**1.3 Photosensitizers for PACT**

Hundreds of compounds are currently available for mediating PDT in various areas of medicine, where some have been shown to be suitable for antimicrobial applications. PSs employed for medical uses should be a single pure compound, stable at room temperature and inexpensive. The PS must have a strong absorption peak in the visible spectrum between 600 and 900 nm and should possess a hightriplet quantum yield that will provide high production of ROS upon illumination. It should not be toxic in the dark (especially to mammalian cells), mutagenic or carcinogenic [15–18]. In addition, when talking about PACT, it is very important that the PS will display preferential association with bacteria, accumulate within the cells, or bind to the bacterial cell envelope [14, 19].

PSs can generally be assigned to several chemical classes: tetrapyrroles (which include porphyrins, chlorins, bacteriochlorins, and phthalocyanines), synthetic dyes (phenothiazinium salts, Rose Bengal, squaraines, etc.), and naturally occurring compounds (such as riboflavin or curcumin). Cyclic tetrapyrroles present the most well-known class of clinically relevant PSs used mostly for anticancer applications [20]. This structure can be found naturally in such important biomolecules such as haem, chlorophyll, and bacteriochlorophyll. Unlike other types of PSs, most tetrapyrroles (except for bacteriochlorins) are more likely to react by a Type II reaction with the creation of singlet oxygen [16], whereas bacteriochlorins act *via* a Type I mechanism. Other well-known antimicrobial agents are phenothiaziniumbased synthetic dyes, including methylene blue (MB) and toluidine blue O (TBO), which also act as anticancer agents in PDT. These structures can be synthesized more easily than tetrapyrroles but possess high-dark toxicity compared to other PSs [15, 21]. Another representative of synthetic dyes, Rose Bengal (RB), has already been used successfully in antimicrobial and anticancer applications for a long

**133**

**Figure 1.**

*Aspects of Photodynamic Inactivation of Bacteria DOI: http://dx.doi.org/10.5772/intechopen.89523*

lished by Hamblin and colleagues [15, 16].

**2. Photosensitizer activation modes**

**2.1 Dark activity**

ent ways for various PSs.

time [16]. Photodynamic active compounds isolated from plants arouse particular interest. These natural compounds include curcumin, extracted from the rhizomes of *Curcuma longa*, which was found effective in eradicating oral pathogens [22]. Another representative of this group is hypericin isolated from St. John's wort, which exhibits photodynamic activity against Gram-positive and Gram-negative bacteria. Detailed descriptions of all PS classes can be found in the reviews pub-

The name photosensitizer implies the need for illumination in order to activate PS molecules and trigger their action. However, PSs possess some so-called "dark activity" even in the absence of illumination, leading to cell death in the dark [23–29]. This feature depends on the PS concentration and manifests itself in differ-

Shrestha demonstrated dark toxicity of RB against Gram-positive *Enterococcus faecalis*. Exposure of the cells to 10 μM RB in the absence of illumination for 15 min led to a 0.5 log10 reduction in cell concentration [26]. Furthermore, a marked dark toxicity of RB against clinical isolates of Gram-negative *Pseudomonas aeruginosa* was observed by Nakonieczna [27]. Brovko compared the activity of various PSs against several types of microorganisms and noted high dark toxicity of RB, as well as of phloxine B against Gram-positive *Bacillus sp*. and *Listeria monocytogenes* (more than 5 log10 reduction in the bacterial concentration after 30 min of treatment with the dye) [30]. The toxicity of malachite green in the dark against the same microorganisms was very low (<0.1 log10 reduction in concentration after 30 min of treatment with the dye). High concentrations (>500 μg/mL) of acriflavin neutral in the absence of light were significantly toxic to *E. coli* (more than 6 log10 reduction in concentration after 30 min of treatment with the dye, both under illumination and

*Effect of RB concentration on its cytotoxic activity. S. aureus cells at the initial concentration of 104 CFU mL−1 were incubated for 3 min in dark conditions at various concentrations of RB. After the incubation, bacteria* 

*were tested by viable count. Error bars present standard deviations.*

*Aspects of Photodynamic Inactivation of Bacteria DOI: http://dx.doi.org/10.5772/intechopen.89523*

time [16]. Photodynamic active compounds isolated from plants arouse particular interest. These natural compounds include curcumin, extracted from the rhizomes of *Curcuma longa*, which was found effective in eradicating oral pathogens [22]. Another representative of this group is hypericin isolated from St. John's wort, which exhibits photodynamic activity against Gram-positive and Gram-negative bacteria. Detailed descriptions of all PS classes can be found in the reviews published by Hamblin and colleagues [15, 16].
