Evaluating bacterial resistance and virulence after photodynamic inactivation with zinc(II) 2,9,16,23-tetrakis[4-(N-methylpyridyloxy)]phthalocyanine.

Photodynamic inactivation (PDI) is a treatment that has long been studied for eliminating microorganisms. However, analysis of its effect on repeated sublethal doses and virulence factors has not been completely understood. For this reason, E. coli cells were subjected to repeated PDI treatments using zinc(II) 2,9,16,23-tetrakis[4-(N-methylpyridyloxy)]phthalocyanine iodide (ZnPPc4+) as photosensitizer. Parameters were adjusted to achieve partial inactivation, allowing surviving colonies to undergo subsequent treatments. No significant differences in E. coli inactivation were observed after 10 PDI. Therefore, repeated sublethal exposure does not induce resistance to subsequent PDT treatments. Furthermore, although some cleavage was observed in plasmid DNA, the genomic DNA of E. coli remained largely unaffected. Interestingly, repeated photodynamic treatment did cause some changes in antibiotic susceptibility. On the other hand, although PDI treatment with ZnPPc4+ effectively eliminates S. aureus cells, it did not inactivate virulence factors activity or the ability to ferment mannitol, under the sublethal conditions analyzed. However, this photosensitizer demonstrated significant efficacy in inactivating S. aureus biofilms, even at very low concentrations. This is particularly noteworthy given that biofilms are much more resistant to antimicrobial treatments than planktonic cells, suggesting great potential for ZnPPc4+ in combating biofilm-related infections without generating resistance to this treatment.