The viable cells on MM+P plates in with and without Ampicillin (100 μg/ml). Ampicillin was added onto the plates once a day by threading 1 ml Ampicillin solution (1000 μg/ml) around the whole plate surface. Each time, 3–5 micro-drops, which with no revertant papillae, were dug out and immerged into the tube with 1 milliliter 0.85% NaCl solution. After half an hour, vortex the tubes and diluted the suspension. The viable cells were counted by plating the appropriate aliquots onto LB plates after incubating at 37°C for 24 hours. Solid diamonds, viable cells in micro-drops on MM+P plate in the absence of Amp. Open circles, viable cells in micro-drops on MM+P plate in the presence of Amp. Solid triangles, viable cells in a whole MM+P plate in the absence of Amp. Open squares, viable cells in a whole MM+P plate in the presence of Amp.