Photodynamic inactivation of the phytopathogenic bacterium Xanthomonas citri subsp. citri

The present work intended to evaluate the applicability of photodynamic inactivation (PDI) of Xanthomonas citri subsp. citri with toluidine blue O (TBO), a commercial photosensitizer, as a strategy to control citrus canker. Assays were conducted with cell suspensions and biofilms, constructed either on polypropylene microtubes (in vitro assays) or on the surface of orange leaves (ex vivo assays), in the presence of TBO and under irradiation with artificial white light or natural sunlight. PDI assays using TBO alone caused a maximum 5.8 log10 reduction of Xanthomonas citri viable cells in suspensions, and a much smaller inactivation (1.5 log10) in biofilms. However, concomitant use of KI potentiated the TBO photosensitization. Biofilms were inactivated down to the detection limit (> 6 log10 reduction) with 5.0 µmol l-1 TBO + 10 mmol l-1 KI (in vitro) or 5.0 µmol l-1 TBO + 100 mmol l-1 KI (ex vivo) after artificial white light irradiation. Under natural sunlight, a reduction down to the detection limit of the Miles-Misra method was achieved with 50 µmol l-1 TBO and 100 mmol l-1 KI. PDI has potential to be applied in the control of citrus canker in field conditions although further studies are needed to show that there are no risks to plant physiology or fruit quality.