The Effects of Photodynamic Therapy in Oral Biofilms - Antibiofilm Agents: From Diagnosis to Treatment and Prevention - page 439

Fig. 1 Triad formative
elements of PDT
PS
Light
Oxygen
Photodynamic Therapy
antimicrobial action, PDT has other names, such as photodynamic inactivation
(PI) or photoactivated disinfection (PAD) (Bonsor et al. 2006 ; Nagata et al. 2012 ).
Compared with other cytotoxic therapies, photodynamic therapy has the poten-
tial to act in the infected tissue or the target cell, and the light source may relate
directly to the injury site (Demidova and Hamblin 2004 ). Thus, an important feature
of photodynamic therapy is the dual selectivity, first through concentration of the
photosensitiser by specific binding to target tissue, and second by restricting the
irradiation to a specific volume. In photodynamic antibacterial therapy, the
photodestruction is primarily caused by damage to the cell membrane and DNA
(Schafer et al. 1998 ; Bertoloni et al. 2000 ; Romanova et al. 2003 ; Soukos and
Goodson 2011 ).
A fundamental difference in susceptibility to PDT between Gram-positive and
Gram-negative bacteria was recognised in the 1990s (Nitzan et al. 1992 ). Generally,
neutral, anionic and cationic photosensitising molecules can efficiently destroy
Gram-positive bacteria, whereas only cationic photosensitisers or strategies,
which alter the permeability of Gram-negative bacteria, in combination with
non-cationic photosensitisers, are capable of inactivating Gram-negative bacteria.
This is due to the presence of a cytoplasmic membrane that is surrounded by a
relatively porous cell wall composed of peptidoglycan and lipoteichoic acid in
Gram-positive bacteria; this structure allows for the diffusion of the photosensitiser
(Nagata et al. 2012 ). The cell wall of Gram-negative bacteria comprises the inner
cytoplasmic membrane and the outer membrane, a periplasmic space that is inter-
spersed with peptidoglycan. The outer membrane forms an effective permeability
barrier between the external environment and the cell, binding and limiting the
penetration of the photosensitiser (Minnock et al. 2000 ). Studies have shown that
Gram-positive bacteria are more susceptible to photoinactivation (Malik
et al. 1992 ; Bertoloni et al. 1992 ; Soukos et al. 1998 ) than Gram-negative bacteria
(Nitzan et al. 1995 ; Soukos et al. 1998 ).