The World Health Organization has been alerting that the “post-antibiotic era” is approaching, making the search for alternative antimicrobial therapies mandatory. Antimicrobial photodynamic therapy (aPDT) has been gaining prominence due to its non-specific mechanism of action, which relies on the generation of reactive oxygen species upon the activation of a photosensitizer (PS) by light of a specific wavelength in the presence of oxygen. However, the discussion of whether or not aPDT can induce bacterial resistance is controversial. In that sense, the aim of this study was to determine if multiple cycles of suboptimal doses of aPDT could induce resistance in Enterococcus faecalis. First, we determined optimal and suboptimal conditions of aPDT employing chlorin-e6 (Ce6) and methylene (MB) for planktonic E. faecalis. The combinations of light dose and PS concentration that induced bacterial reductions between 1 log10 and 3 log10 of CFU/mL were selected to start the cycles (21 µM of Ce6 + 45 J/cm²; and 78 µM of MB + 80 J/cm²). The cycling consisted of treatment, plating on PS-free blood agar and recovering grown colonies to start again. By the end of four cycles, cells treated with both Ce6-aPDT or MB-aPDT were completely eliminated, after sustaining a stable survival rate through the first three cycles. We employed two different PS and observed the same outcomes for both of them, indicating the results were not PS-dependent. Our findings are of paramount importance since they are in the way to prove that bacterial resistance cannot be induced by this approach.
The efficacy of fluorescence spectroscopy to detect squamous cell carcinoma is evaluated in an animal model following laser excitation at 442 and 532 nm. Lesions are chemically induced with a topical DMBA application at the left lateral tongue of Golden Syrian hamsters. The animals are investigated every 2 weeks after the 4th week of induction until a total of 26 weeks. The right lateral tongue of each animal is considered as a control site (normal contralateral tissue) and the induced lesions are analyzed as a set of points covering the entire clinically detectable area. Based on fluorescence spectral differences, four indices are determined to discriminate normal and carcinoma tissues, based on intraspectral analysis. The spectral data are also analyzed using a multivariate data analysis and the results are compared with histology as the diagnostic gold standard. The best result achieved is for blue excitation using the KNN (K-nearest neighbor, a interspectral analysis) algorithm with a sensitivity of 95.7% and a specificity of 91.6%. These high indices indicate that fluorescence spectroscopy may constitute a fast noninvasive auxiliary tool for diagnostic of cancer within the oral cavity.
Fluorescence diagnosis of malignant lesions has been showed as an attractive optical technique due especially to its real-time response and a more objective and quantitative evaluation. Even though the
oral cavity allows a direct examination many lesions are diagnosed when it is already in advanced stage, compromising the patient prognosis. In this study, the fluorescence spectroscopy was used to the detection of chemically induced carcinoma at the lateral border of the tongue in a hamster model. Two excitations wavelengths in visible region were applied: 442 and 532 nm. All the spectra results
were analyzed comparing with the histopathological diagnosis. The better results were achieved with the 442 nm laser excitation. The spectra from carcinoma showed new emission bands and these were
used to determined different ratios for a quantitative analysis. Using the 625-645 nm fluorescence range under 442 nm excitation (A<sub>3</sub> coefficient) the percentage of false negative was of 9.1%, however the false positive percentage was of 18.5%. The 532 nm excitation provided a better normal tissue detection compared to 442 nm excitation. The ideal clinical condition is probably the use of multiple wavelengths excitation for a broader tissue fluorescence investigation.