Isolation and characterization of a Serratia marcescens with insecticidal activity from Polyphylla olivieri (Col.: Scarabaeidae)

Members of the genus Serratia are known for their abilities to infect insects. In this study, a red-pigmented S.marcescens was isolated and characterized from the infected larvae of Polyphylla olivieri using bacterial cultivation, phylogenetic analysis as well as bioassays against larvae of the two insect pests, Plodia interpunctella and Ephestia kuehniella. Comparative 16S rRNA and groEL gene sequence BLAST analyses strongly suggested that the isolated strain should be placed in the genus Serratia, sharing high sequence similarities with several strain of S.marcescens associated with insects. Phylogenetic analysis placed the isolated bacterium with other S.marcescens bacteria in a clade with high bootstrapping values. To assess pathogenicity of the S.marcescens isolate, the bacterial cells were either injected into the haemolymph of the fifth-instar larvae or added to the diets of insects. Survival curves of the control insects and those challenged with six different concentrations of S.marcescens showed that the S.marcescens isolate significantly reduced survival rates of the larvae. The LC(50)s of the bacterium on P.interpunctella and E.kuehniella were 1992.26 and 1.09x10(4) (CFU/l) for injection bioassays at 6h post-injection, and 4.48x10(4) and 1.96x10(5) (CFU/10l) for feeding bioassays at 24h post-feeding, respectively. Injection of the bacterial culture supernatant into the larvae led to continuous bleeding from the site of injection, while injection of heat-treated culture supernatant of the bacterium did not cause continuous bleeding. Together, our results showed the possibility of using this S.marcescens isolate in microbial control of the insect pests after addressing the safety concerns. Moreover, it might be considered as a source of useful bioactive molecules and genes with application in insect control and biotechnology via developing insect-resistant plants.