True about diphtheria is

The toxigenicity of the diphtheria bacillus depends on the presence of corynephages(tox+) in it which acts as the genetic determinant controlling toxin production. Non-toxigenic strains may 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 bacillus is lysogenic. When the bacillus is cured of its phage, as by growing it in the presence of antiphase serum, it loses its toxigenic capacity. Reference: Textbook of Microbiology; Anathanarayan and panicker's; 10th edition; Page no: 240