1967 sale effect of high electric fields on microorganisms_killing bacteria and yeasts
BIOCHI1VflCA ET BIOPHYSICA ACTA 781
BBA 25 876
EFFECTS OF HIGH ELECTRIC FIELDS ON MICROORGANISMS
I. KILLING OF BACTERIA AND YEASTS
A. J. H. SALE AND W. A. HAMILTON*
Unilever Research Laboratory, ColworthHouse, Sharnbrook, Bedford (GreatBritain)
(Received July 7th, 1967)
A lethal effect of high electric fields on a number of species of vegetative bacteria
and yeasts has been demonstrated. Fields up to 25 kV/cm have been applied as a
series of direct current pulses to suspensions of the organisms. Death of the organisms
was not due to tile products of electrolysis; the temperature rise of the suspension
was small and did not cause the lethal effect. The degree of kill of a population was
determined by the product of the pulse length and number of pulses, and by tile
field strength in the suspension. The various species differed in their sensitivity to
the electric field, the yeasts being more sensitive than the vegetative bacteria.
Most of the investigations into tile bactericidal action of electric fields have
been carried out at radio frequencies. In I949 BUI~TON~ reviewed the literature, in
whiclh there are accounts both for and against the existence of bactericidal effects.
There is some uncertainty concerning the heating effects, but it seems that a non-
thermal bactericidal effect might occur if high enough fields are applied. Amongst
the more recent literature the highest field strength appears to be 2 kV/cm reported
by INGRAMAND PAGE 2, who found no lethal effect.
Few reports of the effects of direct current (d.c.) pulses have appeared.
GOSSLING3 claimed that, if the power density is high enough, micro-organisms may
have mutations induced and may be destroyed. FEDOROVAND ROGOV4 reported a
bactericidal effect in milk, attributing the effect to impact and cavitation. Inactivation
of micro-organisms is also reported by DOEVENSPECK5.
We have extended these studies to high field strengths using d.c. pulses up to
25 kV/cm to try to find out whether a non-thermal bactericidal effect exists. In
particular we have attempted to establish which of a number of parameters controlled
the lethal effect on micro-organisms.
If a bactericidal effect due specifically to very high electric fields is to be demon-
strated, the fields must be applied in very short pulses with long intervals between
pulses, to minimise the temperature rise.
* Present address: Department of Biological Chemistry, University of Aberdeen, Scotland.
Biochim. Biophys. Acta, 148 (1967) 781-788
782 A.J. lifo SALE; W. A. ;frA2vflLTON
METHODS AND MATERIALS
A pulse generator was built to provide approximately rectangular d.c. pulses
into a resistive toad; the design followed conventional practice with no notable novel
features. The pulse voltage was continuously adjustable up to io kV and the pulse
length could be varied from 2 to 20 ~sec in steps of 2/zsec. The pulse repetition rate
was one per second and the number of pulses per treatment was varied as required.
The pulses were monitored by an oscilioscope and could be photographed. Fig. z
illustrates the shapes of the voltage pulse applied to the treatment cell and the current
pulse passing through it.
The treatment cell, which forms the load, is illustrated in Fig. 2. The sample
was bounded by the carbon electrodes, the poiythene spacer and air, so that apart
from the meniscus and any crevices, the whole of the sample received uniform treat-
ment. By using a variety of spacers the electrode area and thickness of sample could
be varied, and hence the electrical resistance and electric field. The field strength
was limited to less than 3o kV/cm by the electrical breakdown of the air above the
sample. Provision was made for temperature control by the circulation of water
through the brass blocks that support the carbon electrodes. The water was at room
temperature (approx. 2o°) unless otherwise stated.
Fig. I. Voltage and current pulse shapes 20 #sec).
