Size effect of SnO2 nanoparticles on bacteria toxicity and their membrane damage

• Spherical SnO2 NPs of smaller size produced greatest toxicity for bacteria.
• SnO2 NPs had specific toxicity to B. subtilis which was caused solely by NPs.
• Cell surface charge interactions of NPs appear to be a key mechanism for NPs toxicity.
• Bacteria membrane damage by SnO2 NPs is shown for the first time.

Semiconductor SnO2 nanoparticles (NPs) are being exploited for various applications, including those in the environmental context. However, toxicity studies of SnO2 NPs are very limited. This study evaluated the toxic effect of two sizes of spherical SnO2 NPs (2 and 40 nm) and one size of flower-like SnO2 NPs (800 nm) towards the environmental bacteria E. coli and B. subtilis. SnO2 NPs were synthesized using a hydrothermal or calcination method and they were well characterized prior to toxicity assessment. To evaluate toxicity, cell viability and membrane damage were determined in cells (1 × 109 CFU mL−1) exposed to up to 1000 mg L−1 of NPs, using the plate counting method and confocal laser scanning microscopy. Spherical NPs of smaller primary size (E2) had the lowest hydrodynamic size (226 ± 96 nm) and highest negative charge (−30.3 ± 10.1 mV). Smaller spherical NPs also showed greatest effect on viability (IC50 > 500 mg L−1) and membrane damage of B. subtilis, whereas E. coli was unaffected. Scanning electron microscopy confirmed the membrane damage of exposed B. subtilis and also exhibited the attachment of E2 NPs to the cell surface, as well as the elongation of cells. It was also apparent that toxicity was caused solely by NPs, as released Sn4+ was not toxic to B. subtilis. Thus, surface charge interaction between negatively charged SnO2 NPs and positively charged molecules on the membrane of the Gram positive B. subtilis was indicated as the key mechanism related to toxicity of NPs.