A one milliliter sample of the milk sample is diluted in a 1:10 series. One milliliter of the third dilution tube is plated mi pour plate. fter incubation, the plate has 54 colonies which indicates that the original 29. A sample of milk is tested for bacterial content in a plate count assay. sample contained 54,000 cells C. 540 cells D. 54 cells B. 5400 cells 30. A culture medium on which only Gram-positive organisms grow and a yellow halo surrounds Staphylococcus aureus colonies is called a/an A. differential medium B. enrichment medium C. selective and differential medium D. selective medium