Inactivation of Persister Cells by Low-Level Direct Currents

Recent research has shown that a small percentage of cells in bacterial cultures are non-growing persister cells. Although the mechanism of persister formation remains elusive, it is well accepted that the persister cells are not mutants with growth defects, but rather phenotypic variants of the wild-type strain. The dormant nature of persister cells allows them to escape the killing by antibiotics, presenting a great challenge to curing chronic infections. In this presentation, we report for the first time that the persister cells can be effectively inactivated by low-level direct currents (DCs). After treatment with 75 µA/cm² DC using graphite electrodes for 60 minutes, the viable persister cells of Escherichia coli HM22 in 0.85% NaCl buffer decreased from 5.7×10⁵ to 9.4×10² CFU/mL. In contrast, incubation for 60 minutes in 0.85% NaCl pretreated with the same level and duration of DC only reduced the viable cells from 1.2×10⁶ to 1.2×10⁵ CFU/mL. In addition to inactivation by applying DC alone, synergistic effects were observed when treating persister cells with DC and antibiotic together. As expected, treatment with 100 µg/mL ampicillin (Amp), 20 µg/mL cinoxacin (Cn) or 20 µg/mL tobramycin (Tb) alone only caused slight reduction in the number of viable persister cells. In the presence of 75 µA/cm² DC, however, the viable persister cells were reduced from 5.2×10⁵ to 1.7×10⁴ CFU/mL by 100 µg/mL Amp, from 1.1×10⁶ to 2.3 CFU/mL by 20 µg/mL Cn, and from 1.0×10⁶ to 23 CFU/mL by 20 µg/mL Tb. These data indicate that the efficacy of Amp, Cn and Tb increased approximately by 30, 70000 and 12000 times respectively by 75 µA/cm² DC, compared to the treatments without DC. The possible mechanism of these effects will be discussed. These results are helpful for understanding the mechanism of persister formation and for developing novel control methods.