Work published in BDS in partnership with colleagues from UNESP, Prof. Dr. Cláudio Talge de Carvalho and PhD student Prof. Amjad Abu Hasna.
In: Machado, Ricardo. Endodontics: Biological and Technical Principles. Available at: GEN Group, GEN Group, 2022:
Susceptibility of microorganisms to photodynamic therapy
The photodynamic therapy presents distinct effectiveness according to the microbial population. Gram-positive bacteria are more susceptible to its action than gram-negative bacteria, which is due to physiological differences between these microorganisms. Gram-positive bacteria have a relatively porous outer membrane, formed by a thick layer of peptidioglycans and lipoteic acid. These characteristics allow greater diffusion of the photosensitizer into the bacteria. Gram-positive bacteria can therefore be killed by various dyes and lower doses of irradiation. Gram-negative bacteria have thinner but more structurally complex outer membranes, containing porin proteins, lipopolysaccharides and lipoproteins. These components make their outer membrane more resistant to penetration of substances.66, Fungi have a cell wall consisting of a thick layer of betaglucan and chitin, which provides an intermediate permeability barrier compared to gram-positive and gram-negative bacteria.
However, photosensitization of bacteria is associated with the charge of the photosensitizer. Being positively charged, low molecular weight, and hydrophilic, methylene blue is able to interact with anionic lipopolysaccharide macromolecules and penetrate through less permeable outer membranes, such as those found in gram-negative bacteria.85
Soukos et al. (2006) investigated the effects of photodynamic therapy on root canals of extracted teeth experimentally infected with endodontic pathogens. Methylene blue with 25 μg/mℓ concentration irradiated with a diode laser with 30 J/cm2 power and 665 nm wavelength coupled to an optical fiber was used. All bacterial species were eliminated, with the exception of Enterococcus faecalis (only 53% lysis). However, by increasing the energy fluence to 222 J/cm2, this microorganism was eliminated in 97% of the specimens.
Differences in bacterial susceptibility are also observed according to the manner in which the microorganisms are arranged. Organized in biofilms, they are protected by a polysaccharide matrix; therefore, they are less susceptible to different antimicrobial strategies, including photodynamic therapy.68,86
Bergmans et al. (2008) evaluated the antimicrobial ability of photodynamic therapy against strains of Streptococcus anginosus, Enterococcus faecalis and mixed cultures of Enterococcus faecalis and Fusobacterium nucleatum in root canals of extracted teeth. In planktonic form or in monolayers, the microorganisms were easily eliminated. However, photodynamic therapy proved to be much less effective when they were organized in biofilms.
Upadya and Kishen (2010) compared the efficacy of photodynamic therapy on planktonic and mono-species suspensions of biofilms containing Enterococcus faecalis and Pseudomonas aeruginosa. According to the results of the study, modifications in the formulation of the photosensitizer increased the antibacterial efficacy of photodynamic therapy against microorganisms organized in biofilms.
The reduced sensitivity of biofilms to photodynamic therapy is attributed to the difficulty in penetrating the dye. Thus, several methods have been studied with the aim of minimizing this limitation.
According to George and Kishen (2008), the inclusion of an oxidizing agent and an oxygen carrier in the formulation of methylene blue increased the potential for photo-oxidation and generation of singletoxic oxygen, facilitating the disruption of the polysaccharide matrix of Enterococcus faecalis biofilms. The same authors, in another study also conducted in 2008, concluded that the use of a specific microbial efflux pump inhibitor added to methylene blue was able to increase the efficacy of photodynamic therapy against Enterococcus faecalis organized in biofilm.
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