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SUMMARYThe diversity of colonial types of Pseudomonas aeruginosa which may be encountered is described, together with a series of biochemical tests and the application of serological and pyocine typing which are of use in identifying atypical strains. These methods are particularly recommended for strains which do not form pigment. It is suggested that absence of pigment production even on special media does not preclude the possibility that a strain is P. aeruginosa.

Certain strains ofPs. aeruginosaproduce, in addition to pyocines, substances which inhibit pyocine activity. These pyocine inhibitors are probably proteolytic enzymes.In order to investigate the production of pyocines by various strains ofPs. aeruginosa,aeruginosa, nutrient agar medium was devised in which the action of the pyocine-inhibiting substances is suppressed by incorporating 10–5M iodoacetic acid, 0·1 % sodium citrate and 0·1 % dipotassium hydrogen phosphate. This medium also diminished slime production.Pyocine production is a stable characteristic which is not lost on repeated sub-culture or prolonged storage, and might form the basis of a typing system forPs. aeruginosa.I wish to thank Dr M. T. Parker, Director of the Cross-Infection Reference Laboratory, Public Health Laboratory Service, for constant encouragement and helpful advice.

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