The E. coli escape wave in response to external Zn²⁺ is zinc reserve-dependent

The chemotaxis response of E. coli to metal cations is less understood than their response to organic molecules. Using dark-field videomicroscopy, E. coli behavior was analyzed in a 17 mm-long microfluidic channel exposed to a Zn(NO₃)₂ chemorepellent gradient, generated by a 250 mM solution placed in a well at the channel extremity, with or without prior Zn²⁺ pre-exposure of the cultures (10 µM). The bacteria exhibited an escape wave away from the zinc source. Compared to unexposed cultures, zinc pre-exposure resulted in a constant and shorter passage time at a given position of the wave peak, despite unchanged growth and swimming speed. The time lag decreased with growth duration. Given the one-dimensional gradient setup, this decrease is associated to a reduced diffusion duration from the Zn²⁺ source. The content of Zn²⁺ in the extracellular medium at the peak of the wave is therefore lower, but allows bacteria to escape more rapidly. These findings suggested an increase in bacterial Zn²⁺ sensitivity. By analogy to Ni²⁺ binding to the cytoplasmic HAMP domain of the Tar receptor, Zn²⁺ likely triggers a chemorepellent response through a cytoplasmic receptor. The activation of this receptor relies on the available zinc pool, which is specifically buffered by substantial other intracellular zinc reservoirs. In this model, saturating the reservoirs in pre-exposed cultures would enable the fastest response time, and a gradual filling of the reservoirs in unexposed cells would reduce a delay in chemotactic escape.