In this study, rapid one step facile synthesis of silver nanoparticles (AgNPs) was done using culture supernatant of two Gram positive (B. thuringiensis and S. aureus) and Gram negative (E. coli and Salmonella typhimurium [STAgNP]) bacterial strains and were termed as "Bacillus thuringiensis," "Staphylococcus aureus," "Escherichia coli," and "STAgNP," respectively. Synthesized AgNPs were well characterized with the help of different standard techniques like FESEM, DLS, UV-Vis spectroscopy, and Fourier transform infrared. Mechanism of AgNPs synthesis was elucidated using in silico approach. In vivo cytotoxicity of synthesized AgNPs was assessed in embryonic Zebrafish model with the help of uptake, oxidative stress, and apoptosis induction experimental assays, and the mechanism was investigated through in silico approach at the molecular level. The result showed successful biosynthesis of 20-40 nm sized AgNPs stable with zeta potential of - 45 to - 35 mV having standard silver nanoparticles SPR peaks due to the interaction of reduced silver particles with amino acid residues of bapA proteins of the bacterial supernatant. In vivo cytotoxicity with embryonic Zebrafish was found to be dependent on biogenicity and concentration of biosynthesized AgNPs as consequence of oxidative stress induction and apoptosis due to the influential regulation of sod1 and tp53 genes clarified by pathway analysis with reference to experimental and computational results. The study suggested that cytotoxicity of biologically synthesized silver nanoparticles from bacteria depends on strain specificity with significant difference in use of Gram positive and Gram negative bacterial strains.