What inhibitory substances are present in the EMB plate?

The reason B is the correct choice is that Eosin Y and methylene blue are the dyes incorporated in the EMB (Eosin Methylene Blue) agar, which is a selective and differential medium primarily used for the isolation and differentiation of gram-negative bacteria, particularly enteric bacteria.

These dyes serve a dual purpose in the EMB medium: they inhibit the growth of gram-positive bacteria and help to differentiate between lactose fermenters and non-fermenters based on the color of the colonies that form. Lactose fermenters will produce acid, which reacts with the dyes, resulting in a color change that helps in the identification of coliforms, while non-fermenters remain colorless.

The other substances listed are not relevant to EMB agar. Crystal violet is a dye used in gram staining and is not part of the EMB formulation. Penicillin and streptomycin are antibiotics that are not included in EMB and do not function as inhibitors in this context. Thallium acetate may have uses as a selective agent in some media, but it is not a component of EMB agar. Hence, the presence of Eosin Y and methylene blue makes B the correct answer regarding the inhibitory substances in an EMB plate.