FIG. 4.

Exposure to metallic copper surfaces does not lead to increased mutations in E. coli. A total of 10⁸ E. coli cells were exposed for 5 s to copper surfaces, stainless steel surfaces, or surfaces containing 0.25% (wt/vol) of the mutagen formaldehyde (CH₂O) plus stainless steel, removed, concentrated, and spread on solid medium containing 20 μg·ml⁻¹ of the bacteriostatic compound d-cycloserine. After 24 h of incubation at 37°C, colonies were counted as originating from mutation events leading to resistance via inactivation of CycA, a d-cycloserine uptake permease. Shown are averages from triplicate experiments, with standard deviations (error bars). The asterisk denotes significantly different values (P ≤ 0.05) for formaldehyde-challenged cells.