BM2120 – Kohn’s Medium No.2

One of two media usually used as a pair, the other being Kohn’s No 1 (BM2110), to differentiate members of the enterobacteriacae. Kohn’s No 2 contains Salicin and Sucrose as substrates and Bromothymol Blue as the indicator. The medium is inoculated by stabbing the inoculum into one third of the depth of the medium.  After incubation, sugar fermentation of either or both sugar is indicated by the medium changing colour from blue/green to yellow. It is also possible to determine motility in this medium as a diffuse growth spreading from the line of inoculum. The production of Indole and/or Hydrogen Sulphide can also be tested for using this medium. To do so suspend appropriate Lead Acetate and Indole test papers in the neck of the tube at time of inoculation. After incubation examine the papers for colour changes, blackening of the lower end of the Lead Acetate Paper for H2S and the Indole strip changing from yellow to red.