Insect Pathogenic Properties of Senatia mrcescens: Phageresistant Mutants with a Decreased Resistance to Cecropia Immunity and a Dtxreased Virulence to Drosophila

A non-pigmented strain of Serratia marcescens (Db 10) was isolated from moribund Droso-phila flies. From this strain were isolated spontaneous mutants resistant to streptomycin (Dbll) and nalidixic acid (Db12). Mutant Dbl 1 was used for the isolation of two phages, q5J and #K, which grew on DblO, Dbll and Db12, but not on three reference strains of S. marcescens. Mutant Dbl 1 was demonstrated to fulfil Koch’s postulates. Strain DblO and its antibiotic-resistant derivatives were lethal to Drosophila whether given in the food or by injection. Evidence for toxin(s) was found only in sterile supernatants from 7 d cultures. Such extracts contained proteolytic activity and inactivated the antibacterial activity in immune haemolymph from Cecropia. Phages $J and #K were used to isolate phage-resistant mutants of Dbl 1. Three such mutants and their parental strain were investigated for their susceptibility to immune haemolymph from Cecropia. The parental strain was resistant to incubation with 90 % haemolymph for 2 h at 37 “C; all phage-resistant mutants were susceptible to the immune haemolymph with ‘killing times ’ (i.e. the time required to kill 90 % of the viable cells) ranging from 15 to 55 min. When the same strains were compared for their virulence to Drosophila, the phage-resistant mutants had significantly reduced virulence. It is concluded that resistance to insect immunity plays an important role in the overall pathogenicity of S. marcescens