Inhibition of Bacterial Growth In Vitro Following ... - Physical Therapy

The current waveform from the
instrument changed greatly as it
flowed through the test medium. The
resistance to flow varied, making it
impossible to measure actual current
flow. Figure 2 shows a reduction in
total current and resistance to flow,as
indicated by the baseline not returning
to zero, as the waveform leaves
the stimulator and passes through the
medium.
Discussion
High voltage pulsed current can be
effective in killing common woundinfecting
bacteria in vitro. All organisms
tested were equally affected by
2 hours of exposure to HVPC above
250 V. The data analysis revealed a
strong positive linear relationship
between the voltage and the duration
of exposure to HVPC.
Exposure to HVPC at the cathode
accounted for most or all killing of
bacterial cells. The increasing pH
observed at the cathode was transient
and probably did not reach levels
extreme enough to directly kill bacterial
cells. No effect on skin pH following
a 30-minute application of HVPC at
100 V was reported by Newton and
Karselis. 20 Such a rise in pH could
have a static effect on growth that,
combined with the lethal effect of
HVPC, would help to keep bacterial
population levels down and enable
body defenses to fightoff the infection.
Effects of HVPC at the anode were
complicated by production of some
toxic electrochemical end products
created by passing current through
the wire. Zones of inhibition around
the cathode were recolonized by
motile bacteria, suggesting that no
permanent change had occurred
there. In contrast, organisms were
unable to recolonize the zone of discoloration
around the anode, suggesting
that lethal end products had accumulated
and persisted. These results
are comparable to those reported by
Barranco et al. 14
Caution must be exercised when
attempting to extrapolate the findings
of in vitro studies to predict results
Fig. A. Width of zone of inhibition at cathode after exposure to high voltage pulse
current at 300, 250, 200, and 150 V versus duration of exposure. Points represent
mean (± range) of pooled data from Escherichia coli, Staphylococcus aureus, and
Pseudomonas aeruginosa.
when applying the same intervention
to infected wounds in human subjects.
Present treatment protocols for
use of HVPC on infected wounds,
however, generally indicate a treatment
duration of 20 to 45 minutes
once or twice a day, with voltage
amplitude adjusted to a subthreshold
level for muscle contraction. Our
study used a much higher voltage
setting for a much longer exposure
duration than those used in current
clinical practice. Human subjects may
not be able to tolerate such high voltage
applications. Alternatively, the
actual current flow through the petri
plates was very low because of resistance
from the test medium. If there
is less resistance to current flow in
human skin, then lower settings might
be bactericidally effective in a clinical
setting.
Future avenues of investigation
include in vivo application of HVPC to
infected wounds as well as the application
of other types of electrical current
to microorganisms in vitro. If
future studies indicate that the exposure
of infected wounds to electrical
current results in decreased infection,
then it may be possible to effectively
treat infected wounds on a home-care
basis with portable electrical stimulators,
thereby making wound management
more cost effective.
Conclusion
Some clinicians use HVPC to inhibit
bacterial growth in infected wounds.
The results of this study indicate that,
although there is inhibition or killing
of bacteria in vitro at the cathode with
the application of HVPC, either the
voltage applied or duration of treatment,
or both, may need to be substantially
increased to achieve healing
of infected wounds. Studies
conducted in vivo are necessary to
32/654 Physical Therapy/Volume 69, Number 8/August 1989
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