Monday, January 21, 2013

It's not CSI

Most of us have seen the odd episode, or at least been aware of the popular CSI-Miami or other incarnations of the crime drama TV series.  Those of us who are scientists are somewhat cynical about the ease with which samples from crime scenes can be analysed, seemingly within hours, by whizzy machines in gleaming laboratories.

In most cases, this is far from the truth; analysis of DNA samples, for example, requires painstaking care during the collection and processing of the material.  You only have to see the real world courtroom challenges to forensic laboratory evidence to realise that the whole process is much more complicated than the TV programmes would have us believe.

However, the detection of pathogenic bacteria in foods, or even the deliberate adulteration of beef burgers with other meats, is now benefiting from these molecular techniques.

The ability to sequence whole genomes of bacteria, coupled with the cheap synthesis of primers (probes) that will bind to specific parts of the bacterial DNA has enabled us to test for the presence of pathogenic bacteria in food samples. 

In principle, we extract the bacterial DNA and add our probes to bind to unique sequences in the DNA.  If the probe binds to the DNA, then we can use the Polymerase Chain Reaction** to amplify that piece of DNA and then detect it, separating it on a gel to produce a pattern of bands similar to what we see being examined on CSI.  If there is no binding, no amplification occurs and no detection, so the bacteria are absent from the food.

In practice, it's a bit more complicated and time-consuming.  We normally have to 'selectively enrich' our sample to increase the number of bacteria to a level at which we can detect them.  We do this by adding the food to a culture medium that inhibits most other bacteria and encourages our target bacteria to grow.  The actual preparation of the media, weighing the sample and putting them together takes only a couple of hours.  However, we need to incubate the mixture for up to 48 hours under controlled conditions before we can run the PCR.

Over the past few weeks, I have been working with my research assistant, testing a new PCR-based method of detecting Listeria.  It looks as though the method will be quicker and easier than existing methods and we'll publish our results in the near future.

Manufacturers continue to develop new rapid methods, many based on DNA and using automated equipment, making the detection of pathogens in food easier and quicker, allowing products to be checked and released to the market earlier.  These methods can also be used to track down sources of contamination, such as in the European E. coli O104:H4 outbreak of 2011.

**  For those readers keen to know more about PCR, I'll post a more complete description of the technique, trying to keep it relatively simple.

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