrhagic colitis (HC) or haemolytic-uremic syndrome (HUS).  In most cases, the consumption of raw or undercooked animal products (milk, cheese, meat, etc.) contaminated by this bacterium was responsible for these outbreaks.  Water supplies, and fruits and vegetables contaminated by the spreading of  manure from livestock carrying EHEC, have also been implicated in cases of food poisoning. 

 The digestive tract of cattle: a natural reservoir for EHEC

Although O157:H7 is pathogenic in humans, it can colonise the digestive tract of cattle without triggering any clinical signs, referred to as healthy carriage.  The bacteria preferentially establish themselves in the terminal portion of the intestine (recto-anal junction), then in the colon and small intestine.  However, recent studies have shown that this pathogen is capable of colonising the entire digestive tract.  
The pathogens are dispersed into the animal's environment through its faecal material, and may then be transferred to milk or meat during milking or meat cutting operations after slaughter.  A reduction in the healthy carriage of O157:H7 by animals is thus a key step forward in reducing the risk of food contamination by EHEC.  

Carbohydrates: an essential factor in the survival of EHEC

Although the organs where EHEC establish themselves have been clearly identified, the mechanisms by which they survive and grow in the intestine are poorly understood.  In the context of the European ProsafeBeef project (FP6), INRA researchers have been studying, in collaboration with scientists from the Lallemand Animal Nutrition company, and the Universities of Lille (France), Montreal (Canada), Rhode Island (USA) and Oklahoma (USA), the strategy developed by EHEC to detect the carbohydrates that are available and can be used for their growth.  Indeed, the intestine contains low levels of carbohydrates, mono or disaccharides, which are absorbed by intestinal cells or by the surrounding microbiota, and the O157:H7 bacteria preferentially use some of them to grow. 

 A survival strategy linked to carbohydrates in the intestinal mucosa

The scientists have shown that EHEC are capable of using the carbohydrates that form part of mucin, a glycoprotein that is secreted by intestinal cells to constitute mucus.  This mucus, which coats the intestinal wall, protects it from digestive enzymes and pathogenic bacteria.  Degraded by some commensal micro-organisms within the intestine, mucin releases a variety of carbohydrates which are then found in the intestinal lumen.  
The EHEC can then make use of these carbohydrates.  To achieve this, the bacteria very rapidly deploy an enzymatic arsenal that is capable of degrading these molecules.  The scientists have demonstrated that EHEC utilise these carbohydrates more rapidly than the bacteria present naturally in the intestine - the microbiota - which include commensal Escherichia coli, thus endowing them with a certain competitive advantage.
The researchers will now be working to identify bacterial strains that could be administered to animals - probiotics - and outcompete EHEC regarding the utilisation of these carbohydrates.  This competition for nutrients could then compromise the survival of EHEC within their natural reservoir, and enable a reduction in the attendant health risks.  

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Find out more

  • Carbohydrate utilization by enterohaemorrhagic Escherichia coli O157:H7 in bovine intestinal content. Bertin, Yolande; Chaucheyras-Durand, Frederique; Robbe-Masselot, Catherine; et al. ENVIRONMENTAL MICROBIOLOGY  Volume: 15 DOI: 10.1111/1462-2920.12019   Published: FEB 2013