Leeds Public Health Laboratory Service. Samples, including food products, milk & water are sent to the laboratory to be tested for bacterial or other microbial contamination. The tests are varied & include growing the specimens in petri dishes, using agar as a nutrient. A swab (as seen here) or a metal loop, is used to streak the sample across the surface of the agar plate. A sample such as this contains a mixed population of organisms. Streaking is a method designed to separate & isolate individual organisms for purposes of identification or transferral for further testing.