Insect Pathogenic Properties of Serratia marcescens: Phage-resistant Mutants with a Decreased Resistance to Cecropia Immunity and a Decreased Virulence to Drosophila

Abstract

A non-pigmented strain of Serratia marcescens (Db10) was isolated from moribund Drosophila flies. From this strain were isolated spontaneous mutants resistant to streptomycin (Db11) and nalidixic acid (Db12). Mutant Db11 was used for the isolation of two phages, phi J and phi K, which grew on Db10, Db11 and Db12, but not on three reference strains of S. marcescens. Mutant Db11 was demonstrated to fulfil koch's postulates. Strain Db10 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 phi J and phi K were used to isolate phage-resistant mutants of Db11. 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 degrees 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.

Insect Immunity: Isolation and Structure of Cecropins B and D from Pupae of the Chinese Oak Silk Moth, Antheraea pernyi

“…Assuming a survival value of the D form it follows that we have not yet found a bacterium or condition for which the insect will need cecropin D. The predominance of the D form may also be the result of a long period of domestication. It is known that silk worms have been maintained in culture for about 4000 years Its is interesting to note that three of the species reported to be pathogens for insects, Streptococcus fecalis AD-4 [Ill, Serratia marcescens D b l l [18] and Bacillus thuringiensis all turned out to be highly resistant to the action of both cecropins. This supports an earlier notion that a very important property of a pathogen is to withstand the immune defence of the host.…”

The phage abortive infection system, ToxIN, functions as a protein–RNA toxin–antitoxin pair

“…Eca 1043 (21) and 1039 (37) were grown at 25°C, E. coli DH5␣ (Gibco/BRL) was grown at 37°C, and S. marcescens Db11 (38) was grown at 30°C in Luria broth (LB) at 300 rpm or on LB agar (LBA) containing 1.5% (w v Ϫ1 ) agar, and growth (OD600) was measured as described previously (39). When required, LB was supplemented with the following antibiotics: kanamycin 50 g mL Ϫ1 , spectinomycin (Sp) 50 g mL Ϫ1 , ampicillin (Ap) 100 g mL Ϫ1 , and tetracycline 35 g mL Ϫ1 .…”

QscR, a modulator of quorum-sensing signal synthesis and virulence in Pseudomonas aeruginosa

“…Approximately 8 ϫ 10 9 viable bacterial cells were pelleted from cultures by centrifugation. These cells were suspended in 170 l of 5% sucrose solution (13) and added to a filter disk (2.3-cm diameter Whatman paper) that completely covered the agar surface (5 ml of 2.4% agar with 5% sucrose) at the bottom of a standard glass fly culture vial. Adult male 3-to 5-day-old fruit flies (Drosophila melanogaster Canton S) that had been starved for food and water for 5 h were added to each vial (9-12 flies per vial).…”