Photodynamic inactivation of methicillin-resistant Staphylococcus aureus by using Giemsa dye as a photosensitizer.

The rise of antibiotic-resistant bacteria calls for innovative approaches to combat multidrug-resistant strains. Here, the potential of the standard histological stain, Giemsa, to act as a photosensitizer (PS) for antimicrobial photodynamic inactivation (aPDI) against methicillin-sensitive Staphylococcus aureus (MSSA) and methicillin-resistant Staphylococcus aureus (MRSA) strains is reported. Bioassays were performed using various Giemsa concentrations (ranging from 0.0 to 20.0 µM) under 625 nm illumination at a light dose of 30 J cm-2. Remarkably, Giemsa completely inhibited the growth of MSSA and MRSA bacterial colonies for concentrations at 10 µM and higher but exhibited no inhibitory effect without light exposure. Partition coefficient analysis revealed Giemsa's affinity for membranes. Furthermore, we quantified the production of reactive oxygen species (ROS) and singlet oxygen (1O2) to elucidate the aPDI mechanisms underlying bacterial inactivation mediated by Giemsa. These findings highlight Giemsa stain's potential as a PS in aPDI for targeting multidrug-resistant bacteria.

Antimicrobial and Photoantimicrobial Activities of Chitosan/CNPPV Nanocomposites.

Multidrug-resistant bacteria represent a global health and economic burden that urgently calls for new technologies to combat bacterial antimicrobial resistance. Here, we developed novel nanocomposites (NCPs) based on chitosan that display different degrees of acetylation (DAs), and conjugated polymer cyano-substituted poly(p-phenylene vinylene) (CNPPV) as an alternative approach to inactivate Gram-negative (E. coli) and Gram-positive (S. aureus) bacteria. Chitosan's structure was confirmed through FT-Raman spectroscopy. Bactericidal and photobactericidal activities of NCPs were tested under dark and blue-light irradiation conditions, respectively. Hydrodynamic size and aqueous stability were determined by DLS, zeta potential (ZP) and time-domain NMR. TEM micrographs of NCPs were obtained, and their capacity of generating reactive oxygen species (ROS) under blue illumination was also characterized. Meaningful variations on ZP and relaxation time T2 confirmed successful physical attachment of chitosan/CNPPV. All NCPs exhibited a similar and shrunken spherical shape according to TEM. A lower DA is responsible for driving higher bactericidal performance alongside the synergistic effect from CNPPV, lower nanosized distribution profile and higher positive charged surface. ROS production was proportionally found in NCPs with and without CNPPV by decreasing the DA, leading to a remarkable photobactericidal effect under blue-light irradiation. Overall, our findings indicate that chitosan/CNPPV NCPs may constitute a valuable asset for the development of innovative strategies for inactivation and/or photoinactivation of bacteria.

Photodynamic Inactivation of Methicillin-Resistant Staphylococcus aureus by a Natural Food Colorant (E-141ii).

This study evaluates the photosensitizing effectiveness of sodium copper chlorophyllin, a natural green colorant commonly used as a food additive (E-141ii), to inactivate methicillin-sensitive and methicillin-resistant Staphylococcus aureus under red-light illumination. Antimicrobial photodynamic inactivation (aPDI) was tested on a methicillin-sensitive reference strain (ATCC 25923) and a methicillin-resistant Staphylococcus aureus strain (GenBank accession number Mh087437) isolated from a clinical sample. The photoinactivation efficacy was investigated by exposing the bacterial strains to different E-141ii concentrations (0.0, 1.0, 2.5, 5.0, 10.0, and 20.0 µM) and to red light (625 nm) at 30 J cm-2. The results showed that E-141ii itself did not prevent bacterial growth for all tested concentrations when cultures were placed in the dark. By contrast, E-141ii photoinactivated both methicillin-sensitive Staphylococcus aureus (MSSA) and methicillin-resistant Staphylococcus aureus (MRSA) under red-light illumination. However, different dose responses were observed for MSSA and MRSA. Whilst the MSSA growth was inhibited to the detection limit of the method with E-141ii at 2.5 µM, >10 µM concentrations were required to inhibit the growth of MRSA. The data also suggest that E-141ii can produce reactive oxygen species (ROS) via Type I reaction by electron transfer from its first excited singlet state to oxygen molecules. Our findings demonstrate that the tested food colorant has great potential to be used in aPDI of MRSA.