The contribution of temperature, exposure intensity and visible light to the inhibitory effect of irradiation on acute chlamydial infection

• Water-filtered infrared A combined with visible light was applied to acute chlamydial infection.
• Non-thermal effects contribute to the anti-chlamydial effect of wIRA/VIS irradiation.
• Visible light markedly enhances the inhibitory effect of wIRA on extracellular Chlamydia.
• The effect of irradiation on extracellular Chlamydia depends on the exposure intensity.
• The infectivity of mature Chlamydia is reduced regardless of the irradiation dose.

Water-filtered infrared A (wIRA) is radiation with a spectrum ranging from 780 to 1400 nm. Chlamydiaceae are obligate intracellular bacteria associated with various diseases in both animals and humans. A recent in vitro study demonstrated that wIRA combined with visible light (wIRA/VIS) has potential as a non-chemical method for the treatment of chlamydial infections without adversely affecting the cell viability. The aim of this study was to investigate the influence of various factors on the effect of wIRA/VIS on acute chlamydial infection, namely the impact of temperature, exposure intensity and infectious dose (multiplicity of infection) as well as the efficacy of the visible light component. We demonstrate that non-thermal effects contribute to the inhibition of acute chlamydial infection. Visible light enhances the inhibitory effect of wIRA on extracellular bacteria (elementary bodies or EBs). Moreover, the inhibitory effect of wIRA/VIS following treatment of EBs prior to infection correlated with increased irradiation intensity. The infectivity of mature chlamydial inclusions was significantly reduced upon wIRA/VIS exposure at all irradiation intensities investigated, suggesting the contribution of host cell factors to the anti-chlamydial effect of wIRA/VIS in the late stage of the developmental cycle. The effect of irradiation was not influenced by the infectious dose.