Visual observation of resistance to doxycycline. SAGE plates were incubated at 37 °C for seven days. Shown here is one representative lane, containing up to 25x the initial MIC of doxycycline at a slope of 12.5 mm. Nodules of resistant cells observed on day 2 are marked by arrows. With high concentrations of doxycycline, many of these mutations conferred only a small increase in MIC, leading to small populations of moderately resistant bacteria. Large bacterial populations did not reform until day 5.