a, Ultrasound images of arg1-expressing E. coli at various cellular concentrations, before and after acoustic collapse. b, Mean ultrasound contrast from E. coli expressing arg1 and GFP at various cell densities. Data are from three biological replicates, lines indicate the mean. AU, arbitrary units. c, Ultrasound images of E. coli expressing arg1 after induction with various concentrations of IPTG. Cell concentration was 5 × 108 cells ml−1. d, Normalized ultrasound contrast as a function of IPTG concentration. Data are from three biological replicates, line shows a fit of the data with the Hill equation to facilitate visualization. Each imaging experiment was repeated three times with similar results. Scale bars, 2 mm.