The combination of Ocimum sanctum and Trigonella foenum-graecum L leaf water extract synergistically acts as a reducing and capping agent for the synthesis of narrow polydisperse silver nanoparticles (Ag NPs) with controlled sizes depending on the precursor (AgNO₃) concentration in the plant extract. The toxicity of 40 nm-sized green synthesized Ag NPs is less than that of 10 nm-sized NPs. The Ag NP solution in Ocimum sanctum and Trigonella foenum-graecum L leaf water extract shows synergic antibacterial effect on Gram-negative bacteria by effecting the ester group of the lipids (hydrolysis) and also breaking the amide bonds of the bacterial chemical constituents, which leads to their rapid death.