Pseudomonas aeruginosa is a Gram-negative bacterium widely distributed in nature and causing opportunistic infections in humans. P. aeruginosa is an important bacterial pathogen of nosocomial (hospital derived) infections, and it can also cause life threatening diseases in patients with cancer, burn wounds, cystic fibrosis and those that have received immunosuppressive therapy. Classification of P. aeruginosa isolates is an important and routine task in hospitals. The basis for the serotyping system for P. aeruginosa is the differences among the O antigen side chains of lipopolysaccharide (LPS). A large number of O antigen side chains have been established by various investigators around the world which led to the creation of a standardized serotyping system known as the International Antigenic Typing Scheme (IATS) with twenty characterized O serotype strains of P. aeruginosa.