Technology: Gene probe picks the quirk from the dead

 作者:黎哑     |      日期:2019-03-01 08:17:03
By ANDY COGHLAN Public health inspectors and food manufacturers need to know quickly whether food is safe to eat or process. Food contaminated with bacteria such as Listeria monocytogenes or Escherichia coli is only unsafe if the organisms are alive, and standard tests to distinguish live bacteria from dead ones usually take between one and three days. But researchers are developing a test based on a gene probe that can do the job in a matter of hours. Gene probes are strands of nucleotide bases, the building blocks of DNA. If the bases are in the right order in the strand, they can bind to other ‘complementary’ strands of DNA in a test sample. Probes are designed to bind exclusively to DNA from a target organism such as L. monocytogenes. Radioactive tracers or substances that change colour in certain circumstances can be attached to the probe to show if the target DNA is present. Many gene probes are now available commercially for detecting microorganisms. Most are put to use with the help of the polymerase chain reaction (PCR), a method of turning a tiny initial amount of DNA into enough material to analyse. However, DNA exists in dead organisms as well as living ones. ‘DNA can last for thousands of years, and therefore you can’t tell if the organism it comes from is alive or dead,’ says Ian Masters, a member of the team at the Institute of Food Research in Reading, which is developing the test. Masters and his colleagues are developing probes that detect messenger RNA (mRNA) instead of DNA. These single strands of nucleotide bases only exist in living cells, where they play a critical role in the synthesis of proteins and enzymes. In bacterial cells, mRNA is short-lived: a single molecule seldom survives for longer than a few minutes, so its presence in a cell is an indication that the cell has very recently been active. Masters and his colleagues have already developed probes that detect mRNA in live L. monocytogenes and E. coli. They first break up bacterial cells with chemical agents or detergents, then use probes to detect fragments of mRNA in the debris. They have adapted the PCR to copy only mRNA, not DNA. They say it will probably take them a few more years to perfect the technique. So far, it is only tested on bacterial cultures, not on contaminated food. They also want to increase its sensitivity,