Toxin production by Corynebacterium diphtheriae is due to

The toxigenicity of diphtheria bacilli depends on the presence in it of corynephages, which act as the genetic determinant controlling toxin production. Non-toxigenic strains Kay be rendered toxigenic by infecting them with the beta or some other phage. This is known as lysogenic or phage conversion. The toxigenicity remains only as long as the bacilli are lysogenic. Reference: Textbook of Microbiology; Anathanarayan and paniker's; 10th edition; page no 240