In this task, you are given a sentence. You are expected to write the name of the organism substances in the body mentioned in the input. Although there might be several correct answers, you need to write one of them.

 Four weeks after the last calf stopped shedding , all calves were necropsied and samples from the gastrointestinal tract were taken for the detection of the nalidixic - acid resistant E . coli O157 .
samples