Charge-associated effects of fullerene derivatives on microbial structural integrity and central metabolism

The effects of four types of fullerene compounds (C₆₀, C ₆₀-OH, C₆₀-COOH, C₆₀-NH₂) were examined on two model microorganisms (Escherichia coli W3110 and Shewanella oneidensis MR-1). Positively charged C₆₀-NH₂ at concentrations as low as 10 mg/L inhibited growth and reduced substrate uptake for both microorganisms. Scanning electron microscopy (SEM) revealed damage to cellular structures. Neutrally charged C₆₀ and C₆₀-OH had mild negative effects on S. oneidensis MR-1, whereas the negatively charged C₆₀-COOH did not affect either microorganism's growth. The effect of fullerene compounds on global metabolism was further investigated using [3- ¹³C]L-iactate isotopic labeling, which tracks perturbations to metabolic reaction rates in bacteria by examining the change in the isotopic labeling pattern in the resulting metabolites (often amino acids).^1-3 The ¹³C isotopomer analysis from all fullerene-exposed cultures revealed no significant differences in isotopomer distributions from unstressed cells. This result indicates that microbial central metabolism is robust to environmental stress inflicted by fullerene nanoparticles. In addition, although C₆₀-NH₂ compounds caused mechanical stress on the cell wall or membrane, both S. oneidensis MR-1 and E. coli W3110 can efficiently alleviate such stress by cell aggregation and precipitation of the toxic nanoparticles. The results presented here favor the hypothesis that fullerenes cause more membrane stress^4-6 than perturbation to energy metabolism.⁷