Elek's gel precipitation test is used for detecting of toxins of which of the following organism?

It is an in vitro test for diphtheria. A rectangular strip of filter paper impregnated with diphtheria antitoxin is placed on the surface of 20% normal horse serum agar in a petri dish while the medium is still fluid. If horse serum is not available, sheep / rabbit serum may be used. When the agar has set, the surface is dried and narrow streaks of the strains are made at right angles to the filter paper strip. A positive and negative control should be set up. The plate is incubated at 37 degree for 24-48 hrs. Toxins produced by the bacterial growth will diffuse in agar and where it meets the antitoxin at optimum concentration will produce a line of precipitation. This precipitation lines indicate the strain is toxigenic. Ref: Ananthanarayanan & paniker's 8th ed Page 236.