Background The industrial production of silver nanoparticles (AgNPs) and its commercial applications are being considerably increased in recent times, resulting in the release of AgNPs in the environment and enhanced probability of contaminations and their adverse effects on living systems. Based on this, the present study was conducted to evaluate the in vitro cytotoxicity of actinomycete-synthesized AgNPs on Allium cepa (A. cepa) root tip cells. A green synthesis method was employed for biosynthesis of AgNPs from Streptomyces sp. NS-33. However, morphological, physiological, biochemical, and molecular analysis were carried out to characterize the strain NS-33. Later, the synthesized AgNPs were characterized and antibacterial activity was also carried out against pathogenic bacteria. Finally, cytotoxic activity was evaluated on A. cepa root tip cells. Results Results showed the synthesis of spherical and polydispersed AgNPs with a characteristic UV-visible (UV-Vis.) spectral peak at 397 nm and average size was 32.40 nm. Energy dispersive spectroscopy (EDS) depicted the presence of silver, whereas Fourier transform infrared (FTIR) studies indicated the presence of various functional groups. The phylogenetic relatedness of Streptomyces sp. NS-33 was found with Streptomyces luteosporeus through gene sequencing. A good antibacterial potential of AgNPs was observed against two pathogenic bacteria. Concerning cytotoxicity, a gradually decreased mitotic index (MI) and increased chromosomal aberrations were observed along with the successive increase of AgNPs concentration. Conclusions Therefore, the release of AgNPs into the environment must be prevented, so that it cannot harm plants and other beneficial microorganisms.

In the present study, a Streptomyces sp. KAS-1 was isolated from sediment soil samples of the Kali river estuary, Karwar, Karnataka, India. The isolated Actinomycetes strains were screened against pathogenic microorganisms and for the production of hydrolytic enzymes. Later, the strongest strain Streptomyces sp. KAS-1 was taken for further analysis based on primary characterizations. The morphological, physiological and biochemical characterizations of Streptomyces sp. KAS-1 were carried out, followed by molecular characterization through 16S rRNA gene sequencing. The 16S ribosomal RNA gene sequencing and analysis of the phylogenetic tree showed a 98.97% sequence similarity with Streptomyces thermocarboxydus strain SDT64 (KJ018992). Finally, the intracellular methanol extract was characterized through UV-Visible and FTIR spectrophotometer. The Streptomyces sp. is a gram-positive, aerobic, non-motile, spore-forming bacterium and it was designated as Streptomyces sp. KAS-1 and was found to possess a strong antimicrobial activity against pathogenic microorganisms. The characterizations revealed the Streptomyces sp. KAS-1 was moderately thermophilic, neutrophile organism having the capacity to degrade a broad range of carbon sources. The UV-Visible spectrum of methanol extract of Streptomyces sp. KAS-1 revealed the presence of Muconomycin-B and the presence of different functional groups such as aldehydes, amines, and alkyl halides were indicated by the FTIR spectrum.