Doubling times and minicell production by ΔminC E Doubling times and minicell production by ΔminC E. coli with streptomycin. Doubling times and minicell production by ΔminC E. coli with streptomycin. Doubling times were obtained by tracking single cells from birth to division into two proliferating cells from the phase-contrast images. The production of minicells was not regarded as division since minicells lack chromosomal DNA and cannot grow and divide. (A) Average doubling times of ΔminC strain and wild-type ΔmalT control measured by microscopy on agar pads at 0 and 6 µg streptomycin ml−1. The doubling times of the wild type increased significantly in going from the lower to the higher concentration (unpaired t test; n = 144 and 163; P  < 0.001). The doubling times of the ΔminC bacteria were not significantly different at the two streptomycin levels (unpaired t test; n = 186 and 185; P  = 0.79). (B) Phase-contrast microscopy image of a growing colony of ΔminC E. coli on an agar pad with 6 µg streptomycin ml−1 to illustrate the presence of minicells. (C) Numbers of whole cells and minicells in ΔminC colonies on agar pads with 0 and 6 µg streptomycin ml−1. The frequency (or percentage) of minicells at 6 µg streptomycin ml−1 was significantly greater than at 0 µg by a randomization test for differences (P = 0.0013; StatKey statistical package). Error bars show standard errors of the means. *, **, and *** denote P values less than 0.05, 0.01, and 0.001, respectively. n.s., not significant. Camilla U. Rang et al. mSphere 2018; doi:10.1128/mSphere.00428-18