A Possible Mechanism for Treating Staphylococcus aureus -Induced Chronic Osteomyelitis in Rats Using 808-nm Light

Dear Editor,

I am writing to comment on the article of Kaya et al., ¹ which has recently been published. In this article the authors report that rats with Staphylococcus. aureus-induced osteomyelitis were treated successfully with an 808-nm, 127.3 mW/cm², continuous-wave diode laser. The authors attribute the healing effect to the bactericidal effect of 808-nm diode on S. aureus, an assumption I would like to dispute.

Broadband visible light has been found ^2,3 to inactivate various bacteria, including S. aureus, in the absence of exogenous photosensitizers. The phototoxic effect was found to depend upon reactive oxygen species (ROS) production by the bacteria, and the amount of produced ROS correlated with the degree of the phototoxic effect. However, irradiation of bacteria in vitro, using near-infrared (IR) diode lasers at 780 or 830 nm, did not result in their inactivation. ^4,5 High-power near IR diode lasers were found to kill bacteria, but through a thermal effect and not by the photochemical effect on which light therapy is based.

On the other hand, bacterial killing induced by blue light was found to be successful by several groups. ^{4 –9} The phototoxicity was found to decrease as a function of the wavelength, 415-nm light being more toxic than 455-nm light. ¹⁰

It is true that there is almost no research on in vivo light-induced bacterial inactivation. But based on the in vitro results discussed previously, we believe that the healing effect of 808-nm light on rats with osteomyelitis is a result of immune system regulation and not caused by bacteria killing. Many studies have shown that red or near-IR light can reduce inflammatory conditions such as arthritis ^11,12 or gingival inflammation. ¹³ For example, Peplow et al., ¹⁴ summarize in vitro studies demonstrating that visible and near-IR light promote an increase in gene expression and release of growth factors that are anti-inflammatory. New results concerning the anti-inflammatory effects of 810-nm light have recently been published by Chen et al., ¹⁵ preceding the article of Kaya et al. ¹ The authors show that 810-nm light has an anti-inflammatory effect on activated dendritic cells, possibly mediated by cyclic adenosine monophosphate (cAMP) and reduced NF-κB signaling.

It therefore appears that it is possible that the healing effect of 808-nm light on osteomyelitis induced in rats is not the result of S. aureus killing but through upregulating the immune system.

More detailed in vivo studies must be performed to assess the mechanism of the healing effects of near-IR light.