Single-cell phenotypic characteristics of tolerance under recurring antibiotic exposure in Escherichia coli

Non-heritable drug resistance, such as tolerance and persistence towards
antibiotics, is little characterized compared to genetic resistance.
Tolerance and persistence allow cells to survive application of
antibiotics that are bactericidal to non-tolerant cells. Non-heritable
drug resistance challenges antibiotic treatment, particularly of recurrent
infections, and have implications towards heritable resistance evolution.
Tolerant cells have commonly been characterized as growth arrested cells
prior and during antibiotic application that quickly resume growth
post-application. Here, we explore characteristic of tolerant and
susceptible E. coli single bacteria cells to different levels of recurrent
antibiotic exposure and quantify their occurrence. In using a high
throughput single-cell microfluidic device, we find that tolerant cells
reduce their growth rate by about 50%, but contrary to previous findings
do not go into growth arrest or near growth arrest. The growth reduction
is induced by antibiotic exposure and not caused by a stochastic switch or
predetermined state as previously described. Cells exhibiting constant
intermediate growth survived best under antibiotic exposure and selection
did not primarily act on fast growing cells, as expected for a β-lactam
antibiotic. Control experiments on population cultures confirmed and
challenged scaling of single cell findings to population level processes.
Our findings suggest a prevalent type of tolerance that differs from
previously described tolerance and persister characteristics. Our
described characteristics and its high frequency of occurrence supports
acclaims of an underappreciated role of tolerant cells towards resistance
evolution.