of High-Frequency Fields on Micro-Organisms
OF THE MANY VAGUE AND CONFLICTING REPORTS ON THE EFFECT OF SHORT
WAVES ON MICRO-ORGANISMS, THE LABORATORY ANALYSIS REPORTED IN
THIS ARTICLE WAS UNDERTAKEN; THE AUTHOR HAS ENDEAVORED TO
CORRELATE THE ELECTRICAL DATA WITH MEDICAL AND BACTERIOLOGICAL
DATA. BECAUSE OF TIME AND EQUIPMENT LIMITATIONS, THE STUDY SHOWS
TRENDS IN ELECTROBIOLOGICAL TREATMENT AT HIGH FREQUENCIES RATHER
THAN CONCLUSIVE EVIDENCE.
substance of a paper, " A Study of the Effect of
High-Frequency Fields on Micro-Organisms," which was
awarded the AIEE national prize for Branch paper for the
academic year ending June 30, 1942. The paper was presented at a
joint meeting on the Portland ( Oreg.) Section and the Oregon
State College Branch, Corballis, May 16, 1942.
Hugh Fleming was an enrolled student at Oregon State
College, Corvallis, when the paper was presented. He is now a
sutdent engineer, General Electric Company, Schenectady, N. Y.
The author acknowledges the assistance of the Oregon State
College department of bacteriology.
of high-frequency fields in producing artificial fevers has been
recognized in Europe for 30 years. Medical men in the United
States have used short-wave diathermy extensively for the last
ten years. The basic pattern of use involves the exposure to a
high-frequency field of an ailing portion of the body until the
local temperature has been increased considerably. Heat is
generated by the passage of eddy currents, hysteresis losses,
and dielectric losses. The temporary rise in temperature is
advantageous primarily because of the increased activity of
disease-fighting components of the blood stream. Undoubtedly
secondary effects exist, such as the outright killing of
bacterial bodies unable to survive the rapid change of
temperature and the rapid reversals of potential.
general methods of enveloping the body in the high-frequency
field are used. The most common of these is placing the
structure to be treated in the dielectric field existing between
two small applicator plates. The second method is to cause an
inductive circuit to conform to the necessary dimensions so that
it completely envelops a body part. Here the magnetic field
would play a large part, and hysteresis losses would seem to be
large. The Fisher Scientific Company has made extensive test on
the relative heating efficiency of the two methods and has found
the inductive method greatly superior.
the value of high-frequency treatment of humans was credited
only to the uniform and localized fever effects made possible.
More recently, a study of the effects of short waves upon
bacteria and other micro-organisms has shown that lethal effects
are evidenced without appreciable rise in temperature. Haase and
Schliephake reported that no comparison could be made between
the temperature of water passing high-frequency currents of
lethal value and the temperature of hot water required to
produce the same immobilizing action. This makes evident the
existence of additional effects of germicidal value.
currents between the limits of 9,000 and 100,000 kilocycles are
of recognized medical value. Generation is accomplished by means
of vacuum-tube oscillators much in the same manner as for radio
broadcast purposes. (See circuits in Figures 1 and 2.) In the
literature, there are limited reports of attempts to immobilize
micro-organisms in water media by the passage of high-frequency
currents. Most of the work has been done by European doctors of
medicine, with little presentation of what would be termed
be stated definitely that all forms of microscopic life can be
immobilized with short waves if treatment is severe enough.
Bacteria, in particular, are said to be immobilized rather than
killed because the only determination that can be made is a test
of their power of reproduction. Failing to reproduce, bacteria
are considered dead. If microscopic examination shows bacteria
broken into component parts, it can be assumed that they have
been destroyed. However, merely turning on enough power to
obtain a lethal effect is not economically sound, and it is
reasonable that there should be a search for optimum germicidal
Nearly all cell
life can be speeded up in its rate of growth under the stimulus
o high frequency. This action takes place at a progressively
faster rate as the power is increased, until a point of
overstimulation is reached and lethal effects occur. This point
of overstimulation differs widely depending upon the age,
virulence, and extent of colonization of the organism under
study. A curve illustrating the rate of growth as a function of
the power applied is given in Figure 3.
