New DNA-based methods of detecting and identifying microbial pathogens offer benefits to both manufacturers and consumers. The microbiological safety of foods continues to be of great concern to everyone in the food chain, from farm to fork. Producers, manufacturers and everyone preparing food take all possible precautions to prevent illness through consumption of their food. Food safety management systems that have proved to give good results include the Good Manufacturing Practices and Hazard Analysis Critical Control Point systems. An important aspect of such preventative, safety assurance systems is to establish whether potential pathogens may be present in a raw material, or in the environment of a food production line.

Precision and timing

Testing for pathogens has traditionally required a lot of time-consuming analysis by highly qualified personnel. These analyses have been supplemented by a number of quicker, DNA-based methods during the last decade. A technician can carry out these procedures in a matter of hours, compared with the several days that might be spent by highly trained microbiologists on the older techniques. Since these new methods use genetic information to detect bacteria, the results are more precise than those from traditional tests relying on biochemical or immunological characteristics, which are subject to environmental conditions.

A lot from so little

Many of the new methods depend upon the polymerase chain reaction (PCR), which has revolutionised much of molecular biology in recent years, providing a means of obtaining significant quantities of DNA from very small samples. Other methods rely on a technique called DNA hybridisation. PCR-based tests are very useful in confirming the presence or absence of particular pathogens in food. In order to screen for a particular pathogen such as Salmonella bacteria, a sample of the food in question is treated to encourage growth of any bacteria present. The DNA is extracted from the bacteria and PCR then used to amplify these minute quantities of DNA into amounts that can easily be identified. The PCR will only amplify the DNA from the organism of interest (in this case Salmonella) and if it is present, it can easily be detected. Several commercially produced kits based on this method are now available, for a whole range of pathogenic bacteria.

Finding the perfect match

Where the problem involves the precise identification of pathogens in raw materials, production line environments or finished products, techniques based on DNA hybridisation are used which result in DNA "fingerprints" of microbes. An automated version of this system is available which compares the DNA fingerprint of any sample to an existing database and stores the pattern along with reference to its origin.

By matching these characteristic fingerprints throughout the food production process, it is possible to track the source of a pathogen. One prepared food company for example recently found Staphylococcus epidermidis in its products using its standard quality assurance procedures. With the new technology it was possible to trace the contamination source to the hands of one employee, from among several possible sources of the same pathogen species in the plant enabling an immediate inexpensive corrective measure rather than a costly shut-down and full scale decontamination.

Using these new DNA-based methods, rapid tracing of the source becomes possible, and the incriminated food can be withdrawn from the market. Clearly, both producers and consumers of food may benefit from these new techniques by becoming smarter than the bugs.

FOOD TODAY 05/1999