Background:The resistance of Pseudomonas aeruginosa to antibiotics is a big problem, especially in burns and wound infections. Laser irradiation affects microorganisms by denaturing their proteins, which involves changes in the chemical or physical properties of the protein.Objectives: The aim of this study was to investigate the effect of caffeic acid and low-power laser light co-exposure on Pseudomonas aeruginosa isolated from burn wounds. Materials and Methods: Ten bacterial samples were collected from patients with burn wound infections at Shahid Motahhari medical center of Tehran. The He-Ne laser was used in this study with output power of 2 mW. Results: The data significantly indicated that both the caffeic acid and laser treatment alone reduced the number of colony-forming units compared to control cultures. Co-exposure of bacterial suspensions to caffeic acid and laser at three time points showed the following number of colony-forming units 240.23 ± 60.15, 148.13 ± 52.66 and 84.57 ± 35, respectively. The best concentrations of caffeic acid to achieve countable colonies were 1.5 and 1.75 mM. At the concentration of 1.5 mM of caffeic acid, the number of colonies significantly reduced to 280.78 ± 59 (P = 0.008) while at 1.75 mM the number of colonies reduced to 234.07 ± 72.28 (P = 0.0001). Conclusions: Caffeic acid treatment reduced bacterial growth and resulted in a decreased number of colony formation. The simultaneous effect of caffeic acid and laser at three time courses showed a synergic effect in reducing colony formation compared to the control and caffeic acid, and laser alone.

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