and Schliephake were able to immobilize both streptococci and
staphylococci with waves from 3 to 20 meters. Hicks was unable
to destroy these same bacteria with 4.5 meter waves. It is
interesting to note that he reported cultures contained salt,
which increased the conductivity of the water. Liebesny, using
20- to 30-meter waves, stated at the same time that bacteria
were killed in reciprocal proportion to the increase of
electrolyte introduced. From his experiments he claimed that the
effect on a certain electrolyte is optimum (using a rise in
temperature as the standard), when the dielectric properties
have a definite mathematical relationship to frequency. He wrote
an equation for this: K=fl (ex), where f
represents frequency, the dielectric constant, and x ,
and Hoensdorf found that streptococci and escherichia coli were
materially retarded in their growth in a moderate high-frequency
field if localized. They reported good results at 4.6 meters in
the presence of physiological salt or boric-acid solutions.
Fabian and Graham tested bacillus coli and found that 30 meters
with a current of 0.8 amphere produced death. They cautioned
against concluding immediately on the immobilizing of bacteria,
that they had been successfully killed as..."a seemingly
lethal effect may be followed in a few days by a rapid and
intensive growth." Tibor de Cholnoky indicates a
correlation of media and lethal dosage. He states that
water-borne bacteria are more difficult to immobilize than those
in some other media such as air.
theories have been proposed explaining the mechanics of
rendering a culture of micro-organisms nonviable. The first of
these is that the instantaneous temperature of the bacteria, or
other life, can be raised to a very high value without
necessarily affecting the surrounding media to a great extent.
The assumptions are that the hysteresis, eddy current and IR
losses are very great in the matter making up the cell. If this
theory were true exclusively, it would appear that the
effectiveness should be proportional to the frequency. As was
pointed out already, there is a definite frequency band of
optimum value above which the immobilizing effects diminish.
Figure 1 Schematic diagram of a vacum - tube oscillator
for 90-centimeter waves.
Figure 2 Schematic diagram of a vacuum-tube oscillator for 10-25
theory explaining the lethal action of short waves involves the
possibilities of mechanical stresses. Nearly all bacteria have a
negative charge. It is believed that a condition of mechanical
resonance is reached at certain frequencies that so distort the
organic matter of the micro-organism that it is rendered
non-viable. This would imply that the high-frequency field acts
powerfully enough upon a bacterium, for instance, to cause it to
travel a minute distance first in one direction and then in the
reverse direction as the field is changed. These oscillations of
the tiny bits of matter might well become so rapid as to exceed
the elastic limits of the structure of the organism. Since
different types of bacteria have entirely different physical
dimensions as well as properties, it follows that the frequency
of best destructive power would vary for each different type of
cell treated. This is true although the optimum frequency for a
given type of single cell life is not as critical as might be
supposed. Some microscopic evidence of actual bacterial
destruction due to distorted masses has been reported.
variables studied in the laboratory were: frequency, voltage,
current, time, and conductivity of the media containing the
micro-organisms. All were studied as a function of the
immobilizing power of the high-frequency currents.
variable-frequency tests, numerical concentrations of bacteria
were maintained as constant as was possible. An attempt was made
to keep power outputs constant for all tests by means of
flashlight lamps in parallel with a nonresonant line and the
loading network. Measurement of light intensity of the lamps was
made with a light meter. It is evident that such an arrangement
would not measure exactly the output, but would give
sufficiently relative values to enable significant data to be
taken regarding the remainder of the variables. In all cases,
standard six-inch fermentation tubes filled with a total of ten
cubic centimeters of media and culture were used. Two
oscillators were used. For the range 10-25 meters a tuned-plate
tuned-grid Gammatron 54 supplied the power, and below the range
of five meters to 90 centimeters a resonant-line oscillator was
used. (See Figures 1 and 2.) Utilized power output on all
frequencies was approximately ten watts. In no case was a
significant rise in temperature permitted. The standard type of
bacteria tested were Escherichia coli, which are
nonspore-forming vegetative cells found quite commonly in
polluted rivers and streams. All tests were made in the electric
field existing between two condenser plates.
