A different type of tolerance: selection for and phenotypic characteristics of antibiotic tolerance evaluated at the single cell level under recurring antibiotic exposure in Escherichia coli.

Despite increasing interest, non-heritable, phenotypic drug resistance, such as tolerance and persistence towards antibiotics, remains less characterized compared to genetic resistance. Non-heritable drug resistance challenges antibiotic treatment and have implications towards heritable resistance evolution. Phenotypically resistant cells have commonly been characterized as growth arrested cells prior and during antibiotic application that quickly resume growth post-application. Here, we add novel combinations of characteristics of phenotypic resistant E. coli single cells--that are of particular interest towards genetically fixed resistance--, and contrast those to susceptible cells of the isoclonal initial population by exposure to different levels of recurrent antibiotic. We find that phenotypic resistant cells reduced their growth rate by about 50% compared to growth rates prior to antibiotic exposure, but cells do not go into near growth arrest. The growth reduction is induced by antibiotic exposure and not caused by a stochastic switch or predetermined state as frequently described. Cells exhibiting constant intermediate growth survived best under antibiotic exposure and, contrary to expectations, selection did not primarily act on fast growing cells. Our findings support diverse modes of phenotypic resistance, and we revealed resistant cell characteristics that supports acclaims of an underappreciated role of phenotypic resistant cells towards resistance evolution.