Saturday, July 2, 2011

Can we control toxigenic E. coli?

Last week at the New Zealand Institute of Food Science and Technology annual conference, I presented a paper on non-O157:H7 shigatoxigenic E. coli.  That's a bit of a mouthful, if you'll pardon the unfortunate pun.  These bacteria have gained the ability to produce a toxin that can destroy intestinal epithelial cells and also damage the kidney.  E. coli O157:H7 is probably the best known of the Enterohaemorrhagic E. coli  (EHEC) types.  The strain causing the outbreak in Germany and other parts of Europe is a non-O157 type and indeed appears to be from a different group - the Entero-Aggregtive EC.

During the session, two other papers were presented, one on Cronobacter sakazakii - the neonate nasty, and one on Salmonella.  All three presenters talked about control of these food borne disease-causing organisms.

During the discussions, it became apparent that there was some difference of opinion on the effectiveness of control measures during production and processing of foods.

For example, the poultry industry in New Zealand has been remarkably successful in controlling Salmonella in poultry flocks by strict management of biosecurity, and thus eggs and chicken meat are essentially free of Salmonella.  Unfortunately, these controls have not worked for Campylobacter and there is still a significant rate of C. jejuni illness that can be traced to poultry, though this rate has fallen over the past two years.

Similarly, when I suggested that control of Salmonella would likely also control E. coli, a friend and colleague stood up and said that these two organisms are significantly different and controls would need to be tailored to each.  I invited him to come outside and we'd sort it out by fisticuffs!  However, it became obvious that we were talking at cross purposes and we were both right.

Salmonella, Cronobacter and E. coli are all vegetative bacteria.  That is, they do not form spores and are not particularly heat or chemical resistant.  Thus, during food processing or in the hands of the consumer, control is easy - heating to about 75-80C for a few seconds will kill them all.  If we prevent cross contamination, then the finished food will be safe to eat.

It is much more difficult to set conditions during primary production that will control these bacteria.  Vegetables are usually grown in soil.  They therefore become contaminated with soil organisms.  Even hydroponic growing systems may become colonised by bacterial biofilms and thus contaminate the products.  Animals and birds all carry populations of bacteria in their guts or on hides, hair, feathers and feet.  It is impossible to eradicate these bacteria, so they must be controlled during processing. 

But what do we do with vegetable sprouts?  These products must be regarded as hazardous - they are grown in conditions of high humidity and at temperatures that support the rapid growth of bacteria.  If pathogens, such as E. coli O104:H4 are present in the seeds, in the water or the equipment, they can grow rapidly.  Stir-frying will probably kill the bacteria, provided the temperature gets high enough.  Sprouts are also eaten raw, so there is no controlled lethal process step and the consumer ingests the bacteria, sometimes with fatal results.  If people are to continue to eat raw sprouts, we need to develop some means of decontaminating them, such as a rinse in a bactericidal chemical solution.  Nothing is perfect, but control of production, processing and distribution, together with consumer education, should decrease the likelihood of a similar outbreak to that currently occurring in Germany.

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