Adaptive resistance and growth of Escherichia coli bacteria in an antibiotic gradient

Using a microfluidic device we show that high-density motile E. coli populations composed of non-resistant bacteria can invade environments where a lethal concentration of the antibiotic kanamycin is present. The invading bacteria establish an adaptively resistant population which remains viable for over 24 hours while exposed to the antibiotic. Invading cells are not dormant but show both growth and swimming motility in the presence of the antibiotic. Our results highlight the importance of motility and population density in facilitating adaptive resistance, and indicate that adaptive resistance may be a first step to the acquisition of genetically encoded resistance in landscapes of antibiotic gradients.