Thursday, January 1, 2009

Is Food less safe these days? Part 3

The final in this three part New Year soliloquy on food safety

You'll find the earlier parts in the panels below this one.

: Globalisation has had another effect: within hours we know what has happened in other parts of the world. Multimedia cell phones allow people to send pictures and text; the Internet lets ordinary people put their thoughts in front of anyone who has a computer and of course the news media have worldwide coverage and transmit reports via satellite to every continent. The effect of all this is that we know almost immediately of large scale or unusual cases of food poisoning and food fraud as they occur throughout the world, thus increasing the perceived frequency of incidents.

Evolution: When I was an undergraduate microbiologist in the '60s, E. coli O157:H7 was unheard of. It was first identified as a human pathogen in 1982. Other new serotypes have also been recognized, usually as a result of an outbreak. It appears that evolution is continuing at a visible rate, at least in the microbial world. Dr Chris Bell and Alec Kyriakides† have expressed it beautifully: “Genetic promiscuity is facilitated by a range of genetic elements including plasmids, transposons, conjugative transposons and bacteriophages* . The ability to evolve through horizontal gene transfer and acquire ‘foreign’ DNA, has resulted in novel phenotypes and genotypes emerging”. This mix-and-match behaviour has resulted in the formation of diarrhoea-causing strains that possess previously unreported combinations of virulence factors. The study of DNA sequences in old lineages of E. coli has shown that these lines have acquired the same virulence factors in parallel. Natural selection has thus favoured an ordered acquisition of genes and a progressive build-up of molecular mechanisms that increase virulence (Reid et al., 2000. Nature, 406 64-7). Just this week in the journal Science, John Chen and Richard P Novick have reported that staphylococcal bacteriophage can transfer staphylococcal pathogenicity "islands", pieces of DNA containing superantigen genes and other transferable elements, to Listeria monocytogenes at the same high frequencies as they transfer within Staphylococcus aureus. See here

This might sound esoteric, but the practical result is that we will see more novel virulent bacteria that will in the future cause new food borne illnesses.

Obviously, the examples I have given above contain elements of more than one of the highlighted causes. Does this all add up to an answer to my original question? I think that we can draw some general conclusions.

• Modern food manufacturing processes may be technically more advanced than traditional food production
• These processes can be very reliable and make vast amounts of safe food at affordable prices
• When something does go wrong, the results may be catastrophic because of the scale of operations
• Human error and fraud are ever-present hazards to a safe food supply
• Our perception of the frequency of food poisoning or food fraud incidents may be influenced by the ease of international communication
• Microbial evolution means that we will never produce totally safe food

On balance, I think that our food is actually safer than it was 25 years ago.

†Bell, C. & Kyriakides, A. (2002) Pathogenic Escherichia coli. IN
Foodborne pathogens: Hazards, risk analysis and control. Blackburn, C. de W. & McClure, P.J. Woodhead Publishing, Cambridge, UK.

* Plasmid – a small circular independently replicating piece of DNA in bacteria; Plasmids often carry virulence factors, antibiotic resistance or toxin coding genes
Transposon - sequence of DNA that can move around to different positions within the genome of a single cell, possibly causing mutation
Conjugative transposon - integrated DNA elements that excise themselves to form a circular intermediate, which can transfer by conjugation to a recipient and integrate into the recipient's genome. Conjugative transposons have a broad host range and are probably as important as plasmids in the spread of antibiotic resistance genes in some genera of disease-causing bacteria (A A Salyers et al., (1995) Microbiol Rev. 59(4): 579–590).
Bacteriophage – a virus that infects bacteria and may ferry small sequences of bacterial DNA from one host cell to another. The DNA may integrate into the recipient’s genome and confer new characteristics, such as the ability to synthesise new enzymes.

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