We characterized how cells grow and form micro-colonies on LB agar plates containing different concentrations of antibiotics using time-lapse microscopy. At time zero, we transferred cells growing in antibiotic-free LB liquid medium to a LB agar plate, and confirmed that individual cells were spread out and isolated. Setting up a time-lapse imaging experiment after the transfer took 1 ~ 2 hr, which is why there is a gap in the data immediately after time zero (dashed lines). The experiments were independently repeated twice (biological repeats) and the data from one experiment are shown here. (A, B) We counted the number of growing cells in each micro-colony (represented by a different color). In the absence of antibiotics (panel A) or with a sub-MIC level of a bacteriostatic drug (panel B, 0.7 × MIC of chloramphenicol), the number increased similarly for different colonies, revealing homogeneous population growth. Example image sequences are shown in Videos 1 and 2. The data are replotted on a semi-log scale (insets). Such homogeneous population growth was observed for other bacteriostatic drugs (thiolutin and tetracycline) as well; see Figure 2—figure supplement 1. When we repeated the experiment using a microfluidic device, we again observed the same homogeneous population dynamics (Figure 2—figure supplement 2A). (C) In contrast, the population dynamics of bacteria exposed to a bactericidal drug were highly heterogeneous. An example image sequence was shown in Video 3. The number of growing cells within micro-colonies at 0.8 × MIC of cefsulodin is plotted here; the result shows stochastic population fluctuations. Such population fluctuations were again observed when experiments were repeated using other bactericidal drugs (ofloxacin, kanamycin, and 6-APA); see Figure 2—figure supplement 3A–C. The light red-shaded region indicates the number equal to zero (i.e., population extinction). When we repeated these experiments using a microfluidic device, we again observed population fluctuations (Figure 2—figure supplement 2B–D). (D) Our model predicts that the probability of population extinction increases linearly with death rate, with the slope being 1 / λ (the solid line). We experimentally characterized the extinction probability (Figure 2—figure supplement 4), and the death rate (Figure 2—figure supplements 5–6), at different concentrations of bactericidal drugs. Using these data, we obtained the relationship between the extinction probability and the death rate, and plotted it here (green circles: cefsulodin, pink squares: ofloxacin, and grey triangles: 6-APA). We found good agreement between the model prediction and experimental data. Note that at increasing drug concentrations, λ changed little (Figure 2—figure supplement 6B), and thus was taken as a constant in the analysis here.