3 Rate of growth of cell life under stimulus of high frequency.
referring to the curve in Figure 4, it can be seen that all
frequencies tested had somewhat of a lethal effect upon the
Escherichia coli. This effect became noticeably peaked around 60
megacycles. A standard time interval of one minute was used on
all cultures. The frequency measurements were made by means of a
radio receiver below 28 megacycles and by Lecher wires above 28
megacycles. Three sample and a control were taken for each of
six frequency determinations (350, 200, 60, 28, 14, and 11
megacycles). Each fermentation tube was shaken before exposure
to the high-frequency field to distribute the contents evenly
and to assure uniform conductivity, and was plated immediately
4 Effect of frequency on Escherichia coli.
Samples: 10 cubic centimeters; control: 450,000,000 per cubic
centimeter. Maximum temperature 85 degrees Fahrenheit.
Exposure in dielectric field.
AND CURRENT TESTS
holding that hysteresis and eddy-current losses in the
high-frequency field raise the instantaneous temperature of a
cell to a greater value than the surrounding medium would seem
to indicate that the magnetic field plays the important part: On
the other hand the theory of mechanical resonance implies that
voltage gradients about the bacteria are important. Therefore a
resonant line operating on 90 centimeters was devised of such
dimensions that a fermentation tube could be placed either at a
voltage node or a current node. Considerable unbalance in the
oscillator was caused by the former instance, and relative
values of power had to be supplied by adjusting the oscillator
to a specific increase of plate-power input. For the tuning
operations, sample tube of Escherichia coli had to be used to
simulate the anticipated load. Test conditions again involved a
standard time interval of one minute and a wave length of 90
centimeters. From the following data it is evident that a large
voltage gradient across the bacteria is more important that a
high current or an intense magnetic field: In two of the tests,
the tubes of Escherichia coli were completely sterilized. The
fermentation tubes filled with Escherichia coli acted very much
like a portion of the resonant circuit. In one case a high
current at a low potential drop made the tubes appear as a
conductance, and in the other case a high potential drop caused
the tubes to appear as a high resistance. An error was
undoubtedly introduced by virtue of radiation from the large
fins required for the current treatment. (See Figure 5.)
However, it is believed that the tests made are sufficient
evidence to support the generalization that obtaining voltage
gradients within the solution is far more important that
adjustment so as to pass a large radio-frequency current.
Figure 5 Arrangements for (left) passsing a high
current and (right) obtaining large voltage gradients.
Average Count Per Cubic Centimeter
with the element of time as a variable were made with
constant-current values as indicated by a light meter and
flashlight lamps as in the variable frequency tests previously
considered. The power used was approximately ten watts. Standard
six-inch fermentation tubes filled with ten cubic centimeters of
Escherichia coli and media were used. During the first fraction
of a minute almost all the bacteria are rendered nonviable. For
the next few minutes, until the solution is completely
sterilized, the lethal action appears to go on at a
progressively slower rate. Thus, the instantaneous killing power
might be written as a reciprocal function y=1/1_, where I
represents time. This is in accord with a cluster theory of
bacteria which indicates that colonies become more numerous as
smaller groups are considered. The larger colonies, while
representing a small percentage of the total, present a
different mechanical and electrical characteristic, and reduce
the effectiveness of the high frequency treatment. The
difference in numbers of the bacteria making up a colony as well
as their individual size could explain very well the lack of a
definite optimum frequency for treatment.
Figure 6 Effect of conductivity upon treatment of
The question of
the effect of variations in conductivity of the medium
containing the bacteria to be tested could be approached a
number of ways. Variation in the number of bacteria per cubic
centimeter causes variation in the conductivity. The type of
media in which they were grown may contain varying amounts of
salt. In addition, electrolytes such a sodium chloride or boric
acid may be added in appreciable quantities without harm to the
Escherichia coli used.
latter method was used to demonstrate the effect of varying
conductivity upon the immobilizing power of the high frequency.
A frequency of 28 megacycles was used with an approximate power
of ten watts. The dielectric field was used, and three
concentrations of salt were added, (0.5, 1.0 and 1.5 milligrams
per cubic centimeter). See Figure 6. The time of treatment was
were evident at once. A higher concentration of electrolyte
decreased the effectiveness of treatment. Again it appeared that
high currents, although they generated heat more rapidly, would
not give the results that a high-voltage stress would when
conductivity and current were lowered.
