Merocyanine 540 (MC540) is used as a photosensitizer for the inactivation of
microorganisms. The following is already known about MC540: firstly, MC540
exists in distilled water in both monomeric and dimeric forms, and the addition
of salts into a MC540 solution leads to the formation of large aggregates that
can be detected by the resonance light scattering technique. Secondly, singlet
oxygen can only be photogenerated by MC540 monomers. In the present work, we
studied the effect of MC540 in the aggregated state on the rate of
photosensitized inactivation ofStaphylococcus
aureusandPseudomonas aeruginosa. To this end,
bacteria either in MC540-containing distilled water or in a 0.25 M sodium
chloride aqueous solution also containing MC540 are irradiated (546 nm). The
results show that, in the presence of salt, the aggregation of MC540 greatly
increases the efficiency of the MC540-photosensitized inactivation ofP.
aeruginosaandS. aureus. In the presence of salt,
the rates ofP. aeruginosaandS.
aureusinactivation increase by factors of 10 and 30, respectively, in
comparison with the rate of inactivation observed in the case of distilled
water. Our results suggest that a salt-induced photosensitization mechanism can
switch from the singlet oxygen to the free-radical pathway.