β -Galactosidase Leakage from Escherichia coli Points to Mechanical Damageas Likely Cause of Carbon Nanotube Toxicity

Abstract

We show that the cytotoxic effect of carbon nanotubes (CNTs) on bacteria is mediated by mechanical damage to the cell wall and membrane. Two ?-galactosidase-producing strains of Escherichia coli harboringgenomically integrated reporter gene constructs, namely pchbB:lacZand prpoS:lacZ, were used for the purpose. We first verified that CNTs result in an inhibition of cell growth. Enzyme activity was determined using a reporter gene assay in which CNTs were used without the lysis buffer (containing detergent). ?-galactosidase activity in the presence of CNTs alone measured several fold more than the controls used (without nanotubes). This suggests that CNTs damage the cell membrane in a manner similar to the detergent in the lysis buffer and render E. coli cell walls porous, causing cell contents including enzymes to leak out into the medium. Our results support the hypothesis that mechanical damage to bacterial cell mem- branes is the prevailing cause of CNT-cytotoxicity.