Assessing Different Reactive Oxygen Species as Potential Antibiotics: Selectivity of Intracellular Superoxide Generation Using Quantum Dots

Abstract

Reactive oxygen species (ROS) represent a broad range of chemical species including superoxide, hydroxyl, singlet oxygen, and hydrogen peroxide. Each species behaves differently in the cellular environment. Some can play specific roles as intracellular signaling molecules, while others act primarily as indiscriminate oxidants. Several recent reports have promoted the use of exogenous ROS as therapeutic agents with applications from cancer therapies to novel antimicrobials. However, therapeutics, specifically antibiotics, should either kill or inhibit the growth of harmful cells (bacteria here) without harming the host cells, and hence selectivity of action is of vital importance. Here, we show that among different ROS, only superoxide was found to be bactericidal, killing a range of multidrug-resistant (MDR) pathogens without affecting the viability or growth of mammalian cells. Superoxide has a high thermodynamic capacity to be a strong oxidant. However, its lack of reactivity with cellular components at a physiological pH, except for the inactivation of biosynthetic enzymes containing labile iron−sulfur clusters, is key to its selectivity. The role of iron in bacterial pathogenesis also makes superoxide a strong candidate for antimicrobial therapy. Additionally, using a series of selective scavengers, we show that the superoxide radical is therapeutically effective and selective compared to other ROS like hydroxyl radicals, confirming previous results that used Escherichia coli gene knockouts to show that superoxide selectively deactivates some enzymes rather than causing indiscriminate damage of cellular components. In our in vitro studies, intracellular superoxide generation using light-activated quantum dots yielded highly selective and effective antimicrobial action. We screened 45 clinical MDR bacterial isolates and observed inhibition/therapeutic action in all strains, highlighting the applicability of such nanoparticle superoxide therapy. These results can pave the way for rational design of nanoscale therapies as precision medicine.

Near‐Infrared‐Light‐Triggered Antimicrobial Indium Phosphide Quantum Dots

“…We next sought to evaluate the suitability of Red InP and NIR InP for treating MDR infections. The literature suggests that MDR pathogens should also be sensitive to superoxide, and we obtained clinical isolates to verify this assumption in vitro. Our strain of carbapenem‐resistant Enterobacteriaceae E. coli (CRE E. coli ) is classified as the most critical pathogen, according to the WHO, because it is resistant to multiple classes of antibiotics, including carbapenem—a last‐resort antibiotic.…”

“…This allows QDs to act synergistically with existing antibiotics, or even potentiate failed antibiotics, making them effective again . Such dual use as an effective antimicrobial and antibiotic potentiator opens new avenues for addressing the imminent problem of rapidly developing MDR superbugs …”

“…Previous studies have used QDs in combination with visible light to eliminate bacteria, including 45 MDR strains, many of which are resistant to almost all known and last‐resort antibiotics . Upon illumination, absorbed light triggers the photo‐electrochemical reduction of oxygen to generate therapeutic superoxide intracellularly . The bottom‐up design of QD nanotherapeutics involves optimizing several aspects: material selection, reduction and oxidation potentials, biocompatibility, and surface chemistry.…”

“…These variables tightly control the desired phototherapeutic effect, eliminate undesired material toxicity, can modulate the innate host immune response, and ensure efficient QD transport . While cadmium telluride (CdTe) QDs demonstrate selective bacterial killing (even of MDR superbugs), its green‐light absorbing band gap and cadmium content present some concerns. Since longer wavelengths of light penetrate deeper through human tissue, owing to the optical window of biological transparency for light with wavelengths between 650 and 1350 nm, it is desirable to create heavy‐metal‐free QDs triggered by near‐infrared (NIR) light .…”

“…Based on the clear evidence of superoxide generation from the InP QDs, toxicity to bacteria was expected. Bacterial toxicity thresholds are lower for superoxide than for other reactive oxygen species (ROS), and bacteria are more susceptible to superoxide than human cells are . Further, since pathogens sequester iron from their hosts and environment in order to proliferate, they become conveniently iron‐rich targets for superoxide therapy .…”