Appl Microbiol Biotechnol (2011) 90:1681–1690 Brynildsen and Liao (2009) reported that 8 g/L (1%) of isobutanol caused a growth arrest in E. coli, but even a lower concentration of solvent (0.8%, v/v, or 6.4 g/L of n- butanol) caused a 50% growth decrease, disruption in respiratory efficiency, increased permeability of the cell wall, and oxidative stress response on E. coli (Rutherford et al. 2010). In this work, cell growth of JCL260 and JCL16 stopped around 10 h (when isobutanol concentra- tion in the broth was roughly 7 g/L) and cells did not grow even after feeding. Such a phenomenon might be attributed to solvent toxicity because isobutanol concen- tration at 10 h was similar to those tested by Brynildsen and Liao (2009) and Atsumi et al. (2010b). Even though the present work is not a complete optimi- zation study, the results show that a very simple strategy (gas stripping) significantly improved the effective titer of iso- butanol. 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