Sae regulator factor impairs the response to photodynamic inactivation mediated by Toluidine blue in Staphylococcus aureus

Abstract

Photodynamic inactivation (PDI) involves the combined use of light and aphotosensitizer, which, in the presence of oxygen, originates cytotoxic species capableof inactivating bacteria. Since the emergence of multi-resistant bacterial strains isbecoming an increasing public health concern, PDI becomes an attractive choice.The aim of this work was to study the differential susceptibility to Toluidine blue (TB)mediated PDI (TB-PDI) of S. aureus mutants (RN6390 and Newman backgrounds) fordifferent key regulators of virulence factors related to some extent to oxidative stress.Complete bacteria eradication of planktonic cultures of RN6390 S. aureusphotosensitized with 13 μM TB was obtained upon illumination with a low light dose of4.2 J/cm2 from a non-coherent light source. Similarly, complete cell death was achievedapplying 1.3 μM TB and 19 J/cm2 light dose, showing that higher light doses can lead toequal cell death employing low photosensitizer concentrations. Interestingly, RN6390 inplanktonic culture responded significantly better to TB-PDI than the Newman strain.3We showed that deficiencies in rsbU, mgrA (transcription factors related to stressresponse) or agr (quorum sensing system involved in copper resistance to oxidativestress) did not modify the response of planktonic S. aureus to PDI. On the other hand,the two component system sae impaired the response to TB-PDI through a mechanismnot related to the Eap adhesin.More severe conditions were needed to inactivate S. aureus biofilms (0.5 mM TB, 157J/cm2 laser light). In mutant sae biofilms, strain dependant differential susceptibilitiesare not noticed.