Viral message
While firms routinely test for bacteria, yeasts and moulds, foodborne viruses largely go unchecked and they are on the increase, says the Health Protection Agency. There were 1,922 reported cases of norovirus in the year 2000 in England and Wales and 11.776 in 2010.
But virus tests may soon be made more widely available in the UK. Polymerase chain reaction (PCR) tests for viruses are already in use at food testing laboratory Eurofins' partner laboratory in Nantes, France, and it is looking at introducing the tests in the UK. Keith Watkins, microbiology specialist at Eurofins, says that could happen this year, but there are big cost barriers. Watkins says virus testing is likely to cost around £150 a test. By comparison, a standard test for salmonella costs just £4.
Noroviruses are the single biggest cause of gastro-intestinal disease in the western world. Foodborne viruses are usually associated with contaminated fresh produce particularly shellfish.
The Food Standards Agency (FSA) has included norovirus in its list of the top six pathogens in its 2010-2015 strategy for reducing foodborne illness, but Watkins says demand from industry is still uncertain. "We can introduce it but we can't make people test, and if they won't test then it's not worth us introducing it."
Unless retailers insist, virus testing may remain a rarity. However, he says the scare last autumn means testing on shellfish is likely to increase and testing may also be introduced for soft fruit. There could also be demand for tests for other viruses, such as Hepatitis A, Hepatitis E and rotovirus, says Watkins
Catherine Cockcroft, head of microbiological services at Exova, another food testing laboratory, agrees the lack of testing facilities need to be addressed. "The viruses are not new, but we didn't previously have the information to suggest they were a big issue," she says. "We as an industry and the EU are trying to get a handle on the problem."
There is also controversy over the efficacy of existing testing methods. "The difficulty with detecting human noroviruses is they do not grow in culture, so we have to rely on molecular techniques for their detection and rather complicated extraction and concentration procedures to detect them in the first place," says Angus Knight, principle consultant at Leatherhead Food Research.
There are significant problems with then interpreting the results, he adds. "The technique that is used is a molecular technique known as the reverse transcribed quantitative polymerase chain reaction (RT-qPCR)," says Knight. "Most people are familiar with PCR but the problem with it is it doesn't distinguish between infective virus particles and virus particles that are not infective. So detection on its own really doesn't tell us very much." Knight would prefer test results to be confirmed "either by a separate assay, or by a characterisation of the PCR product".
Test for Bacteria
While virus testing is still an emerging area, testing for bacterial pathogens is becoming more sophisticated all the time. Rapid techniques such as enzyme-linked immuno-sorbent assay (Elisa), which stimulates the production of antibodies and antigens in reaction to bacteria, and PCR, which analyses the DNA and RNA of pathogens, continue to compete with slower but cheaper standard agar cultural testing methods.
Recent developments have included a new xylose lysine tergitol-4 (XLT4) agar from Lab M, which allows labs to identify salmonella species in heavily contaminated samples.
In the world of PCR, the new DuPont Qualicon Bax System Q7 is designed to eliminate as many steps as possible. This allows food labs to quickly detect organisms, with the results displayed on a screen.
Despite the proliferation of rapid testing techniques, most manufacturers still opt for cultural techniques, says Watkins, which take on average three to four days to deliver results, compared with just eight hours for the fastest PCR tests.This is purely down to cost.
According to Tom Weschler, lead author of Strategic Consulting's Food Microbiology Testing in Europe, which was published in January 2012, 64% of tests carried out in Europe last year were conventional cultural tests, while 36% were newer methods such as molecular, immunoassay, chromogenic and automated routine methods.
Most tests are for total bacteria, total coliforms, yeast and mould, which are indicators of product freshness or cleanliness. These make up 82% of all microbiology tests being done, says Weschler. Testing for pathogens such as salmonella and listeria makes up 18% of tests.
Labs usually carry out tests for specific pathogens, such as listeria and salmonella. But, according to Don Brown of Exova, there are testing techniques currently being developed that will allow the industry to blanket test for a huge range of different pathogens. "We are often asked to test for knowns," says Brown, "but there are techniques we are beginning to use that can look in non-targeted ways at all sorts of things the unknown unknowns."
Unkown contaminants
The detection of new or unknown contaminants requires a different approach to more traditional 'targeted' analysis. "A strategy more akin to forensic work is required," says Brown, "utilising sophisticated instrumentation, systematic sample preparation techniques and, most importantly, knowledge about the history of the sample."
To test for contaminants, adulteration and authenticity, Brown says: "We use high resolution time-of-flight mass spectrometry coupled to liquid chromatography which enables us to acquire the full spectral profile of sample components to very high mass accuracy. From this we can elucidate identities of unusual or unexpected compounds either directly from predicted chemical formulae, from study of expected metabolites, or from comparison of 'spectral profiles'.
"In addition, high resolution full scan mass spectroscopy can be utilised to screen for very large target lists of contaminants (1,000s)." Because all the spectral data is collected, it is possible to review the data retrospectively to search for previously unlooked-for compounds. "So as soon as the unknown becomes known we can look to see if it was a problem before we knew about it!"
While manufacturers now have access to far more tests than ever before, it remains to be seen whether testing for viruses will ever become as commonplace as it currently is for bacteria.
