Toxigenicity of Corynebacterium diphtheria is determined by

Elek's gel precipitation test: a rectangular strip of filter paper impregnated with diphtheria antitoxin is placed on the surface of a 20% normal horse serum agar in a Petri dish while the medium is still fluid. -when agar has set, the surface is dried and narrow streaks of strains are made at right angles to the filter paper strip. A positive and negative control should be put up. The plate is incubated at 37degC for 24-48 hours. The toxin produced by the bacterial growth will diffuse in agar and where it meets the antitoxin at optimum concentration, will produce a line of precipitation. The presence of such arrow headlines of a precipitate indicates the strain is toxigenic. No precipitate is formed in the case of non-toxigenic strains. Reference: Textbook of Microbiology; Anathanarayan and panicker's; 10th edition; Page no: 243