One of two media usually used as a pair, the other being Kohn’s No 2 (BM2120), to differentiate members of the enterobacteriacae. Kohn’s No 1 contains Mannitol and Dextrose as substrates and Phenol Red as the indicator. To inoculate the medium, smear the inoculum onto the surface of the slope and also stab it deep into the butt. After incubation the production of acid by both substrates is shown by the medium turning yellow. If fermentation is of Dextrose only the butt will have become yellow and the surface red, while if only Mannitol is fermented the slope will turn yellow. The production of gas is demonstrated by the appearance of bubbles within the medium. Urease positive organisms turn the medium bright pink. For details of the many reactions that may arise during the use of this medium reference should be made to one of the many standard textbooks.