In both cases, the conjugates displayed low toxicity in vitro and strong in vitro and in vivo antimicrobial activity against intracellular bacteria, with the conjugates conferring complete subject survival compared to the complete lack thereof in untreated mice [37,38]. practice. [31,32,33]. 2.2. Endogenous Targeting For direct endogenous targeting, specific infection-associated ligands can be exploited. Vancomycin [34,35], antibodies targeting bacterial surface proteins , aptamers [37,38], and lectins [39,40] have all been conjugated to nanoparticles for bacteria-specific targeting. For example, conjugating an antibody specifically binding to staphylococcal protein A, a species-specific surface protein, to daptomycin-loaded polydopamine-coated platinum nanocages enabled targeting of with no binding to mammalian cells . However, ligand-based targeting can be limited by the convenience of the target cells as well the structural heterogeneity of the targeted molecules . SB-568849 Similarly, the negative surface charge of bacteria can provide a means of targeting through the electrostatic interactions of cationic nanoparticles . In this is one of the ways, AMPs SB-568849 themselves are being designed to promote specificity. A number of characteristics unique to the microenvironment of the contamination site can also be utilized for targeted delivery of antibiotics. These include pH, redox gradients, and enzyme concentration. Firstly, pH-sensitive linkers can take advantage of the local low pH environment associated with bacterial infection and biofilms. This pH can reach as low as 4.5 in the case of biofilms [42,43,44] and is associated with anaerobic fermentation and inflammation, both of which produce acidic products [41,42]. Targeting infection-associated pH allows for indiscriminate bacterial targeting, which can be useful in broad-spectrum targeting. For example, Radovic-Morena et al. developed vancomycin-encapsulating pH-responsive poly(d,l-lactic-or [51,52], and bacterial lipases  have been examined for this purpose. Enzymes secreted by the host during the inflammatory response to contamination can also potentially be harnessed here (e.g., matrix metalloproteinases ). Targeting Intracellular Bacteria It is becoming increasingly apparent that intracellular bacteria play a large role in recurrent and prolonged bacterial infection. During contamination, the bodys most immediate defence against pathogenic bacteria is the innate immune system . This system houses a number of immune cells that identify the invading pathogen and remove it from the body. In particular, macrophages and neutrophils, phagocytic cells acting as the first line of defence against contamination, engulf bacteria within minutes of contamination . The engulfed bacteria are then damaged by fusion with acidic lysosomes, which deliver digestive enzymes, bactericidal proteins (e.g., lysozyme), proton pumps, ROS and reactive nitrogen species to the phagosome [57,58]. Although phagolysosomal killing is usually quite effective, a number of bacteria, including and  and Mycobacteria SB-568849 [62,63], suggesting they could be encouraging therapeutics for intracellular bacteria killing. One unique approach to targeting bacterial infection is usually to take advantage of the innate targeting of phagocytic immune cells that spontaneously scavenge and eliminate bacteria within the body [28,58,64]. For example, Xiong et al. developed vancomycin-loaded nanogels covered in mannosyl ligands for targeted delivery to macrophages, which SAT1 express high levels of mannose receptors . Once the nanogel-containing macrophages engulfed bacteria at the site of contamination, the nanogels polyphosphoester core was degraded by bacterially produced phosphatase or phospholipase, causing release of the vancomycin and subsequent bacterial destruction. More recently, in a method using adoptive macrophage therapy, Hou et al. utilized vitamin lipid nanoparticles to deliver an mRNA strand encoding an AMP-cathepsin B conjugate to cultured macrophages . When delivered to the cytosol of the cell, the mRNA was translated, and the SB-568849 producing protein conjugate was subsequently trafficked to the lysosome by its cathepsin B tag, where it was cleaved by the enzyme to release the AMP. These macrophages, enhanced with a lysosomally localized broad spectrum AMP, displayed efficient killing of drug resistant intracellular and and recovered the immune system of immunocompromised septic.