Demonstration of cell envelope-bound exotoxin A in Pseudomonas aeruginosa

The quantity of membrane-bound and extracellular exotoxin A in four strains of Pseudomonas aeruginosa was investigated. In strain PA-103, which is the prototype strain used for toxin production, all of the toxin was released into the growth medium and little toxin remained with the cell envelope. In a virulent strain from a clinical source, strain 119, exotoxin A was found in equal amounts in the growth medium and in the cell envelopes. An avirulent mutant of this strain, strain 119 (AP-), which was resistant to 800 microgram of polymyxin B per ml, had all the exotoxin A in a membrane-bound state and was unable to release exotoxin A into the growth medium. Thus, exotoxin A can be found in the membrane-bound form, in the extracellular form, or in both. The quantities of membrane-bound toxin and extracellular toxin vary with the strain P. aeruginosa. The polymyxin B-resistant mutant which is blocked in toxin release will be useful in studies of exotoxin A secretion.