Biochim. Biophys. Acta, 148 ~i967) 781-788
784 A.j.H. SALE, W. A. HA2VflLTON
Lethal effect of d.c. pulses
In preliminary experiments d.c. pulses -wereapplied to suspensions of the various
vegetative bacteria and yeasts listed under organisms. There was a tethai effect when
the pulse amplitude was high enough. E. coli was chosen as the test organism, for
a study of the parameters likely to influence the kill. The sensitivity of the cel!s to
d.c. pulses was not influenced by the stage of growth at which the ceils were harvested,
by the presence or absence of oxygen during growth, nor by varying the pH of the
suspending medium from 4 to 9- Therefore approximately neutra! NaC1 solutions were
used as the suspending media for the study of the effects of the electrical conditions
on the degree of kill, which was measured in terms of percentage survival of the
Insig~cifica~ce of electrolysis
Electrolysis occurs under d.c. conditions and this was visible as a burst of gas
generated at the electrode surface by each pulse; simultaneously a marked disturbance
of the liquid could be seen. The possibility that the products of electrolysis might be
causing the lethal effect had to be examined. If the pulse treatment was carried out
with the organisms suspended in a gel, one would expect that the gases produced
by electrolysis would only contact the gel surfaces adjacent to the electrodes and
not penetrate the gel. So examination of the organisms in the gel should reveal
whether electrolysis was responsible for the lethal effect. First it was necessary to
show that the products of electroiysis did not penetrate the gel. A molten nutrient
agar containing Io % rezazurin was put into the treatment cell and allowed to set.
After a number of pulses the gel was removed. Only the surface next to the cathode
had become pink, so the nascent hydrogen had not diffused into the agar. Next a
suspension of E. coZiin o.1% peptone water was mixed with molten agar at 48 ° and
put in the treatment cell and allowed to set. After a number of pulses the gel was
removed and thin slices cut so that. viewing across the agar. one edge had been
adjacent to the cathode and the other to the anode. The slices were placed on a
microscope slide and under a cover slip, and incubated in moist conditions at 37 °
for 4 h. The agar was then examined for microcolonies and compared with similar
microcultures of untreated suspensions Live organisms would give rise to micro-
colonies whereas dead ones would remain as single organisms. There were many
colonies throughout the untreated suspension. In the treated preparation there were
few colonies but many single orgamsms, which could be seen uniformly distributed
throughout the agar. Therefore the kill occurred even where the products of elec-
trolysis were absent.
Insig~ifiea~ee of heating
In many experiments it would have been possible for the amount of energy
supplied in pulses to the suspensions in the treatment cell to have markedly heated
the liquid if there had not been any cooling by the thermal capacity of the cell and
the cooling water. It was not practicable to have a thermoeouple present during the
treatment and when one was introduced into the suspension afterwards no significant
rise was registered.
l?iochim, Biophys. AclG 148 (i967) 781-788
KILLING OF MICROORGANISMS BY ELECTRIC FIELDS 785
However, it was possible to obtain the temperature rise of the suspension itself
during an actual treatment from the change of its electrical resistance. The suspending
medium was an NaCl solution, of which the resistance changes by 2.4 % per degree.
The resistance change was found from the amplitudes of the voltage and current
pulses of the treatment. Although the accuracy was limited, a temperature change
of 5° was readily detected. The maximum temperature rise found in this way was IO°,
which appears among the examples of a number of treatments of IO pulses of 2o/,see
quoted in Table I.
In the table the energy input for each treatment has also been calculated. The
energies expressed in cal/cm~ would be numerically equal to the temperature rises
in degrees that could have occurred if there had not been any cooling, but the actual
temperature rises were found to be much less. The energy inputs for very many con-
ditions of voltage, current, conductivity, number of pulses and pulse length have
been gathered together in Fig. 3, where the percent survival has been plotted against
the energy input. There was no correlation between the energy and the degree of kill.
Effect of time of treatment
When the number of pulses was increased, while keeping the other conditions
constant, the degree of kill increased rapidly at first and then more slowly. The effect
of altering the pulse length was that when the length was shortened more pulses
were required to achieve the same degree of kill. If these two effects were combined,
it was found that the degree of kill appeared related to the product of the number
of pulses and the pulse length, that is, to the total time for which the voltage is
actually applied to the cell. This is illustrated by the example in Fig. 4, which shows
EFFECT OF VARYING THE ELECTRIC FIELD AND CONDUCTIVITY OF THE 1VIEDIU1ViON CURRENT DENSITY,
ENERGY, TEMPERATURE RISE AND DEGREE OF KILL OF E. coli
Treatment consisted of I0 pulses of 20 #sec at room temp. (approx. 20°).
