The detection of E. coli 0157:H7 in food products involves selective enrichment. Selective enrichment is performed to encourage growth of your target pathogen and inhibit growth of background organisms. Selective enrichment is used when the level of the organism is not a priority, instead, the researcher is interested in its presence or absence. Enrichments are usually used for serious human pathogens, such as Salmonella, E. coli 0157:H7, and Listeria monocytogenes. Enrichments generally use samples of food products (often 25 g), which are homogenized in a selective enrichment broth. This food homogenate is incubated for 18 to 24 h to allow levels of the pathogen of interest to increase and multiply, while inhibiting other organisms. Theoretically, after selective enrichment, your pathogen had time to grow to very large numbers, even if it was in your original food product at very low levels. It is much easier to isolate and identify your pathogen of choice from a selective enrichment broth due to inhibition of other microflora (due to selective agents) and increased numbers of target organisms.