Biosynthesis of bismuth nanoparticles and its synergistic effect with antibiotics against Escherichia coli and Klebsiella pneumoniae

Today, the biosynthesis of nanoparticles (NPs) assisted by microorganisms (particularly bacteria) received increasing attention. In this study, Bacillus subtilis strain SFTS, a bismuth-reducing bacterium, was isolated from the soil of a copper mine in the South of Iran and used for biosynthesis of bismuth NPs (Bi NPs).

Bacillus subtilis strain SFTS was identified by conventional identification tests and the 16S rDNA fragment amplification method. Characterizations of the bio-fabricated Bi NPs were examined using FTIR, EDS, XRD, TEM, and SEM analysis after purification of Bi NPs. In addition, the synergistic effect of biogenic Bi NPs in combination with different antibiotics was also investigated.

The attained results revealed that the biosynthesized Bi NPs average size was 22.36 nm and spherical in shape. The XRD pattern showed that the biosynthesized nanoparticles consisted only of Bi4 and monoclinic crystals. Furthermore, the results of antibacterial effect of Bi NPs in combination with various antibiotics showed that the nanoparticles represented the highest synergistic effect together with imipenem and the lowest effect in combination with tetracycline against clinical strains of E. coli and K. pneumoniae. Significant difference between synergistic effect of Bi NPs with antibiotics compared to antibiotics disc alone against E. coli and K. pneumoniae strains was observed (P<0.001).

This study showed that Bi NPs biologically synthesized by Bacillus subtilis strain SFTS had a small size and different structure. However, finding about their antibacterial effect and related mechanism merit further investigations.