We report a simple one step microwave irradiation method for the synthesis of gold nanoparticles using citric acid as reducing agent and cetyl trimethyl ammonium bromide (CTAB) as binding agent. The reaction was completed under two different microwave irradiation times (40 s and 70 s) for the production of two types of gold nanoparticles. The synthesized nanoparticles were characterized using UV-Vis absorption spectroscopy and transmission electron microscopy (TEM) measurements. UV-Vis study revealed the formation of gold nanoparticles with surface plasmon absorption maxima at 590 and 560 nm for 40 and 70 s irradiation time respectively. From TEM analysis, it is observed that the gold nanoparticles have spherical shape with particle size distribution in the range 1-10 nm and 1-2 nm for 40 s and 70 s irradiation time respectively. Antibacterial activity of gold nanoparticles as a function of particle concentration against gram-negative bacterium Escherichia coli (E. coli) was carried out in solid growth media. The two types of gold nanoparticles show high antibacterial activity with zone of inhibition of about 22 mm against E. coli (ATCC 25922 strain). Very small difference in the antibacterial activity for the two types of gold nanoparticles were observed. Though nanoparticles synthesized for 70 s irradiation time show slightly better antibacterial activity. (C) 2010 Elsevier B.V. All rights reserved.