Isolating the Escherichia coli Transcriptomic Response to Superoxide Generation from Cadmium Chalcogenide Quantum Dots

Nanomaterials have been extensively used in the biomedical field and have recently garnered attention as potential antimicrobial agents. Cadmium telluride quantum dots (QDs) with a bandgap of 2.4 eV (CdTe-2.4) were previously shown to inhibit multidrug-resistant clinical isolates of bacterial pathogens via light-activated superoxide generation. Here we investigate the transcriptomic response of Escherichia coli to phototherapeutic CdTe-2.4 QDs both with and without illumination, as well as in comparison with the non-superoxide-generating cadmium selenide QDs (CdSe-2.4) as a negative control. Our analysis sought to separate the transcriptomic response of E. coli to the generation of superoxide by the CdTe-2.4 QDs from the presence of cadmium chalcogenide nanoparticles alone. We used comparisons between illuminated CdTe-2.4 conditions and all others to establish the superoxide generation response and used comparisons between all QD conditions and the no treatment condition to establish the cadmium chalcogenide QD response. In our analysis of the gene expression experiments, we found eight genes to be consistently differentially expressed as a response to superoxide generation, and these genes demonstrate a consistent association with the DNA damage response and deactivation of iron-sulfur clusters. Each of these responses is characteristic of a bacterial superoxide response. We found 18 genes associated with the presence of cadmium chalcogenide QDs but not the generation of superoxide by CdTe-2.4, including several that implicated metabolism of amino acids in the E. coli response. To explore each of these gene sets further, we performed both gene knockout and amino acid supplementation experiments. We identified the importance of leucyl-tRNA downregulation as a cadmium chalcogenide QD response and reinforced the relationship between CdTe-2.4 stress and iron-sulfur clusters through examination of the gene tusA. This study demonstrates the transcriptomic response of E. coli to CdTe-2.4 and CdSe-2.4 QDs and parses the different effects of superoxide versus material effects on the bacteria. Our findings may provide useful information toward the development of QD-based antibacterial therapy in the future.