Comparative analyses of the Serratia marcescens complex reveal variations in their pathogenicity against a fruit pest

BACKGROUND Members of the Serratia marcescens complex are opportunistic pathogens affecting a wide range of plants and animals. They are often present in the intestinal tract of Bactrocera dorsalis (Hendel) (Diptera: Tephritidae). However, their virulence and interactions with the host remain poorly understood. RESULTS Four strains of S. marcescens complex from diverse sources were isolated and characterized. Hemolymph injection caused significantly higher mortality in B. dorsalis compared with oral infection across all four strains. The gut‐derived Bd_SM strain from B. dorsalis showed the lowest virulence, whereas strains from the gut of Bactrocera correcta (Bezzi) (Diptera: Tephritidae) (Bc_SM), Aedes aegypti (L.) (Diptera: Culicidae) (Aa_SM), and a dead B. dorsalis adult (DBd_SM) demonstrated markedly higher virulence. Host immune responses varied with infection route and bacterial strain. Specifically, oral infection induced dose‐dependent gut immune responses, whereas hemolymph injection triggered strain‐specific antimicrobial peptide (AMP) gene activation. Consistent with these phenotypic observations, integrated in vitro assays and comparative genomic analyses revealed substantial variation in predicted virulence determinants, with Bd_SM possessing the fewest such factors. CONCLUSION Strains of the S. marcescens complex exhibited differences in virulence and triggered distinct immune responses in B. dorsalis . These findings enhance our understanding of strain‐specific host–microbe interactions and elucidate genomic determinants underlying pathogenicity.