Clinicians often have to deal with
infections that are difficult to control because they are caused by superbugs resistant to many
antibiotics. Alternatives to
antibiotic treatment include antimicrobial
photodynamic therapy (aPDT). The photodynamic process causes bacterial death, inducing oxidative stress through the photoactivation of
photosensitizer molecules in the presence of
oxygen. No
PDT-resistant bacteria have been selected to date, thus the response to photo-oxidative stress in non-phototrophic bacteria needs further investigation. The opportunistic pathogen Pseudomonas aeruginosa, in particular, has been shown to be more tolerant to
PDT than other micro-organisms. In order to find any genetic determinants involved in
PDT-tolerance, a panel of transposon mutants of P. aeruginosa PAO1 involved in the quorum sensing signalling system and membrane cytoplasmic transport were photoinactivated as part of this study. Two pseudomonas
quinolone signalling (PQS) knock-out mutants, pqsH- and pqsC-, were as
PDT-sensitive as the PAO1 wild-type strains. Two PQS hyperproducer variants, pqsA- and rsaL-, were shown to be more tolerant to photo-oxidative stress than the wild-type strain. In the pqsA- mutant, the
hyperpigmentation due to the presence of
phenazines could protect cells against
PDT stress, while in rsaL- no pigmentation was detectable. Furthermore, a mutant impaired in an
ATP-binding cassette transport involved in maintaining the asymmetry of the outer membrane was significantly more tolerant to photo-oxidative stress than the wild-type strain. These observations support the involvement of quorum sensing and the importance of the bacterial cell envelope when dealing with photo-oxidative stress induced by photodynamic treatment.