Resistant bacterial strains are responsible of serious infections, in particular in nosocomial settings, impossible to be treated with traditional antibiotics. Furthermore, the particular form of resistance supported by biofilm production, hinder the durability, reliability, and performance of many medical devices and implants. In this regard, the development of new antibacterial agents and antibiofilm strategies, such as antibacterial coatings that repel bacteria and prevent biofilm formation are highly urgent. Natural cationic antimicrobial peptides (CAMPs), which are capable to kill pathogens simply through external contact, without the need to address the numerous resistance mechanisms due to genetic mutations that bacteria can develop, represent a promising option to conventional antibiotics. Unfortunately, high costs of production, instability and low selectivity for bacteria, limit their clinical application. Inspired by natural CAMPs, cationic antimicrobial macromolecules including dendrimers (Ds) have shown to function as antibacterial agents and as antimicrobial surface coatings as well. Different types of dendrimers, including cationic ones, have been developed for the treatment of infections sustained by multidrug-resistant bacteria, mainly during the last decade. Commercially available PAMAM and PPI-based dendrimers, as well as several their dendrimer derivatives which proved to possess in vitro broad-spectrum activities, have been reviewed in a recent entry on Encyclopedia (https://encyclopedia.pub/7167). In a second entry, we have provided an updated overview concerning the most studied class of cationic antibacterial dendrimers (CADs), i.e. the antimicrobial peptide ones. Now, we have herein collected the main case studies concerning the antibacterial cationic dendrimers having different internal scaffolds, but peripherally positively charged due to the presence of terminal ammonium groups.

Ammonium-Terminated Dendrimers

As happens among cationic antimicrobial polymers ^[1], also among CADs a class is represented by dendrimers which, despite having structurally different internal matrices, possess peripheral protonated amino groups, responsible for their cationic character and essential for their antibacterial potency. Second-generation (G2) ammonium-terminated Ds were prepared by synthesizing Boc-protected precursor Ds (G2-Boc), with a divergent synthetic strategy through polymerization of t-butyloxycarbonyl-aminoethyl acrylate (Boc-AEA), followed by deprotection treatment with trifluoroacetic acid (TFA) to remove Boc groups ^[2]. In plate killing essays, the G2 dendrimer obtained showed high antimicrobial activity against E. coli and S. aureus species (MBC_99.9 = 3–8 μg/mL and 4 μg/mL respectively), associated with very low hemolytic toxicity (<10% at concentration of 1024 μg/mL). Interestingly, since net cationic membrane-disruptive antimicrobials may elicit an immune attack when administered intravenously, Xu’s same group, having cloaked a G2 dendrimer with poly(caprolactone-b-ethylene glycol) (PCL-b-PEG), obtained a new antimicrobial, G2-g-(PCL-b-PEG), which exhibits neutral surface charge. The neutral dendrimer was able to kill >99.9% of the inoculated bacterial cells, at ≤8 μg/mL, it exhibited good colloidal stability in the presence of serum and it showed insignificant hemolytic toxicity, even at concentrations ≥2048 μg/mL. The authors attributed the maintained antimicrobial activity of the neutral dendrimer to the degradation of the neutral shell due to bacterial lipase and the consequent exposure of the membrane-disruptive bactericidal cationic G2 core.

Semisynthetic ammonium-terminated gallic acid-triethylene glycol Ds (GATG) developed by Leire et al. (2016) have been shown to be able to interact with bacteria, thus attracting researchers as potential multivalent macromolecules for the development of new antimicrobials^[3]. GATG Ds, due to the presence of primary amines in their periphery, were capable of sequestering bacteria cells and to induce the formation of clusters in Vibrio harveyi, an opportunistic marine luminescent pathogen. This property was dependent on dendrimer generations and was more potent than that of poly(N-[3-(dimethylamino)propyl]methacrylamide) [p(DMAPMAm)], a cationic linear polymer previously shown to cluster bacteria. The authors also investigated the bacteria viability within the formed clusters and the quorum sensing activity, responsible for light production in V. harveyi. The results suggested that GATG Ds may activate microbial responses by maintaining a high concentration of quorum sensing signals within the clusters and by increasing permeability of the microbial OM, with the growth of their generation. In particular, when bacteria cells were exposed to third-generation GATG Ds, more than 85% of population had OM damaged. The GATG Ds developed by Leire et al. might constitute a valuable platform for the development of novel antimicrobial materials which can affect microbial viability and/or virulence.

