Biomedical Engineering Reference
In-Depth Information
The fungal cell wall has a relatively thick layer of chitin and beta-glucan that has
intermediate permeability between Gram-positive and Gram-negative bacteria (Dai
et al.
2009
).
3 Mechanism of Action of PDT
Photodynamic therapy is based on the concept that a particular compound, or
photosensitiser curing unit, can be preferably located in certain tissues and subse-
quently activated by light of the appropriate wavelength to generate singlet oxygen
and free radicals that are cytotoxic to the microorganisms in the target tissue
(Soukos and Goodson
2011
). Photodynamic therapy involves two stages: the first
stage involves the application of a photosensitising agent, and the second stage
involves the application of light directly to the treated area. When light is combined
with the photosensitising agent, phototoxic reactions are induced to destroy the
microbial cells (Malik et al.
2010
).
In this process, photon absorption only occurs when the wavelength of irradiated
light belongs to the absorption spectrum of the photosensitive substance. After
absorption of light, the photosensitiser ground state goes to the excited singlet state
with a short half-life. The compound excited singlet can return to the ground state
by emitting light in the form of fluorescence or can pass to a triplet excited state
with a long half-life through a process referred to as crossing between systems
(Ochsner
1997b
; Henderson and Dougherty
1992
; Malik et al.
2010
). In the excited
triplet state, the photosensitiser can undergo two types of reactions: type I and type
II. In a type I reaction, there is a transfer of a proton or an electron from the
photosensitiser excited triplet state to the substrate or solvent molecules generating
an ion radical anion or cation, which reacts with the oxygen in the ground state to
form reactive species oxygen. In type II reactions, energy transfer occurs from the
photosensitiser triplet state directly to molecular oxygen, generating highly cyto-
toxic singlet oxygen (Ochsner
1997b
; Castano et al.
2004
; Henderson and
Dougherty
1992
). Reactions can occur simultaneously and depend on the type of
photosensitiser used and the concentration of the substrate and oxygen (Castano
et al.
2004
; Soukos and Goodson
2011
), as shown in Fig.
2
.
There are two basic mechanisms proposed to explain the lethal damage caused
by PDT in bacteria: DNA damage and cytoplasmic membrane damage, causing
leakage of cellular contents or inactivation of membrane transport systems and
enzymes. Studies have reported that treatment of bacteria with different photosen-
sitisers and light causes DNA damage. However, while DNA damage occurs, this is
not a major cause of bacterial cell death (Bertoloni et al.
2000
; Hamblin and Hasan
2004
).
Several microbial cells are susceptible to the photooxidation effect caused by
singlet oxygen. The photooxidation effect includes inactivation of enzymes and
other proteins and lipid peroxidation, leading to lysis of cell membranes, mitochon-
dria and lysosomes (Gonzales and Maisch
2012
).
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