TOXICITY EFFECT OF BIOSYNTHESIZED SILVER NANOPARTICLES AGAINST E. COLI AND B. SUBTILISSALWA AL-THAWADI AND ALAA SHUKRALLA A. RASOOLAbstract Silver nanoparticles (AgNPs) are increasingly used in the medical and agricultural sectors for disease management, due to their antimicrobial activity. In the current study, biosynthesized AgNPs produced in a previous study (AgNPs1, AgNPs2 and AgNPs3) were used to test their toxicity on Gram negative (E. coli) and Gram positive (B. subtilis) bacteria. These nanoparticles (NPs) were examined by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDS) and X-ray diffraction (XRD). SEM micrographs show that AgNPs1 were almost spherical and monodispersed, while AgNPs2 were aggregated. More aggregations appeared in AgNPs3. The AgNP peaks were obvious in the EDS analysis and were further confirmed by XRD analysis, indicating the precipitation of AgNPs. The toxic effects of AgNPs were tested by exposing E. coli and B. subtilis to AgNPs1, AgNPs2 and AgNPs3 during growth. Bacterial growth was followed by OD measurements at 600 nm. Unexpectedly, an enhancement effect was shown in the presence of AgNPs1 for both bacterial strains, whereas an inhibition effect of bacterial growth was shown in the presence of AgNPs2 and AgNPs3. AgNPs3 were selected for further tests; the minimum toxic effect against E. coli and B. subtilis was shown to be 10 mg.L-1. To our knowledge, this study is the first to show bacterial growth enhancement by AgNPs. The reasons for this enhancement need further investigation.
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