Modification of Fatty Acids in Membranes of Bacteria: Implication for an Adaptive Mechanism to the Toxicity of Carbon Nanotubes

We explored whether bacteria could respond adaptively to the presence of carbon nanotubes (CNTs) by investigating the influence of CNTs on the viability, composition of fatty acids, and cytoplasmic membrane fluidity of bacteria in aqueous medium for 24 h exposure. The CNTs included long single-walled carbon nanotubes (L-SWCNTs), short single-walled carbon nanotubes (S-SWCNTs), short carboxyl single-walled carbon nanotubes (S-SWCNT-COOH), and aligned multiwalled carbon nanotubes (A-MWCNTs). The bacteria included three common model bacteria, <i>Staphyloccocus aureus</i> (Gram-positive), <i>Bacillus subtilis</i> (Gram-positive), and <i>Escherichia coli</i> (Gram-negative), and one polybrominated diphenyl ether degrading strain, <i>Ochrobactrum</i> sp. (Gram-negative). Generally, L-SWCNTs were the most toxic to bacteria, whereas S-SWCNT-COOH showed the mildest bacterial toxicity. <i>Ochrobactrum</i> sp. was more susceptible to the toxic effect of CNTs than <i>E. coli</i>. Compared to the control in the absence of CNTs, the viability of <i>Ochrobactrum</i> sp. decreased from 71.6−81.4% to 41.8–70.2%, and <i>E. coli</i> from 93.7−104.0% to 67.7–91.0% when CNT concentration increased from 10 to 50 mg L^–1. The cytoplasmic membrane fluidity of bacteria increased with CNT concentration, and a significant negative correlation existed between the bacterial viabilities and membrane fluidity for <i>E. coli</i> and <i>Ochrobactrum</i> sp. (<i>p</i> < 0.05), indicating that the increase in membrane fluidity induced by CNTs was an important factor causing the inactivation of bacteria. In the presence of CNTs, <i>E. coli</i> and <i>Ochrobactrum</i> sp. showed elevation in the level of saturated fatty acids accompanied with reduction in unsaturated fatty acids, compensating for the fluidizing effect of CNTs. This demonstrated that bacteria could modify their composition of fatty acids to adapt to the toxicity of CNTs. In contrast, <i>S. aureus</i> and <i>B. subtilis</i> exposed to CNTs increased the proportion of branched-chain fatty acids and decreased the level of straight-chain fatty acids, which was also favorable to counteract the toxic effect of CNTs. This study suggests that the bacterial tolerances to CNTs are associated with both the adaptive modification of fatty acids in the membrane and the physicochemical properties of CNTs. This is the first report about the physiologically adaptive response of bacteria to CNTs, and may help to further understand the ecotoxicological effects of CNTs