Single-cell imaging and characterization of Escherichia coli persister cells to ofloxacin in exponential cultures.

Bacterial persistence refers to the capacity of small subpopulations within clonal populations to tolerate antibiotics. Persisters are thought to originate from dormant cells in which antibiotic targets are less active and cannot be corrupted. Here, we report that in exponentially growing cultures, ofloxacin persisters originate from metabolically active cells: These cells are dividing before the addition of ofloxacin and do endure DNA damages during the treatment, similar to their nonpersister siblings. We observed that growth rate, DNA content, and SOS induction vary among persisters, as in the bulk of the population and therefore do not constitute predictive markers for persistence. Persister cells typically form long polynucleoid filaments and reach maximum SOS induction after removal of ofloxacin. Eventually, cell division resumes, giving rise to a new population. Our findings highlight the heterogeneity of persister cells and therefore the need to analyze these low-frequency phenotypic variants on a case-by-case basis at the single-cell level.