Interferometeric sensing of β-galactosidase released by recombinant E. coli responding to an endocrine disruptor, tributyltin

A recombinant E. coli ACV1003 releasing β-galactosidase by a SOS regulon system when it is exposed to a DNA-damaging compound, has been used to detect endocrine disruptors such as tributyltin (TBT) and triphenyltin (TPT). Maximum response ratio by E. coli ACV 1003 (recA::lacZ) - indicating the maximum ratio of enzyme produced against an environmental toxicant to that produced in the absence of a toxicant - was estimated as 6.3 with 1.0 μg TBT ml^-1 at 37 °C, which was considerably higher than those with other strains. Extracellular β-galactosidase activity was 51 unit ml^-1, which was 5% of that obtained by the conventional Miller's enzyme assay using solvents. Such a low enzyme activity can be rapidly determined, not by the usual time-consuming and tedious enzyme assay, but by an alternative interferometric biosensor. Heavily-doped porous silicon to apply to an interferometer was fabricated by etching to produce a Fabry-Pérot fringe pattern, which caused the change in the refractive index of the medium including β-galactosidase. The change in the effective optical thickness versus β-galactosidase activity showed a sigmoid increase up to the concentration of 250 unit β-galactosidase ml^-1.