It's very complex. When you have a clinical sample like blood or cerebral spinal fluid, usually you have the organism in a pure culture, so it's very easy to isolate. When you have a food, the food itself is complex, plus you have many, many different types of bacteria in the food. You have to use very selective media and broths to try to enrich for the organism you're trying to detect, in this case listeria, and you try to inhibit the other organisms in the sample. So we use very selective techniques to fish out the listeria from the food sample.
On April 22nd, 2009. See this statement in context.