## 4 Antibacterial Nanotheranostics
### Theranostic Nanoparticles (NPs) and Nanofibers
#### 4.1.2 Nanoparticle-Mediated Therapeutic Antibacterial and Antibiofilm Actions  
This review pioneers our understanding of the mechanisms behind the antibacterial therapeutic effects of NPs. We will begin with the mechanistic story and address a series of confusing questions to elaborate on the different mechanisms as delineated in Figure 4.  
Figure 4: Therapeutic antibacterial and antibiofilm mechanisms of different sizes and shapes of nanoparticle (NP) gunshots. Antibacterial mechanisms are started by large contact areas between the bacterial surface and small-sized NP shots, hitting bacteria forming pits, inducing ROS (damaging membranes), and releasing metal ions (e.g., silver ions, Ag\({}^{+}\)). The positively charged ions (Ag\({}^{+}\)) interact with negatively charged groups (e.g., sulfur, thiol, and phosphorous) in proteins and enzymes, and deoxyribonucleic acid (DNA), inactivating proteins and enzymes and inhibiting replication, respectively. Ag NPs or Ag\({}^{+}\) inhibit the electron transport chain, collapsing respiration. Gentamicin in drug-delivery systems inhibits protein synthesis, binding to the ribosomal 30S subunit. The antibiofilm mechanisms are started by electrostatic interactions between NP shots and bacterial surfaces, preventing surface adhesion. NP shots or Ag\({}^{+}\) downregulate the expression of glucosyltransferase (_gtf_) genes, quorum sensing (QS)-related genes, and _ica_ (intercellular adhesion) genes, inhibiting biofilm formation and proliferation. NP shots or activated photosensitizer (PS) induce ROS, degrading the extracellular polymeric substance (EPS) of mature biofilms. The antibacterial mechanisms of NPs also kill bacterial cells in biofilms. Assuming that Ag NPs or NPs, in general, are similar to therapeutic gunshots, once the shot hits and interacts with the treated bacterial cell membranes, it forms \"pits\" that damage the membranes, increasing the permeability of bacterial membranes and killing the bacteria [152]. Then, the Ag NP shots may inactivate the bacterial respiratory chain dehydrogenases, inhibiting respiration [153], and form ROS, damaging the membranes [154; 155] (more specific roles of ROS are explained below in Section 5.2. PDT). Factors that affect the bacterial killing efficiency of Ag NP shots remain unclear. The size of the Ag NPs is a fundamental factor. Smaller Ag shots (1 to 10 nm) kill bacteria more efficiently than larger shots [156; 157; 158]. These small nanoshots have large surface areas that allow larger contact areas with the bacterial cells, killing them [159; 160; 157]. The immobilization of Ag NP shots on substrates (silica) allows more contact-mode interactions between Ag shots and cells because it inhibits the sequestration of shots within the bacterial cells [161]. Smaller Ag NP shots with high surface areas have higher oxide contents [162]. The oxidized Ag NPs release more Ag\({}^{+}\) ions [163]. Ag\({}^{+}\) ions (as such or released from Ag NPs) are soft acids motivated to interact with soft bases containing sulfur and phosphorous. The interaction of Ag\({}^{+}\) ions with sulfur-containing proteins in bacterial cell walls and thiol groups of bacterial enzymes and proteins inactivates them, damaging bacterial membranes. The interaction of Ag\({}^{+}\) ions with phosphorus parts of DNA prevents bacterial replication [164; 165]. Ag\({}^{+}\) ions also inhibit the bacterial electron transport chain (which is membrane-bound) via facilitating the oxidation of respiratory membrane-bound enzymes (flavoprotein and cytochrome b1), eventually collapsing bacterial respiration [166; 167]. From the mechanisms above, we notice substantial differences and cannot deny the similarities between the antibacterial actions of Ag NPs and Ag\({}^{+}\) ions. Can we attribute the antibacterial mechanisms of Ag NP shots to Ag\({}^{+}\) ions?