(253g) 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 2.3CFU/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.