Synthesis of Bioactive Silver Nanoparticles Using New Bacterial Strains from an Antarctic Consortium

Abstract

In this study, we report on the synthesis of silver nanoparticles (AgNPs) achieved by using three bacterial strains Rhodococcus, Brevundimonas and Bacillus as reducing and capping agents, newly isolated from a consortium associated with the Antarctic marine ciliate Euplotes focardii. After incubation of these bacteria with a 1 mM solution of AgNO₃ at 22 °C, AgNPs were synthesized within 24 h. Unlike Rhodococcus and Bacillus, the reduction of Ag⁺ from AgNO₃ into Ag⁰ has never been reported for a Brevundimonas strain. The maximum absorbances of these AgNPs in the UV-Vis spectra were in the range of 404 nm and 406 nm. EDAX spectra showed strong signals from the Ag atom and medium signals from C, N and O due to capping protein emissions. TEM analysis showed that the NPs were spherical and rod-shaped, with sizes in the range of 20 to 50 nm, and they were clustered, even though not in contact with one another. Besides aggregation, all the AgNPs showed significant antimicrobial activity. This biosynthesis may play a dual role: detoxification of AgNO₃ and pathogen protection against both the bacterium and ciliate. Biosynthetic AgNPs also represent a promising alternative to conventional antibiotics against common nosocomial pathogens.

Keywords: antimicrobial activity; capped nanoparticles; green synthesis; nanomaterials; nosocomial pathogens.

Figure 1

EDAX analysis of AgNPs synthesized from Rhodococcus (A), Brevundimonas (B) and Bacillus (C). Ag, C, N and O indicate the silver (the highest peak, recorded at 3 or 3.2 keV), carbon, nitrogen and oxygen signals (the relative amounts are reported in the tables).

Figure 1

EDAX analysis of AgNPs synthesized from Rhodococcus (A), Brevundimonas (B) and Bacillus (C). Ag, C, N and O indicate the silver (the highest peak, recorded at 3 or 3.2 keV), carbon, nitrogen and oxygen signals (the relative amounts are reported in the tables).

Figure 2

Dynamic light scattering particle size analysis of Rhodococcus (A), Brevundimonas (B) and Bacillus (C) AgNPs. The average particle sizes were 40.8 nm, 46.7 nm and 40.6 nm, respectively.

Figure 3

TEM images of biosynthesized AgNPs from Rhodococcus (A), Bacillus (B), and Brevundimonas (C).

Figure 3

TEM images of biosynthesized AgNPs from Rhodococcus (A), Bacillus (B), and Brevundimonas (C).

Figure 4

Kirby–Bauer test performed with AgNPs from 1. Marinomonas ef1; 2. Rhodococcus ef1; 3. Brevundimonas ef1; 4. Pseudomonas ef1; 5. Bacillus ef1. C indicates the control represented by 1 mM AgNO₃. The test was performed with Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, Pseudomonas aeruginosa, Serratia marcescens, Citrobacter koseri, Acinetobacter baumanii, Candida albicans, and Candida parapsilosis.