Study type: Medical/biological study (experimental study)

Pulsed electric fields cause bacterial envelopes permeabilization depending on the treatment intensity, the treatment medium pH and the microorganism investigated. med./bio.

Published in: Int J Food Microbiol 2007; 113 (2): 219-227

Aim of study (acc. to author)

This study was performed to investigate the effects of pulsed electric fields on the membrane permeabilization in two gram-positive and two gram-negative bacterial strains to obtain an understanding of the mechanism of bacterial inactivation by pulsed electric fields.

Background/further details

The experiments were conducted with different electric field strengths, exposure times, and medium pH values (pH 4.0 and 7.0). Additionally, both gram-positive and gram-negative bacteria were investigated to discover possible differences in their reactions to pulsed electric field exposure due to their different cell membrane properties.
The uptake of the fluorescent dye propidium iodide into the cell was measured as it is a sensitive marker of cell membrane integrity. The occurence of this dye in the cell indicates loss of membrane integrity. Propidium iodide uptake was measured before and after pulsed electric field exposure to characterize whether the membrane permeabilization was reversible or not.

Endpoint

Exposure

Exposure Parameters
Exposure 1: 1 Hz
Modulation type: pulsed
Exposure duration: not given in this article
Exposure 2: 2 Hz
Modulation type: pulsed
Exposure duration: not given in this article
Exposure 3:
Modulation type: single pulse
Exposure duration: single pulse of 50 µs
Exposure 4:
Modulation type: single pulse
Exposure duration: single pulses of 12,5 - 100 µs
Exposure 5:
Modulation type: single pulse
Exposure duration: single pulses of 10 - 400 µs

General information

cells were stained with PI a) before exposure b) after exposure

Exposure 1

Main characteristics
Frequency 1 Hz
Type
Waveform
Exposure duration not given in this article
Modulation
Modulation type pulsed
Pulse width 2 µs
Repetition frequency 1 Hz
Exposure setup
Exposure source
  • discharging of 2 µF capacitor
Setup cylindrical plastic tube with two polished stainless steel electrodes - 0.25 cm apart and with an area of 2.01 cm² - was used as chamber
Parameters
Measurand Value Type Method Mass Remarks
electric field strength 25 kV/cm - measured - -

Exposure 2

Main characteristics
Frequency 2 Hz
Type
Waveform
Exposure duration not given in this article
Modulation
Modulation type pulsed
Pulse width 2 µs
Repetition frequency 2 Hz
Exposure setup
Exposure source
Parameters
Measurand Value Type Method Mass Remarks
electric field strength 12 kV/cm - measured - -
electric field strength 19 kV/cm - measured - -

Exposure 3

Main characteristics
Frequency
Type
Waveform
Exposure duration single pulse of 50 µs
Modulation
Modulation type single pulse
Pulse width 50 µs
Exposure setup
Exposure source
Parameters
Measurand Value Type Method Mass Remarks
electric field strength 10 kV/cm minimum measured - -
electric field strength 25 kV/cm maximum measured - -

Exposure 4

Main characteristics
Frequency
Type
Waveform
Exposure duration single pulses of 12,5 - 100 µs
Modulation
Modulation type single pulse
Pulse width 12.5 µs
Additional info

pulse width: 12.5 - 100 µs

Exposure setup
Exposure source
Parameters
Measurand Value Type Method Mass Remarks
electric field strength 19 kV/cm - measured - -

Exposure 5

Main characteristics
Frequency
Type
Waveform
Exposure duration single pulses of 10 - 400 µs
Modulation
Modulation type single pulse
Pulse width 10 µs
Additional info

pulse width: 10 - 400 µs

Exposure setup
Exposure source
Parameters
Measurand Value Type Method Mass Remarks
electric field strength 25 kV/cm maximum measured - -

Reference articles

Exposed system:

Methods Endpoint/measurement parameters/methodology

Investigated system:
Time of investigation:
  • after exposure

Main outcome of study (acc. to author)

Investiagtions at pH 7.0 showed positive correlation between degree of membrane permeabilization, exposure time, and electric field strength applied in all bacterial strains. In gram-negative bacteria all permeabilized cells died (both reversible and permanent permeabilized cells). In gram-positive bacteria only cells with permanent membrane permeabilization died. The membrane permeabilization was partially reversible in all bacterial strains.
Investigations at pH 4.0 revealed a relationship between loss of cell viability with permanent loss in cell membrane integrity in Listeria monocytogenes, but in Lactobacillus plantarum the degree of permeabilization was higher, and in the gram-negative bacterial strains the permeabilization much lower than the percentage of inactivated cells. The permeabilization in all bacterial strains was permanent.
These results confirm that membrane permeabilization is involved in the mechanism of bacterial inactivation by pulsed electric fields, but the nature of membrane damage and its relationship to cell death depends on the bacterial species and the medium pH value.

Study character:

Study funded by

Related articles