Figure 2.

Exposure to metallic copper (Cu) surfaces does not promote mutations but causes membrane damage. Cells of Staphylococcus haemolyticus (10¹⁰ cells per sample) were exposed for 5 sec to Cu, stainless steel, or 0.25% (wt/vol) of the mutagen formaldehyde (CH₂O) + stainless steel surfaces. Cells were washed from surfaces, concentrated, and spread on solid media containing 80 μg × ml⁻¹D-cycloserine. D-cycloserine is bacteriostatic and colonies arise from inactivating mutations in the gene of the D-cycloserine uptake-permease AapA (A). Cells were exposed to metal surfaces for 0 or 7 min, removed, washed, subjected to Live/dead staining, and observed by fluorescence microscopy (B). Live bacteria with undamaged membranes fluoresce green, cells with damaged membranes fluoresce red. Shown are averages of triplicate experiments with standard deviations (error bars, A) or representative micrographs from three independent experiments with similar results (B). The asterisk denotes significantly (P ≤ 0.05, t -test) different values in the mutagen formaldehyde-treated controls.