Escherichia coli biofilms are a major causative agent of many intestinal infections, and there is ongoing research aimed at E. coli biofilm eradication. Gold nanoclusters (AuNCs) conjugated with various surface ligands have been extensively investigated for antimicrobial properties and provide a potential solution. There is little known about their in vivo safety because current standards of nanosafety research involve incubation of AuNCs with cells in vitro to confirm biocompatibility. In addition to systemic administration, nanosafety research on AuNC-based antimicrobials designed to treat gastrointestinal infections must also consider the potential for inducing gastrointestinal disorders. We report the design and application of two AuNCs coated with either hydroxyl (AuNC@PEG-OH)- or amine (AuNC@PEG-NH2)-functionalized poly­(ethylene glycol), which enables the eradication of E. coli biofilms. Gastrointestinal safety of AuNC@PEG-OH and AuNC@PEG-NH2 was evaluated in healthy mice up to 35 days after administration by oral gavage at a dose of 10 mg/kg (or 1 mg/mL) daily for 14 days. No changes were detected in the histopathology of major organs, serum chemistry, hematology, and feces. Thus, oral administration of AuNCs is unlikely to be of concern for systemic toxicity or in the induction of gastrointestinal illnesses. Further studies on increasing time exposure and doses are necessary to determine whether toxicity occurs at higher doses or whether there is no adverse effect limit.

Anionic AuNC@CPP enters the cells and dissipates the proton gradient (ΔpH), which is compensated by an increase in electrical potential (ΔΨ) that leads to membrane hyperpolarization and enhances the susceptibility of persisters to antibiotics.

Persister cells are an important medical concern, leading to the overuse of antibiotics and ultimately contributing to antimicrobial resistance. The use of an adjuvant that augments the antibiotic efficacy has yet to be explored for combating persister cells within biofilm infections. Here we demonstrate a paradigm shift in targeting bacteria in chronic infections by coadministration of a conventional antibiotic with a novel engineered gold nanocluster (AuNC@CPP). AuNC@CPP was found to reduce the minimum biofilm eradication concentration (MBEC) of ofloxacin up to 300-fold (from > 3000 μg/mL to 10 μg/mL). Compared to ofloxacin alone (FLOXIN®Otic), coadministration of ofloxacin with AuNC@CPP induces up to a 10,000-fold reduction in bacterial burden in a validated mouse model of chronic P. aeruginosa of ear infection mimicking chronic suppurative otitis media. The biocompatibility of AuNC@CPP encourages efforts for development as an antibiotic adjunct for combating bacterial biofilm infections.