Conductivity Electric field Current density Energy Temperature Survivors
(mr2-1 ) (k V / cm ) (A /cm ~) (cal/cm3) rise (% )
0.8 lO. 5 8 4 <5 21
0.8 21 17 17 <5 <i
1.6 lO. 5 17 8 o 4°
1.6 21 34 34 5 i
3.2 lO.5 34 17 <5 22
1.6 5.3 8 2 o 94
3 .2 4.9 16 4 o ioo
3 .2 18.5 59 52 io <I
6.4 4-9 31 7 <5 ioo
6.4 9.5 61 27 >5 < IO 44
0.9 18. 5 18 I5 <5 2
1.2 14. 5 17 12 <5 4
1.6 11. 5 18 io <5 17
1.7 11.4 19 Ii <5 25
2.0 io 20 9 <5 5 °
2.6 7-5 19 7 o 80
2.6 15 39 28 5 3
2.6 19.5 51 47 >5 <io 2
3 .2 14.5 46 32 8
Biochim. Biophys. Acta, 148 (1967) 781-788
KILLING OF MICROORGANISMS BY ELECTRIC FIELDS 787
shown by crosses in the same figure are the results of experiments carried out with
the water circulated through the treatment cell at 4°° , so that the temperature of
the suspension was 4o0 instead of 2o° at the start of the treatment; the relationship
between the degree of kill and electric field was not significantly altered.
S ~8 25
Fig. 7. Relationship between degree of kill and electric field for various organisms, io pulses of
20/~sec. S.C., Saccharomyces cerevisiae; C.U., Candida utilis; E.C., Escherichia coli; M.P., motile
pseudomonad; C.W., Clostridium welchii ; M.L., Micrococcus lysodeikticus.
Sensitivity of the various @cries
A few organisms, other than E. coli, were also treated with IO pulses of 2o ~sec
at 20 ° to find out how the degree of kill was related to electric field. The species
were M. lysodeikticus, C. welchii, B. megaterium, S. cerevisiae, C. utilis and the motile
pseudomonad, and tile results are shown in Fig. 7. In all cases the relationships were
similar but the species differed in their sensitivity to the electric field.
In the gel experiment the organisms were killed whether they were in contact
with the products of electrolysis or not, so the products did not cause the kill. Further
evidence is provided by the kill being independent of current density, because the
rate of generation of the products is proportional to current density.
There is evidence that the cooling of the suspension was remarkably effective;
although the energy inputs were as high as 5° cal/cm3 (which would raise the tempera-
ture by 5o° in the absence of cooling), the greatest temperature rise that was recorded
was IO°. This rapid cooling, which occurred in the IO see of the IO pulse treatments,
may be accounted for, not by conduction, but by intense mixing caused by the bursts
of gas generated at the electrodes by each pulse. The small actual temperature rises
were insufficient for temperatures lethal to E. coli to be reached, and the lack of
correlation between the degree of kill and energy input is evidence for the non-thermal
nature of the kill. Further evidence is provided by the lack of any increase of kill
when the starting temperature of treatments was raised from 2o ° to 4o°, and also
provided by the shape of the survivors vs. total time curve (Fig. 3)- If the kill had
been thermal the survival would have been expected to remain at IOO% until enough
Biochim. Biophys. dcta, 148 (1967) 781-788
788 A. ~. H. SALE, ~,V A. HAMILTON
time had elapsed for the suspension to have reached a lethai temperature and ther.
fallen rapidly; but the survival fell immediately and rapidly before leveliing off.
The evidence therefore suggests that the kill was non-thermal, in that it was
not due to heating of the suspension as a whole. The data of Figs. 6 and 7 show that,
for a given time of treatment, the kill was determined by the electric field in the
suspension. This suggests that when an electrical potential in or around the organism
reached a sufficient magnitude, irreversible damage was done. The nature of the
damage to the organism will be discussed in the subsequent paper.
We wish to thank Mrs, C. A. FOULCER, L. STUTTARD and W. L. Km~~ for their
I H. BURTOn-,NaEional Inslitute for Resec~ch in Dairying paper no. zo4z Reading (I949).
2 M. INGRAMANDL. J. PAGE,Proc. Soc. Appl. Bact., 16 (1953) 69.
3 B. S. GOSSLING,Brit. Pat. 845 743-
4 N. E. FEDOROVAND I. A. ROGOV,via Dairy Sci. Abstr., 23 (1963) 312.
5 DOEVENSPECX,Fleischwirtschctfl, 13 (196I) 986.
Biochim. Biophys. Acta, I48 (1967) 781-788