Scientists at the department say they have developed a machine called a "flow cytometer" that can handle up to 100 samples at a time and identify Listeria monocytogenes and Candida albicansbacteria with accuracy within a day.
Listeria is a hardy pathogen that resists freezing, drying and heat processes and has been responsible for numerous and sometimes deadly outbreaks of food poisoning infections around the world.Candida, a yeast, can be lethal to individuals with weakened immune systems. It is one of a number of yeasts that can cause spoilage in food and drinks.
Currently pulsed-field gel electrophoresis (PFGE) is considered the best method for genetically identifying Listeria bacteria that cause food poisoning. But PFGE is difficult to run, takes aboutthree days to produce a result and has several disadvantages that complicate efforts to determine the relationships between different bacteria, say the scientists.
The new machine also target variations of the bacteria, says Todd Ward, a microbiologist who helped develop the system. Such variations can help distinguish one strain of Listeria from another. Bytargeting genes for virulence, the test could enable a user to understand what makes some strains more harmful or better adapted to a particular environment than others.
The feature could prove especially useful in hazards analysis and critical-control-point programs at food-processing plants, he stated.
"The ability to identify Listeria that have colonised your production plant can help determine whether food products are contaminated before coming into the plant or within the plant by resident strains," Ward stated in a report on the research.
The machine uses molecules called "probes" that find and bind to corresponding pieces of species-specific DNA. The researchers created the probes using DNA-sequence information from theirunit's microbial-genomics database. The probes have a fluorescent marker that tells the cytometer which DNA sequence was detected. The machine clearly displays the species' identity as color-codedbar graphs.
The test could also be used to check for yeasts such as Candida that cause food and beverage spoilage. It could also speed the search for yeasts adept at fermenting cornstarch into ethanol or thoseused for the control of fruit-storage rots, he said.
An outbreak of Listeria monocytogenes in Switzerland a few weeks ago was associated with cheese consumption and resulted in the death of two people and two miscarriages.
Foods typically associated with Liseria monocytogenes contamination include ready-to-eat products made with red-meat, poultry-meat and fish, including frankfurters, pâté, smoked salmon andfermented raw meat sausages. Commonly affected dairy products include soft cheeses, such as brie, camembert and roulade, semi-soft farmhouse cheeses and unpasteurised milk. Prepared salads, such ascoleslaw, also support the growth of the pathogen.
The department of agriculture wants to collaborate with private firms to develop the resulting tests in kit form.