Cynthia writes, <Can anyone tell me what irgasan is and what it is used for in PIA(Pseudomonas Isolation Agar)??> Irgasan is a broad-spectrum antimicrobial which is ineffective against Pseudomonas. References: The Difco Manual, p.711 Furia and Schenkel (Soap and Chemical Specialties, January 1968). Infectionately, Yersinia. "Hell no, let 'em grow!"