By using the robust copper (I) catalyzed alkyne-azide cycloaddition (CuAAC) “click reaction” for dendrimer synthesis and for post-synthesis functionalization, Ladd et al. reported a versatile divergent methodology to construct ammonium-terminated Ds from the tetrafunctional core pentaerythritol^[4]. Ds of generations 0–3 (G0–G3) were achieved with from 4 to 32 acetylene surface groups, which in G0–G2 Ds were subsequently used to covalently link cationic amino groups. The authors explored the bactericidal efficacy of the cationic amine terminated Ds G0-NH₃⁺-G3NH₃⁺ determining MIC and MBC values against the E. coli ATCC 11229TM strain. The results asserted that the bacteriostatic and bactericidal activity of the prepared Ds depended on their size, functional end groups and hydrophilicity, as already established in many studies in the field. In particular, G0 dendrimer displayed poor antibacterial activity, with MIC values in the range 29–59 μM and MBC values > 59 μM, while G2 and G1 Ds displayed from good to strong antibacterial activity. In the authors’ opinion, the octacationic first-generation dendrimer was the most potent antimicrobial dendrimer displaying MIC values (0.9 μg/mL, 0.26 μM) and MBC values (4–8 μg/mL, 1.1–2.3 μM) that were lower than those of several previously studied Ds. In our opinion, and considering when the MIC values are expressed in μM (which takes into account the MW of the Ds and which provides the actual equivalents of Ds able to exert a certain effect), rather than in μg/mL, the G2 dendrimer can be considered the most active antimicrobial device (MIC value = 0.12 μM vs. 0.26 μM). Furthermore, although the Ds developed by Ladd and colleagues may appear very promising, the absence of investigations into their cytotoxicity and hemolytic toxicity prevents them from being considered suitable for clinical applications.

Ammonium-Terminated Phosphorous Dendrimers

By performing a sol-gel process, the fruitful association of a fourth-generation phosphorus dendrimer, peripherally modified with quaternized ethylene diamine and titanium dioxide (TiO₂), was realized by Milowska et al. (2015) together with other three ones^[5]. The hybrid mesoporous dendrimer-coated TiO₂ obtained showed to be nanosized, in a crystalline and to possess a surface mimicking that one of PEI-Ds. It was investigated for its toxicity on red blood cells (RBCs), its cytotoxicity toward B14 Chinese fibroblasts and its antimicrobial activity against some strains of bacteria and yeast. The dendrimer proved appreciable antibacterial activity on S. aureus and S. epidermidis MSSE strains at MIC = 500 μg/mL in both cases. Nevertheless, considering that in hemolytic and cytotoxicity essays it showed a HC₅₀ value of 100 μg/mL and an IC₅₀ of 25 μg/mL, its antimicrobial activity must be considered questionable.

Ammonium-Terminated Carbosilane Dendrimers

Cationic carbosilane Ds have shown their potential to be used as devices in several biomedical applications including as bactericides^[5]. Both silicon containing hyperbranched polymers and Ds are interesting as organic-inorganic hybrid materials, due to the presence of C–Si bonds, that confer high stability and very low hydrophilicity, favorable to the processes of bio permeability^[6]. Nevertheless, when preparing cationic materials for antimicrobial uses, hydrophobicity must be tuned by functionalizing the macromolecules with cationic groups, thus achieving good levels of water solubility, a pivotal requirement for antibacterial studies. In this context, among other cationic materials, including hyperbranched polymers, some second- and third-generation (G2 and G3) cationic Ds, based on a carbosilane scaffold, but peripherally decorated with ammonium groups, were prepared by Ortega et al. (2011) and proposed as antibacterial agents. Although slightly less active that the hyperbranched analogous, the cationic carbosilane Ds exhibited considerable antimicrobial activities against the Gram-negative E. coli and against the Gram-positive S. aureus, with MIC values in the range of 16–64 μg/mL and 4–8 μg/mL, respectively. MBC values, also calculated against the same strains, were in the ranges of 32–54 μg/mL and 8 μg/mL, respectively. The materials studied by Ortega and colleagues seem very promising as novel broad-spectrum antimicrobial agents, but, since research on their cytotoxicity, hemolytic toxicity, and selectivity is missing, their suitability for clinical uses cannot be taken for granted.

Recently, Fernandez and colleagues (2019) synthetized G1 and G2 carbosilane dendrons and, using a selection of three different CAMPs, created a series of nanocomposites either by covalent links or by physical non-covalent interactions, with the aim of establishing synergistic or collaborative relations between both types of molecules^[7]. The authors speculated that the carbosilane dendrimer structures, already known for their on contact antibacterial potency against Gram-positive and Gram-negative bacteria strains and for their poor tendency to induce resistances probably due to their easier penetration into the phospholipid bilayer could help to protect or transport AMPs inside bacterial membranes. The antibacterial activity of ammonium-terminated carbosilane Ds (namely 3 and 4) was assessed against E. coli and S. aureus strains and was compared to that of the four nanocomposites [namely 11, 14 (covalent nanoconjugates) and 15, 16 (non-covalent nanoconjugates)] obtained. The MIC values calculated for both groups of compounds were comparable in the case of E. coli, but not in the case of S. aureus, since the nanocomposites (11, 15) displayed a significantly higher antibacterial activity than that of corresponding dendrons (3) (MIC values of 32 and 64 μg/mL vs. 128 μg/mL, respectively). This trend also applied to the nanocomposite (16), when compared to the corresponding dendron (4) (MIC value of 4 μg/mL vs. 8 μg/mL).