MATERIALS AND METHODS
Organism. P. aeruginosa PA103, isolated by Liu (23), was
selected for this study. This strain is naturally elastase
deficient (28). We found that all protease activity in culture
supernatants of strain PA103 was neutralized by specific
alkaline protease antisera, demonstrating that the only extracellular
Isolation of mutants. EMS (methanesulfonic acid ethyl
ester; Sigma) was employed by the method of Carlton and
Brown (2). A solution of Vogel-Bonner minimal medium (36)
containing 4% EMS was equilibrated at 37°C. To this solution
was added an equal volume of a late-exponential-phase
culture of P. aeruginosa PA103 grown in nutrient broth. This
mixture was incubated with shaking for 90 min at 37°C, and
then cells were washed with phosphate-buffered saline
(PBS), grown overnight in nutrient broth, and used to
inoculate skim milk plates at dilutions sufficient to result in
ca. 100 colonies per plate. The conditions of EMS mutagenesis
employed resulted in ca. 90% lethality. After incubation
of these plates at 37°C for 48 h, colonies were examined for
zones of clearing indicative of alkaline protease production.
Colonies which did not produce zones of clearing after 48 h
were chosen for further study.