THE AGE OF “NANO-BIOTICS”: SELF-ASSEMBLED MATERIALS AS NOVEL ANTIBACTER-IAL AGENTS AGAINST DRUG-RESISTANCE BACTERIA

Thomas Jay Webster

Department of Chemical Engineering, Northeastern University, Boston, MA, USA

Abstract

Bacteria drug resistance has become one of the biggest threats to the global health care system. Many conventional antibiotics prevent bacterial growth in a target-specific manner that has led to bacteria developing defensive mechanisms by alternating binding sites of the drugs or hydrolyzing the drug; such events lead to antibiotic ineffectiveness, the formation of antibiotic-resistant bacteria, and long-term infection complications. Moreover, bacterial biofilms (formed by immobilized bacteria on the surfaces of medical devices) continue to plague the medical device community. Herein, we developed and tested (using in vitro and in vivo experiments) a wide range of self-assembled peptide materials (nanoparticles, nanofibers, etc.) as antibacterial and biocompatible agents against drug-resistant bacteria without the use of antibiotics.

In our studies in particular, two types of cationic peptide amphiphiles (PAs) have been designed to self-assemble into nanoparticles with different morphologies in aqueous solution. Each PA molecule we designed contains an aliphatic hydrophobic segment and an oligopeptide hydrophilic segment. As a result, simultaneous self-assembly is driven by hydrophobic interactions, whereas the hydrophilic peptide segment can control the self-assembled structure and display desirable interactions with bacterial membranes. With the antibacterial activities inherited from naturally occurring antimicrobial peptides (AMPs), these self-assembled “nano-biotics” can effectively penetrate bacterial membranes via electrostatic interactions, resulting in a low likelihood for bacteria to develop drug resistance.

First, antibacterial cationic peptide amphiphiles (ACA-PAs) can self-assemble into nanorod structures via intermolecular hydrogen bonds, which enable the formation of transmembrane pores on bacteria membranes. Our results showed that these self-assembled nanorods were 7-10 nm in diameter, as observed by transmission electron microscopy (TEM). In bacterial studies, Gram-positive methicillin-resistant Staphylococcus aureus (MRSA) and Gram- negative multidrug-resistant Escherichia coli (MDR E. coli) were tested. At concentrations over 0.2 mg/ml (80 µM), the ACA nanorods completely inhibited the growth of antibiotic-resistant bacteria, and decreased MRSA and MDR E. coli density by 100-fold and 10,000-fold, respectively. In particular, self-assembly of ACA-PAs was shown to be important for killing Gram-negative bacteria. The self-assembled nanorods also exhibited significantly lower toxicity towards human dermal fibroblast (HDF) cells.

Moreover, PA self-assembled nanoparticles can be coated on medical device surfaces to prevent bacterial colonization. As just ne example, spherical amphiphilic peptide nanoparticles (APNPs) have been coated onto porous chitosan matrices (PCM) using simple lyophilization. APNPs can self-assemble into spherical micelles with arginine-rich peptide sequences exposed on the surface in solution. Followed by lyophilization, nanospheres of APNPs (about 300 nm in diameter) were observed by scanning electron microscopy (SEM). PCM, which was prepared by lyophilizing a 0.02% chitosan solution in 1% acetic acid, possessed a highly distributed porous structure for APNP entrapment. SEM characterization showed that APNPs can physically attached on the inner pores of PCM. In addition, after incubation with Staphylococcus epidermidis (S. epidermidis) for 24 h, PCM scaffolds loaded with 200 μM APNPs reduced bacterial colonization by 100-fold compared with plain PCM. In vivo studies confirmed in vitro results and will also be presented.

In conclusion, the above mentioned self-assembled nanoparticles of PAs exhibit potent antibacterial activity and high biocompatibility. These self-assembling peptides could be promising nanomedicines as alternatives to antibiotics against bacterial drug-resistance.

Keywords: Self-assembly, peptide amphiphiles, antibacterial, and antibiotic-resistant bacteria.