## 3 Controlled Antibiotic Delivery
### Endogenous Stimuli-Responsive Antibiotic Delivery  
Variations in microenvironment characteristics are the important marker for bacterial infections, making them attractive targets for designing stimuli-response delivery systems (summarized in Table 1). In this respect, antibiotic-loaded NPs can be synthesized or modified via various polymers or labile chemical bonds that respond to the specific stimuli to release the therapeutic cargo selectively. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Biological Parameters** & **Normal Tissues** & **Pathological Tissues** \\ \hline pH & 7.4 & 5–7.0 in inflammatory tissue \\  & 7.4 & 4.5–6.5 in endosomes and lysosomes \\  & & Acid-pH gradients in biofilm (4.5–6.5) \\ \hline Enzyme & Low expression & Overexpression (Lipase, Protease) \\  & & Or specific enzyme expression (\(\beta\)-lactamase, elastase LasB) \\ \hline Reduced glutathione (GSH) & Low level (2–20 uM) & High level in intracellular environment of macrophages and bacteria (2–10 mM) \\ \hline Reactive oxygen species (ROS) & Low level & High level in inflammatory environment (Neutrophil production) \\  & & High level in biofilm \\ \hline Nitric oxide (NO) & Low level & High level (Proinflammatory mediator) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Differences in physiological parameters between normal and pathological tissues. One of the most exploited stimuli in controlled-release antibiotic applications is **pH**. Indeed, the pH value of pathological tissues subjected to inflammation is significantly different from healthy tissue. The anaerobic fermentation in some bacteria, in which the products are organic acids such as lactic and acetic acids, and the inflammation response cause the pH of the site of infection to be significantly lower (pH 5-7) than the healthy tissue (pH 7.4). Based on this characteristic, many pH-sensitive nano-antibiotic systems have been widely designed by incorporating the ionizable groups or acid-labile bonds (hydrazine, amide, and ester linkages) in their polymer matrix [46; 47]. These systems retain their stability during the systemic circulation under normal physiological pH conditions (pH 7.4) while collapsing in an acidic microenvironment, resulting in the release of the loaded antibiotic at the desired locations. For instance, polymycin B is an old antibiotic with excellent bactericidal activity against gram-negative \"superbugs\". Unfortunately, the high nephrotoxicity and neurotoxicity occurring during systemic polymycin therapy strongly limited its clinical applications. To address this issue, [PERSON] et al. reported a charge switchable polyion nanocomposite with a pH-sensitive property to improve polymycin's biosafety while retaining its excellent antibacterial activity (Figure 4A) [48]. The nanocomposite was prepared through electrostatic interaction of positively charged polymycin B and negatively charged 2,3-dimethyl maleic anhydride-grafted chitol oligosaccharidesacharide. Upon arriving in infectious sites, the negative charge of the graft polymer will convert to positive due to the hydrolysis of amide bonds in the acidic environment, leading to the disassembly of the nanocomposite and release of polymycin B. As a result, the toxicity of the nanocomposite dramatically decreased compared to free polymycin B while exhibiting superior bactericidal activity against _P. aeruginosa_ in an acute lung infection mouse model. Likewise, in attempts to control variogenic dental biofilms, a pH-sensitive diblock copolymer composed of 2-(dimethylamino)ethyl methacrylate (DMAEMA), butyl methacrylate (BMA), and 2-propylicylic acid (PA) was synthesized to form cationic micellar drug carriers, capable of binding to the biofilm matrix (Figure 4B) [19]. Owing to hydrophobic cores, the micelle system allows the loading of hydrophobic antimicrobials and their release on demand at acidic pH, characteristic of cariogenic biofilm microenvironments (through core destabilization).