A common cause of respiratory infection with a membrane over the tonsil is Corynebacterium diphtheriae. Media appropriate for culture is Tellurite blood agar: Tellurite inhibits the growth of most other bacteria acting as a selective agent for diphtheria bacilli. Diphtheria bacilli reduce Tellurite to metallic tellurium which is incorporated in the colonies giving them a grey or black color. Reference: Textbook of Microbiology; Anathanarayan and panicker's; 10th edition; Page no: 242