For the first time, a comparative analysis of 8 strains of the genus Shewanella isolated from various natural sources is carried out according to the level of the reducing activity of Ag+ to silver-containing nanoparticles (NPs). The characteristics of biogenic NPs in terms of shape, size, crystal-lattice parameters, hydrodynamic diameter, ζ potential, and optical characteristics are obtained using scanning transmission electron microscopy, dynamic light scattering, spectrophotometry, atomic emission spectroscopy, and assessment of the biocidal activity against various types of microorganisms, including the studied strains of the genus Shewanella. It is found that the shape of the NPs is close to spherical, the sizes vary from 2 to 30 nm, the samples are polydisperse colloidal systems. Analysis of crystal lattices of the NPs confirms the presence of AgNPs, as well as Ag2O2NPs and Ag3O4NPs in some samples. The results of atomic emission spectroscopy on the quantitative analysis of the content of Ag in the nanomaterial samples are obtained, on the basis of which strains with a high ability to form a Ag-containing nanomaterial are identified. The high sensitivity of gram-positive bacteria to AgNPs is established. Different levels of resistance of 8 strains of the genus Shewanella to the action of biogenic AgNPs are demonstrated. The results indicate the metabolic diversity of strains of the genus Shewanella isolated from various natural sources. The analysis of strains of the genus Shewanella according to the level of the reducing activity is relevant for the development of an effective technology for the “green” synthesis of biogenic NPs.