Opacity around colonies of clostridium perfringens is due to

Alpha toxin is produced by all thoracic clostridium perfringes. Alpha toxin is lecithinase C. Demonstrated by Nagler's reaction. Nagler reaction; When Clostridium perfringes are grown in media containing 6 % agar, Fildes peptic digest of sheep blood and 20% human serum, with antitoxin spread on one half of plate, colonies in the other half without antitoxin will be surrounded by a zone of opacity. No opacity around the colonies on the half of plate with antitoxin. This is due to the specific neutralization of the alpha-toxin or lecithinase C. Reference: Textbook of Microbiology; Anathanarayan and paniker's; 10th edition; Page no: 259