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The antibacterial effect of light emitting diodes (LEDs) in the visible region (461, 521 and 642 nm) of the electromagnetic spectrum was investigated on Escherichia coli O157:H7, Salmonella typhimurium, Listeria monocytogenes and Staphylococcus aureus. The irradiances of the 461, 521 and 642 nm LEDs were 22.1, 16 and 25.4 mW/cm², respectively. Bacterial cultures suspended in tryptic soy broth were illuminated by 10-watt LEDs at a distance of 4.5 cm for 7.5 h at 20, 15 and 10 °C. Regardless of the bacterial strains, bacterial inactivation was observed with the range of 4.6–5.2 log CFU/ml at 10 and 15 °C after illumination with the 461 nm LED, while illumination with the 521 nm LED resulted in only 1.0–2.0 log reductions after 7.5 h. On the other hand, no antibacterial effect was observed using the 642 nm LED treatment. The photodynamic inactivation by 461 and 521 nm LEDs was found to be greater at the set …