Mechanistic evaluation of the size dependent antimicrobial activity of water soluble QDs

Abstract

Herein, we report the preparation of water soluble CdSe quantum dots (coated with 3-mercaptopropionic acid, MPA) of variable sizes (2.43–5.09 nm) based on hot injection kinetic synthesis. A comprehensive spectroscopic analysis of these nanoparticles by steady state and time resolved fluorescence spectroscopy yielded interesting results. Since the internalization of QDs by prokaryotes has not been well-understood, as yet, the activity of the variable sized QDs was assessed against Escherichia coli DH5α (Gram-negative) and Staphylococcus aureus (Gram-positive bacteria), ATCC 13709 by the agar disc assay and MTT assay. The mechanisms of toxicity were evaluated by measuring the reactive oxygen species (ROS) generated by the QDs alone and investigating the oxidative damage to bacteria. The sensitivity of S. aureus was higher compared to that of E. coli and a significant size dependent increase in intracellular ROS generation compared to the control was noted. Statistically significant size-dependent increase in the zone of inhibition was observed in S. aureus and E. coli by the agar disc diffusion assay. Since, reduced glutathione (GSH) is the primary line of defence against acute toxicities of electrophiles and ROS and has a detrimental role in conferring cellular protection, the GSH depletion was quantified: GSH in S. aureus and E.  coli by QD-A (2.43 nm = 0.055 ± 0.0007, 0.053 ± 0.0002), QD-C (5.09 nm = 0.057 ± 0.001, 0.054 ± 0.0046) followed by QD-B (3.75 nm = 0.060 ± 0.0003, 0.057 ± 0.0005) with respect to the control (0.086 ± 0.001, 0.075 ± 0.0005). ROS were further quantified using DCFH-DA. An increase in the levels of reactive oxygen species was observed [QD-A treatment (119.35% ± 5.77% and 34.58% ± 5.77%), QD-C (62.90% ± 11.54% and 23.3% ± 5.77%) followed by QD-B (14.51% ± 15.27% and 12.03% ± 5.77%)]. Lactate dehydrogenase release determines the integrity of the cell membrane and the LDH release was directly proportional to cellular damage [QD-A (104.05% ± 5.77% and 102.61% ± 13.76%) and C (68.89% ± 7.21% and 29.4% ± 15.12%) followed by B (51.68% ± 11.54% and 18.45% ± 15.46%)]. The QD induced oxidative stress enhanced genotoxicity as evidenced by the DNA damage in S. aureus (QD-A > QD-C > QD-B = 53% ± 1.5%, 49% ± 1.15%, 47% ± 1.15%) and E. coli (49.5% ± 1.73%, 45% ± 0.57%, 42% ± 1.15%). The results confirmed the role of QDs in inducing oxidative stress in the microbial system whereby it can be applied to inflict damage to many unwanted members of the microbial communities.