Despite the high number of legionella infections, there are currently no convincing preventive measures. Photoinactivation with visible light is a promising new approach and the photoinactivation sensitivity properties of planktonic Legionella rubrilucens to 450, 470, and 620 nm irradiation were thus investigated and compared to existing 405 nm inactivation data for obtaining information on responsible endogenous photosensitizers. Legionella were streaked on agar plates and irradiated with different doses by light emitting diodes (LEDs) of different visible wavelengths. When irradiating bacterial samples with blue light of 450 nm, a 5-log reduction could be achieved by applying a dose of 300 J cm−2, whereas at 470 nm, a comparable reduction required about 500 J cm−2. For red irradiation at 620 nm, no inactivation could be observed, even at 500 J cm−2. The declining photoinactivation sensitivity with an increasing wavelength is consistent with the assumption of porphyrins and flavins being among the relevant photosensitizers. These results were obtained for L. rubrilucens, but there is reason to believe that its inactivation behavior is similar to that of pathogenic legionella species. Therefore, this photoinactivation might lead to new future concepts for legionella reduction and prevention in technical applications or even on or inside the human body.

In this study, the photoinactivation of Legionella by visible light is investigated. The success of this approach would offer new prospects for technical water disinfection and maybe even for therapeutic measures in cases of Legionella infections. Therefore, Legionella rubrilucens was dispensed on buffered charcoal yeast extract medium agar plates and illuminated with different doses of violet light generated by 405 nm light-emitting diodes (LEDs). A strong photoinactivation effect was observed. A dose of 125 J/ cm2 reduced the bacterial concentration by more than 5 orders of magnitude compared to Legionella on unirradiated agar plates. The necessary dose for a one log-level reduction was about 24 J/cm2. These results were obtained for extracellular L. rubrilucens, but other Legionella species may exhibit a similar behavior.

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Legionella infections caused by contaminated water are a widespread problem worldwide. Discharge lamps like mercury vapor lamps are widely known for the disinfection properties of their radiation, but they suffer technical disadvantages, like high voltages and toxic content, and are, therefore, not suitable for some infection control applications. New highintensity ultraviolet (UV) and violet LEDs offer new approaches for Legionella control, because these bacteria are significantly light sensitive compared to other pathogens. One of the most important infection pathways is the inhalation of Legionellacontaining aerosols during showering. This problem could be reduced by a single strong UV LED within the shower head, which irradiates the passing water for some milliseconds. This practice can be especially beneficial in hospitals and care facilities. UV light offers only a limited penetration depth, however, even in pure water. To disinfect larger water volumes, e.g., in water dispensers, visible violet LEDs are more appropriate. Unfortunately, up to now, neither approach has been given much attention by potential users.