## 4 Antibacterial Nanotheranostics
### Photodynamic Therapy (PDT)  
Photodynamic therapy (PDT) plays an essential role in selective bacterial illumination via selective PS and the efficient treatment of bacterial diseases via the produced ROS [108]. The meaning of ROS refers to reduced oxygen metabolites with strong oxidizing abilities, deteriorating cells (i.e., oxidizing proteins and lipids and damaging DNA) at high concentrations, and serving as signaling molecules (i.e., regulating cell growth, differentiation, and apoptosis). Well-known examples of ROS are the superoxide anion (O\({}_{2}\)\({}^{-}\)), hydroxyl radical (OH\({}^{\bullet}\)), H\({}_{2}\)O\({}_{2}\), and hypochlorous acid (HOCI) [220]. Three main ROS mechanisms may prove toxic to bacteria (delineated in Figure 6). First, the damage of iron-sulfur (Fe-S) cluster proteins, where O\({}_{2}\)\({}^{-}\) is electrostatically motivated (because of their chemical nature as univalent electron donors), binds the bacterial solvent-exposed Fe-S cluster. ROS converts them to an unstable oxidized form [221; 222] (as displayed in Figure 6). The Fe-S cluster is inherently recruited as an enzymatic cofactor in several cellular processes, such as electron transport and enzymatic catalysis and regulation. The cluster binds \(\alpha\),\(\beta\)-dihydroxy acid substrates, and it is involved in dehydration reactions through catalyzing (activating) dehydatases [223; 224]. The decomposition of the unstable oxidized Fe-S cluster inactivates the dehydatases [221]. Bacterial switching to fermentation (anaerobic growth) reduces the  
Figure 5: Schematic illustration showing the structure of the six-armed star-shaped poly(AA) with pendent methacrylate groups. Reprinted with permission from [219]. Copyright, 2012, Elsevier. damage of the Fe-S cluster [222; 223]. The second ROS mechanism involves the damage of mononuclear iron enzymes, where O\({}_{2}\)\({}^{-}\) and H\({}_{2}\)O\({}_{2}\) inactivate the enzyme family (i.e., epimerases, dehydrogenagenases, deformylases, and deaminases) using the single iron atom as an enzymatic cofactor. Ultimately, a third mechanism might be associated with the indirect damage of DNA, where H\({}_{2}\)O\({}_{2}\) produces OH\({}^{\bullet}\), damaging the nitrogen-containing nucleobases and deoxyribose of DNA oxidatively [193]  
[PERSON] et al. [225] have taken advantage of the molecular antibiotic mechanism of MRSA (expressing \(\beta\)-lactamase) and prepared a PS construct (\(\beta\)-lactamase enzyme-activated PS, \(\beta\)-LEAP) that was cleaved by \(\beta\)-lactamase of MRSA. Following the PS cleavage, local ROS were explicitly produced to MRSA upon activation by laser light at 670 nm, inhibiting the growth of MRSA strains. A direct relationship was observed between the \(\beta\)-lactamase activity of MRSA and the susceptibility of MRSA to \(\beta\)-LEAP inducing PDT. [PERSON] et al. [226] have explored the PDT on a bioluminescent MRSA infected mouse wound using a polyethyleneimine-ce6 (PS) and a red light. They have observed that PDT inactivated MRSA efficiently, abrogating the bioluminescence of wounds, and enhanced wound healing. However, less PDT inactivation of MRSA would be reached in vivo than in vitro because in vivo, MRSA tends to deeply penetrate tissue, forming biofilms that act as a barrier, hindering PS's access, and weakening the penetrated light dose into the deep tissue. Figure 6: Toxicity mechanisms of reactive oxygen species (ROS). ROS exert multiple roles, damaging (1) bacterial membranes, (2) Fe-S cluster proteins, (3) mononuclear iron enzymes, (4) DNA, and (5) biofilm EPS. A highlighting spot on the damage of iron-sulfur (Fe-S) cluster proteins bound to the \(\alpha\),\(\beta\)-dihydroxy acid substrate after exposure to superoxide (O\({}_{2}\)\({}^{-}\)). The cluster is oxidized, forming hydrogen peroxide (H\({}_{2}\) O\({}_{2}\)), and converted to the unstable oxidized form [4 Fe–45]\({}^{3+}\) species [221]. [PERSON] et al.