Effect of sub-lethal photodynamic inactivation on the antibiotic susceptibility and biofilm formation of clinical Staphylococcus aureus isolates
Introduction
Photodynamic inactivation (PDI) of microorganisms is based on the properties of dyes, known as photosensitizers, to be preferentially localized in the bacteria and not in the surrounding tissue. They are subsequently activated by low doses of visible light of an appropriate wavelength, generating free radicals or singlet oxygen that are toxic to target microorganisms [1], [2]. Potential uses for PDI include treatment of localized wound infection [3], [4], onychomycosis [5] and treatment of periodontitis [6].
Staphylococcus aureus is an important human pathogen capable of causing health care associated as well as community acquired infections. It may cause not only local infections, such as postoperative or injury wound infections and osteomyelitis, but also generalized infections [7], [8], [9].
Much is already known about the PDI of both methicillin-sensitive and methicillin-resistant S. aureus strains. Various photosensitizers such as haematoporphyrin [10], porphyrin derivatives [11], [12], phenothiazinium salts [13], [14], chlorin [15], [16] and 5-aminolaevulinic acid-induced porphyrin sensitizers [17] were studied and found to demonstrate high bactericidal effect after illumination with visible light against S. aureus strains. However, almost all investigations concerning PDI of microorganisms are performed with the use of one or two bacterial isolates. In this report the effect of PDI against 40 clinical S. aureus strains was examined to determine whether different isolates of the same species may respond differently to photoinactivation. The effect of sub-lethal PDI on the antibiotic susceptibility and biofilm formation of these strains was also studied.
Section snippets
Materials and methods
A total of 14 methicillin-resistant S. aureus (MRSA) and 26 methicillin-sensitive S. aureus (MSSA) strains recovered from acute and chronic wounds were used in this study. In addition, S. aureus (ATCC 25923) was also included. The oxacillin salt-agar screening-plate procedure was used for the detection of MRSA [18].
Among the MSSA strains, three (strains UTMC 1440, UTMC 1443, and UTMC 1444, recovered from diabetic foot ulcers) were tested for biofilm formation after sub-lethal PDI. All the MSSA
Killing effect of TBO/MB-PDI on S. aureus strains
The PDI was applied to all 14 clinical MRSA, 26 MSSA, and S. aureus (ATCC 25923) strains. In the case of MRSA strains the obtained results for TBO-PDI included viable count reduction ranging from 1 to 1.3 log10-unit, and for MSSA strains approximately from 1.1 to 1.3 log10-unit. For MB-PDI, in the case of MRSA strains the obtained results included viable count reduction ranging from 0.7 to 1 log10-unit, and for MSSA strains approximately from 0.7 to 0.9 log10-unit.
S. aureus (ATCC 25923) was shown
Discussion
The mechanism underlying strain dependent effectiveness of PDI has not been studied so far. In the study of Maish et al. two MRSA and two MSSA strains were used in PDI investigation [22]. Those strains showed some variation in their sensitivity to PDI, however, the observed differences in PDI response did not exceed 0.2 log10-unit reduction. Lambrechts et al. studied PDI against three wild type strains of S. aureus and noticed that those strains showed some significant variation in their
Acknowledgements
This work was supported by the College of Science, University of Tehran and Research Center of Ear, Nose, Throat, Head and Neck Surgery, Hazrat Rasoul Akram Medical Complex, Tehran University of Medical Sciences.
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