Certain strains of Ps. aeruginosa produce, 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 of Ps. 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 for Ps. 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.