The harm of Salmonella typhimurium(S.typhimurium)to public health mainly by contaminating fresh food and water emphasizes the urgent need for rapid detection techniques to help control the spread of the pathogen.In this assay,an newly designed capture probe complex that contained specific S.typhimurium-aptamer and hybridized signal target sequence was used for viable S.typhimurium recognition directly.In the presence of the target S.typhimurium,single-stranded target sequences were liberated and initiated the isothermal amplification,producing numerous DNA-scaffolded AgNC structures with a linker on the 3\'-end.And then,the sensing system took innovative advantage of quadratic linker-induced isothermal amplification for the first time to release target sequence in succession,leading to the cyclic reuse of the target sequences and cascade signal amplification,thereby achieving the successive production of AgNC structures,generating significantly enhanced fluorescent signals to achieve highly sensitive detection of S.typhimurium down to 70 CFU/mL with a linear range from 102 to 107 CFU/mL.By coupling the cascade two-stage target sequences-recyclable toehold strand-displacement with aptamer-based target recognition successfully,it is the first report on a novel non-label,modification-free and DNA extraction-free ultrasensitive fluorescence biosensor for detecting viable S.typhimurium directly,which can discriminate from dead S.typhimurium.