Fig 2. tetA counterselection with NiCl₂.

Wildtype MG1655 (black circles, solid line) and MG1655 rrnB::tetA (red triangles, dashed line) were grown in RDM + 0.5% v/v glycerol with concentrations of NiCl₂ from 0–10 mM, and the effect of NiCl₂ quantified by measuring the growth rate (doublings per hour) in exponential growth. Points are the average of three measurements, and error bars are the SD. The growth rate for both strains at high concentrations of NiCl₂ where no observable growth was detected is set to zero. Because of the large differential in growth rate, counterselection against the tetA landing pad can be effectively performed at 5–7 mM NiCl₂ in K-12 MG1655.