Figure 7 Time of treatment of Eschericha coli
Samples: 10 cubic centimeters; control: 500,000,000 per
cubic centimeter. Maximum temperature variation per sample: 70
to 110 degress Fahrenheit. Exposure in dielectric field.
Frequency: 28 megacycles.
should be emphasized again that the experimental work was only
developed to demonstrate trends in the immobilizing effect of
the variables listed. Exact measurement of the actual energy
dissipated within the fermentation tubes could not be made.
Particularly was this true at the ultrahigh frequencies where
the percentage of radiated power was appreciable.
undertaken showed that germicidal results were easy to obtain
even with small amounts of power, and suggests that for some
applications a consideration of sterilization by means of
exposure to high-frequency fields is warranted economically.
Also the possibilities of greatly speeding up life process such
as in yeast manufacture are evident. The best frequency to use
for treating a given micro-organism depends upon the side of the
organism. However, the bactericidal range can be roughly
assigned to the 1-20 meter band.
It is possible
that additional lethal effects might be experienced in the
ultrahigh frequency range, but control is difficult when it
comes to treating large masses of organic matter.
It is evident
that a high potential gradient should be maintained for maximum
effectiveness. This means that as little electrolyte as possible
should be allowed in the presence of the high-frequency field. A
high voltage necessitates a tuned circuit whose dimensions
conform to the quantity to be treated.
results of the time study suggest that it would be much more
economical to use a large amount of power for a short time. If
sufficient power is used to immobilize the largest cluster of
bacteria in a short interval of time, it appears that the
remainder would be easily disposed of.
close of the experimentation a new technique in testing the
effects of high frequency upon bacteria was devised which
greatly speeded up the analysis. The method is suggested for
further study and outlined as follows: The bacteria to be tested
are plated out in countable concentrations on a number of petri
dishes. Wire can be imbedded in the agar solutions used so as to
remain fixed in the gelatinous mass. Controlled amounts of
energy are then applied to the petri dishes and a direct count
can be made of colonies the next day. A variation of this
consists of omitting the wires and substituting two metal plates
on the top and bottom of the petri dishes, arranged to conform
to the latter’s dimensions.
1. Action of
Radio Waves on Insect Pests, .J.H. Davis. Scientific American,
Volume 148, 1933, page 272.
2. Biological Fundamentals of Radiation Therapy, E. P. Ellinger,
Electronic. Interscience Publishers, New York, N. Y., 1937.
3. Diathermy Measurement Technique, J. D. Kraus and R. W. Tweed.
Electronics, Volume 13, December 1940, pages 39-40.
4. Energy Distribution in Adjacent Zone of a Diapole in Aqua, J.
Patsold and K. Oswald. Strahlentherapir, Volume 66, 1939, pages
5. High-Frequency Practice, MacMillan Company, New York, N. Y.
6. Modified Radio Practice in Guayaquil, Public health Reports,
Volume 56 February 14, 1941, pages 2929-33.
7. Radio ( fifth edition). radio, Ltd., Santa Barbara, Calif.,
8. Radioactivity in Biological Experiments, Science News, Volume
90, December 29, 1939, page 615.
9. Radio Engineering, F. E. Terman. McGraw-Hill Book Company,
Inc., New York, N. Y., 1937
10. Report of the International Conference on Fever Therapy. The
Medical Press, New york, N. Y., 1937.
11. The Relation of Frequency to the Physiological Effects of
Ultrahigh-Frequency Currents. Journal of Experimental Medicine,
Volume 49, pages 301-21.
12. Short-wave Diathermy, Tibar de chqlnoky. Columbia University
13. the Nature of the Effect of a High-Frequency Electric Field
Upon Paramecium. Public Health reports, Volume 4, pages 339-47.
14. Ultrahigh-Frequency Vibrations: There Effects Upon Living
Organisms, A. Brunori and S. S. Torrisi. American Journal of
Phyisical therapy, Volume 7, 1930, pages